🌡️ Kelvin to Celsius Converter

How do you convert Kelvin to Celsius?

Converting Kelvin to Celsius is one of the simplest temperature conversions in science. You simply subtract 273.15 from the Kelvin temperature. The formula is: °C = K - 273.15.

For example: 273.15 K - 273.15 = 0°C (water freezing point), 373.15 K - 273.15 = 100°C (water boiling point), and 298.15 K - 273.15 = 25°C (standard room temperature). This simplicity exists because both scales have identical unit magnitudes—a change of one kelvin equals a change of one degree Celsius in size. The only difference is where each scale starts: the Kelvin scale begins at absolute zero (0 K), while the Celsius scale begins at water's freezing point (0°C), which is 273.15 kelvin above absolute zero.

According to NIST Special Publication 811, the constant 273.15 is exact by definition in the SI system, meaning there's no inherent rounding error in the conversion factor—only in your temperature measurement itself. The value represents the temperature difference between absolute zero and the ice point of water at standard atmospheric pressure (101.325 kPa). Since the 2019 SI redefinition, both Kelvin and Celsius are ultimately defined in terms of the Boltzmann constant, but their numerical relationship (the 273.15 offset) remains unchanged for all practical purposes.

This makes Kelvin-Celsius conversion much simpler than Fahrenheit conversions, which require both multiplication (by 9/5 or 5/9) and addition/subtraction. With Kelvin and Celsius, you only need to add or subtract a single constant.

What is 0 Kelvin in Celsius?

0 Kelvin equals -273.15 degrees Celsius. This is absolute zero, the lowest theoretically possible temperature in the universe. At this point, a system reaches its minimum possible energy state—not truly zero energy (which would violate quantum mechanics), but the lowest quantum mechanical energy level known as zero-point energy.

The conversion is straightforward using the formula °C = K - 273.15, giving us: 0 - 273.15 = -273.15°C. This temperature represents a fundamental limit in physics. According to the third law of thermodynamics, formulated by Walther Nernst in the early 20th century, absolute zero cannot be reached by any finite series of thermodynamic processes. We can approach it arbitrarily closely but never actually attain it.

The popular description that "all molecular motion stops" at absolute zero is a useful simplification but not entirely accurate from a quantum mechanical perspective. According to Heisenberg's uncertainty principle, even at 0 K, particles retain a minimum quantum motion called zero-point motion or vacuum fluctuations. This residual energy is a consequence of wave-particle duality at the quantum scale and cannot be removed.

Modern experimental physics has achieved temperatures remarkably close to absolute zero:

• Dilution refrigerators reach below 0.002 K (2 millikelvin or -273.148°C) for solid-state physics experiments
• Laser cooling techniques achieve microkelvin to nanokelvin temperatures (10⁻⁶ to 10⁻⁹ K) for isolated atoms
• Adiabatic nuclear demagnetization has reached picokelvin temperatures (10⁻¹² K) in nuclear spin systems
• The current record is approximately 38 picokelvin (0.000000000038 K or -273.149999999962°C), achieved by researchers at NIST with magnetically trapped sodium atoms

At these ultra-low temperatures, exotic quantum phenomena emerge: Bose-Einstein condensates (where thousands of atoms occupy the same quantum state), superfluidity (frictionless flow), superconductivity (zero electrical resistance), and quantum phase transitions. This research has applications in quantum computing, precision measurement, and tests of fundamental physics.

The value -273.15°C as absolute zero has been known since the 19th century through extrapolation of gas laws. It represents the temperature at which an ideal gas would theoretically have zero volume and pressure—a physical impossibility (gases liquefy first), but a useful mathematical limit that led to the concept of absolute zero and ultimately to the Kelvin scale.

What is 273.15 Kelvin in Celsius?

273.15 Kelvin equals exactly 0 degrees Celsius. This is the freezing point of water (ice point) at standard atmospheric pressure (101.325 kPa or 1 atmosphere), one of the most fundamental reference temperatures in science and everyday life.

The conversion is simple: °C = 273.15 - 273.15 = 0°C. This temperature represents the phase transition where liquid water transforms into solid ice under normal pressure conditions. It's historically significant because the Celsius scale was originally defined using this as one of its two fixed points (along with the boiling point at 100°C).

The value 273.15 K for water's freezing point emerged from the historical definition of the Kelvin scale. Before 2019, the kelvin was defined using the triple point of water as exactly 273.16 K—the unique temperature and pressure where ice, liquid water, and water vapor coexist in equilibrium. Since the triple point is 0.01°C above the normal freezing point, this established: 0°C = 273.16 - 0.01 = 273.15 K.

Although the 2019 SI redefinition changed the formal basis of the kelvin (now defined via the Boltzmann constant rather than water's triple point), the numerical relationship remains: 0°C is still 273.15 K. This continuity was carefully maintained to avoid disrupting decades of scientific data and measurements.

This temperature is crucial in many fields:

• Thermometer Calibration: 273.15 K (0°C) is one of the easily reproducible fixed points used to calibrate thermometers, using ice-water mixtures at standard pressure
• Climate Science: The 273.15 K threshold determines precipitation type (rain vs snow), sea ice formation, and permafrost boundaries
• Materials Science: Many material properties change dramatically at water's freezing point, affecting concrete curing, road maintenance, and construction practices
• Biology: Cryopreservation of biological samples begins at this temperature, with protocols carefully managing ice crystal formation
• Chemistry: The ice point serves as a convenient reference for thermodynamic calculations and phase diagrams

It's important to note that water's freezing point varies with pressure (decreases by about 0.0074°C per atmosphere of increased pressure) and purity (dissolved salts or impurities lower the freezing point). The 273.15 K value specifically refers to pure water at exactly 101.325 kPa pressure—the formally defined "standard atmospheric pressure."

Why subtract 273.15 when converting Kelvin to Celsius?

We subtract 273.15 because that's the offset between the two scales' zero points. The Kelvin scale starts at absolute zero (0 K = the coldest possible temperature), while the Celsius scale starts at water's freezing point, which happens to be 273.15 kelvins above absolute zero.

Both scales have identical unit sizes—a change of 1 K equals a change of 1°C in magnitude. This means the scales are parallel; they just start at different points. To convert from Kelvin to Celsius, we need to shift by the offset between their starting points: 273.15 units.

Think of it as two rulers measuring the same thing but starting at different reference points. If one ruler (Kelvin) starts at absolute zero and reads 300, and another ruler (Celsius) starts 273.15 units up from there, the second ruler would read 300 - 273.15 = 26.85.

Why specifically 273.15? This value represents the fundamental physical quantity: how many kelvin units separate water's freezing point from absolute zero. This is determined by the thermal energy difference between:

• Absolute zero (0 K): Minimum quantum mechanical energy state; essentially no thermal energy
• Water's freezing point (273.15 K): The kinetic energy of water molecules at the liquid-ice transition at standard pressure

Historically, this value was established when Lord Kelvin (William Thomson) proposed an absolute temperature scale in the 1850s. By extrapolating the behavior of gases (using the ideal gas law), scientists determined that absolute zero would be approximately -273°C. Later, more precise measurements and the definition of the kelvin using water's triple point (273.16 K) established the exact value as 273.15 K for the ice point.

According to NIST SP 811 and the BIPM SI Brochure, the relationship t/°C = T/K − 273.15 is exact by definition in the current SI system. Even after the 2019 SI redefinition changed how the kelvin is fundamentally defined (now based on the Boltzmann constant), this numerical relationship was preserved to maintain continuity with historical data and measurements.

The simplicity of the conversion (just add or subtract a constant) makes Kelvin-Celsius conversions much easier than Fahrenheit conversions, which require both multiplication and addition/subtraction because Fahrenheit has both a different zero point AND a different unit size.

What is 298.15 Kelvin in Celsius?

298.15 Kelvin equals 25 degrees Celsius. This is standard temperature as defined by IUPAC (International Union of Pure and Applied Chemistry) for reporting thermochemical data, combined with a pressure of 1 bar (100 kPa) to constitute "standard ambient temperature and pressure" (SATP).

The conversion is straightforward: °C = 298.15 - 273.15 = 25°C. This temperature is widely used as a reference point in chemistry, materials science, and engineering because it represents typical ambient conditions in many parts of the world—comfortable room temperature in climate-controlled environments.

Why 298.15 K (25°C) as a standard?

• Practical convenience: It's close to typical laboratory conditions in many countries, making experimental data more relevant to standard tables
• Nice round number: 25°C is a convenient value in the Celsius scale, avoiding decimals in most thermochemical calculations
• Historical adoption: The choice emerged from consensus among chemists and was formalized by IUPAC as the standard reference for thermodynamic tables
• Moderate conditions: Many chemical reactions, material properties, and biological processes show representative behavior at this temperature—neither too cold nor too hot

Applications of 298.15 K standard:

• Thermochemical Data: Standard enthalpies of formation (ΔH°f), standard Gibbs free energies (ΔG°f), and standard entropies (S°) are universally tabulated at 298.15 K and 1 bar pressure. This allows chemists worldwide to compare data from different sources and predict reaction spontaneity
• Equilibrium Constants: The standard Gibbs free energy relationship ΔG° = -RT ln(K) typically uses T = 298.15 K for tabulated equilibrium constants
• Electrochemistry: Standard reduction potentials (E°) are measured and tabulated at 298.15 K (25°C) for predicting battery voltages and corrosion behavior
• Material Properties: Density, viscosity, electrical conductivity, and other material properties are often reported at 25°C for standardization
• Biological Sciences: Many biochemical studies use 25°C as a reference, though 37°C (310.15 K, body temperature) is often used for physiologically relevant conditions

It's worth noting that different organizations use slightly different standard temperatures:

• IUPAC: 298.15 K (25°C) for thermochemical data
• NIST: Often uses 293.15 K (20°C) for physical constants and engineering calculations
• STP (Standard Temperature and Pressure): Historically 273.15 K (0°C) and 1 atm (101.325 kPa), though IUPAC now recommends 273.15 K and 1 bar (100 kPa)

When citing or using reference data, always check which standard temperature was used, as thermodynamic properties can vary significantly with temperature. For example, many reactions that are non-spontaneous (ΔG° > 0) at 298.15 K become spontaneous at higher temperatures due to entropy contributions.

What is 373.15 Kelvin in Celsius?

373.15 Kelvin equals 100 degrees Celsius. This is the boiling point of water at standard atmospheric pressure (101.325 kPa or 1 atmosphere), representing the temperature at which liquid water rapidly vaporizes into steam.

The conversion: °C = 373.15 - 273.15 = 100°C. This temperature, along with the freezing point (0°C = 273.15 K), historically defined the Celsius scale. Anders Celsius originally proposed dividing the temperature range between water's freezing and boiling points into 100 equal divisions—hence the name "centigrade" (hundred steps) before it was officially renamed "Celsius" in 1948.

The boiling point of water is particularly important because:

• Thermometer Calibration: Along with the freezing point, it provides an easily reproducible fixed point for calibrating thermometers
• Phase Transition: It marks the temperature where water's vapor pressure equals atmospheric pressure, causing rapid phase change from liquid to gas throughout the liquid (not just at the surface, as in evaporation)
• Cooking & Food Safety: Boiling at 100°C/373.15 K is used to sterilize water and cook food, as most bacteria and pathogens are destroyed at this temperature
• Industrial Processes: Many steam-based industrial processes and power generation systems rely on water's boiling point and latent heat of vaporization

Important note on pressure dependence: Unlike many physical constants, water's boiling point varies significantly with atmospheric pressure. The 100°C (373.15 K) value specifically applies at standard atmospheric pressure of exactly 101.325 kPa (1 atm). At different pressures:

• Higher altitude (lower pressure): Water boils at temperatures below 100°C
  • Denver, Colorado (~1 mile elevation, ~84 kPa): boiling point ≈ 95°C (368.15 K)
  • Mount Everest summit (~34 kPa): boiling point ≈ 72°C (345.15 K)
• Pressure cooker (higher pressure ~200 kPa): Water boils at ≈ 120°C (393.15 K), allowing faster cooking
• Vacuum conditions: Water can boil at room temperature or below if pressure is sufficiently reduced

The relationship between pressure and boiling point is described by the Clausius-Clapeyron equation, which relates vapor pressure to temperature for phase transitions. This is why high-altitude cooking requires recipe adjustments and why pressure cookers reduce cooking time.

For high-precision thermometry, the International Temperature Scale of 1990 (ITS-90) uses the triple point of water (273.16 K = 0.01°C) rather than the boiling point as a fixed point, because the triple point is more precisely reproducible (it occurs at a unique temperature-pressure combination) and doesn't require precise pressure control like the boiling point does.

Can you convert negative Kelvin to Celsius?

In normal circumstances, Kelvin temperatures cannot be negative because the Kelvin scale is an absolute scale starting at absolute zero (0 K), the lowest theoretically possible temperature. According to the third law of thermodynamics and fundamental physics, there is no temperature below absolute zero—0 K represents the minimum energy state of matter (specifically, the quantum mechanical zero-point energy).

If you were to hypothetically apply the conversion formula to a negative Kelvin value, the mathematics would work: °C = K - 273.15, so -1 K would become -1 - 273.15 = -274.15°C. However, this result is physically meaningless in almost all contexts because negative Kelvin temperatures don't exist in normal thermodynamics.

The Exotic Exception: Negative Absolute Temperatures in Quantum Systems

There is, however, a fascinating and highly specialized exception in quantum physics. In certain exotic quantum systems with population inversion (where higher energy states are more populated than lower energy states), physicists have mathematically defined "negative absolute temperatures." But these are profoundly counterintuitive and don't represent cold at all.

Key points about negative absolute temperatures:

• Hotter than infinite positive temperature: The temperature scale actually forms a loop: 0 K → +∞ K → -∞ K → 0 K. Negative temperatures are "hotter" than any positive temperature in the sense that heat flows spontaneously from negative temperature systems to positive temperature systems
• Require bounded energy systems: Negative temperatures can only occur in systems with a maximum possible energy level, unlike normal kinetic energy which is unbounded. Examples include nuclear spin systems and certain ultracold quantum gases
• Created in laboratories: Researchers have achieved negative absolute temperatures in specialized experiments with ultracold atoms (Braun et al., Science 339, 52-55, 2013) and nuclear magnetic resonance systems
• Not "colder than absolute zero": Despite the negative sign, these states have extremely high energy—objects at negative temperatures have more energy than objects at any positive temperature

For example, if a specialized quantum system were at -1 K (negative one kelvin), converting to Celsius would give -274.15°C, but this wouldn't represent extreme cold. Instead, it would represent an extremely high-energy state hotter than infinite positive temperature, due to the way temperature is defined thermodynamically as ∂S/∂E (the change in entropy with respect to energy).

Practical Conclusion: For all everyday applications—weather, cooking, scientific experiments, industrial processes, medical uses, cryogenics, materials science, and virtually all physics and chemistry—Kelvin temperatures are always positive or zero. The Kelvin scale ranges from 0 K (absolute zero) to arbitrarily high positive values. Negative Kelvin temperatures are a fascinating theoretical edge case in specialized quantum systems but have no relevance to temperature conversion for practical purposes.

According to NIST and BIPM standards, thermodynamic temperature is defined as a positive-valued quantity. When using this converter or any temperature conversion in normal contexts, always ensure input temperatures are ≥ 0 K.

What is room temperature in Kelvin and Celsius?

Room temperature typically ranges from 293.15 to 298.15 Kelvin (20-25°C or 68-77°F), depending on the standard being referenced, regional preferences, and the specific application. There isn't a single universal "room temperature" value—different scientific organizations and contexts use slightly different standards.

Official Standards for Room Temperature:

• NIST (National Institute of Standards and Technology): 293.15 K (20°C) is commonly used as the reference temperature for reporting physical constants, material properties, and engineering calculations. This provides a convenient standard that's slightly cooler than typical comfort levels
• IUPAC (International Union of Pure and Applied Chemistry): 298.15 K (25°C) is the standard temperature for thermochemical data, combined with 1 bar pressure (100 kPa). This is used for tabulating enthalpies of formation, Gibbs free energies, and equilibrium constants
• ISO 7730 (International Organization for Standardization): Recommends 293-296 K (20-23°C) for sedentary office work, varying with season, clothing, and activity level
• ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers): Recommends comfort ranges of 294.15-297.15 K (21-24°C) in summer and 293.15-295.15 K (20-22°C) in winter for indoor environments

Converting Between Standards:

• 293.15 K - 273.15 = 20°C (NIST standard)
• 298.15 K - 273.15 = 25°C (IUPAC standard)
• 295.15 K - 273.15 = 22°C (typical comfortable indoor temperature)

Why Different Standards Exist:

• Scientific experiments: Laboratories often maintain 293.15±1 K (20±1°C) for reproducibility, with precise climate control. Some sensitive measurements require even tighter tolerances (±0.1 K)
• Thermochemical data: 298.15 K (25°C) provides a convenient reference close to ambient conditions in many parts of the world, making data more relevant without requiring extreme temperature control
• Human comfort: Optimal comfort depends on humidity, air movement, clothing, and activity. The "comfort zone" generally spans 293-300 K (20-27°C), with individual preferences varying significantly
• Energy efficiency: Building standards balance human comfort with energy costs. Wider acceptable ranges (e.g., 20-25°C) reduce HVAC energy consumption
• Regional variation: Countries in hot climates often prefer cooler indoor temperatures (around 295 K or 22°C), while those in cold climates may use slightly warmer settings

Context-Specific Room Temperatures:

• Electronics testing: Often 291-295 K (18-22°C) to prevent overheating while maintaining realistic operating conditions
• Data centers: Historically 291-295 K (18-22°C), though modern facilities increasingly use higher temperatures (up to 300 K or 27°C) with improved cooling designs for energy efficiency
• Museums and archives: Typically maintain 293-295 K (20-22°C) with controlled humidity to preserve artifacts
• Biological laboratories: Often use 298.15 K (25°C) or 310.15 K (37°C, body temperature) depending on whether studying ambient organisms or mammalian systems

When citing experimental data or using reference values, it's important to specify the exact temperature rather than using the ambiguous term "room temperature." According to NIST guidelines for scientific publications, always state the actual temperature (e.g., "measurements were performed at 293.15 K") rather than generic terms, to ensure reproducibility and proper interpretation of results.

What is the difference between Kelvin and Celsius scales?

The main difference between Kelvin and Celsius is their zero point: Kelvin starts at absolute zero (0 K = -273.15°C), while Celsius starts at water's freezing point (0°C = 273.15 K). However, both scales have identical unit magnitudes—a change of 1 K equals a change of 1°C—so they're parallel scales offset by exactly 273.15 units.

Key Differences:

1. Zero Point (Origin):

• Kelvin: 0 K is absolute zero, the lowest theoretically possible temperature where systems reach their minimum quantum energy state. This is a fundamental physical limit based on thermodynamics and quantum mechanics
• Celsius: 0°C is water's freezing point at standard atmospheric pressure—an arbitrary but practical reference point based on a common substance's phase transition

2. Scale Type:

• Kelvin: An absolute temperature scale measuring thermodynamic temperature from a fundamental physical zero. Temperature ratios are meaningful: 200 K is twice as hot as 100 K in terms of average molecular kinetic energy
• Celsius: A relative temperature scale measuring "degrees" above or below an arbitrary reference. Temperature ratios are NOT meaningful: 40°C is not twice as hot as 20°C in absolute terms

3. Notation:

• Kelvin: Does NOT use the degree symbol. Write "300 K" or "300 kelvin" (lowercase when spelled out)
• Celsius: DOES use the degree symbol. Write "25°C" or "25 degrees Celsius"

4. SI Status:

• Kelvin: One of the seven SI base units, on equal footing with meter, kilogram, second, ampere, mole, and candela. Fundamental to the International System of Units
• Celsius: A derived scale defined in terms of kelvin: t/°C = T/K − 273.15. Officially recognized and widely used, but not a base unit

5. Scientific Use:

• Kelvin: Required for thermodynamic equations where absolute temperature matters:
  • Ideal gas law: PV = nRT (T must be in kelvin)
  • Carnot efficiency: η = 1 - T_cold/T_hot (requires kelvin for correct ratios)
  • Stefan-Boltzmann radiation: P = σT⁴ (T in kelvin)
  • Boltzmann distribution: P(E) ∝ exp(-E/kT) (T in kelvin)
  • Arrhenius equation for reaction rates: k = A·exp(-Ea/RT) (T in kelvin)
• Celsius: Preferred for practical measurements where absolute zero is irrelevant: weather reports, cooking, medical thermometers, material processing temperatures, everyday laboratory measurements

6. Negative Values:

• Kelvin: Cannot be negative in normal circumstances (absolute scale starts at zero); only exotic quantum systems with population inversion have mathematically defined negative temperatures
• Celsius: Commonly negative for cold temperatures. Any temperature below water's freezing point (below 273.15 K) is negative in Celsius: -20°C is a cold winter day (253.15 K)

7. Definition:

• Kelvin: Since 2019, defined by fixing the Boltzmann constant to exactly 1.380649×10⁻²³ J/K, linking temperature directly to energy
• Celsius: Defined by its relationship to kelvin: the magnitude of 1°C equals 1 K, with a 273.15 offset

Practical Example Comparing Both:

• Water freezes: 273.15 K = 0°C
• Room temperature: 293.15 K = 20°C (difference of 20 units in both)
• Water boils: 373.15 K = 100°C (another 80-unit increase in both)
• Temperature change: ΔT = 50 K = Δt = 50°C (identical magnitudes)

When to Use Which Scale:

• Use Kelvin when: Doing thermodynamic calculations, working with gas laws, calculating reaction rates, dealing with ratios of temperatures, publishing in physics/chemistry journals, or when absolute temperature is conceptually important
• Use Celsius when: Reporting weather, measuring body temperature, cooking, describing laboratory ambient conditions, or communicating with non-scientific audiences who find Celsius more intuitive

According to the BIPM SI Brochure and NIST SP 811, both units are acceptable for scientific use, but kelvin is recommended for expressing thermodynamic temperature when absolute temperature is relevant to the calculation or measurement. Many scientists routinely convert between them: measuring and recording in Celsius (more intuitive), then converting to Kelvin for calculations (mathematically necessary), and sometimes converting results back to Celsius for reporting (more accessible to broader audiences).