🌡️ Celsius to Kelvin Converter

How do you convert Celsius to Kelvin?

Converting Celsius to Kelvin is one of the simplest temperature conversions. You simply add 273.15 to the Celsius temperature. The formula is: K = °C + 273.15.

For example: 0°C + 273.15 = 273.15 K (water freezing point), 100°C + 273.15 = 373.15 K (water boiling point), and 25°C + 273.15 = 298.15 K (standard room temperature). This simplicity exists because both scales have the same unit magnitude—one degree Celsius equals one kelvin in size. The only difference is the zero point: 0 K (absolute zero) corresponds to -273.15°C.

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

What is 0 degrees Celsius in Kelvin?

0 degrees Celsius equals exactly 273.15 Kelvin. This is the freezing point of water (ice point) at standard atmospheric pressure (101.325 kPa or 1 atmosphere). It's one of the most fundamental reference points in temperature measurement.

This conversion is straightforward using the formula: K = °C + 273.15, so 0 + 273.15 = 273.15 K. The Kelvin scale starts at absolute zero (0 K = -273.15°C), the theoretical lowest possible temperature where all classical thermal motion ceases. Therefore, all Celsius temperatures are offset by 273.15 kelvin units when converted.

Historically, before the 2019 SI redefinition, the Kelvin scale was defined using the triple point of water (the unique temperature and pressure where ice, liquid water, and water vapor coexist) as exactly 273.16 K. Since the triple point occurs at 0.01°C, this established the ice point at 273.15 K. While the modern definition of the kelvin has changed to be based on the Boltzmann constant, the numerical relationships remain the same: 0°C = 273.15 K is still exact.

This temperature is significant in many fields: it marks the transition between liquid water and ice, it's used as a calibration point for thermometers, and it appears in countless scientific calculations and standards. The BIPM SI Brochure confirms this relationship as part of the official definition linking Celsius to the thermodynamic Kelvin scale.

What is absolute zero in Celsius?

Absolute zero is -273.15°C, which equals 0 Kelvin (0 K). This is the lowest possible temperature in the universe, representing the theoretical point at which all classical thermal motion of particles reaches its minimum quantum mechanical state.

While often simplified as "all molecular motion stops," this description isn't entirely accurate from a quantum mechanical perspective. According to Heisenberg's uncertainty principle, even at absolute zero, particles retain zero-point energy—a minimum quantum mechanical motion that cannot be removed. This residual motion is a consequence of the wave-particle duality of matter at the quantum scale.

The third law of thermodynamics, formulated by Walther Nernst, states that the entropy of a perfect crystal approaches zero as temperature approaches absolute zero. More importantly for practical purposes, this law implies that absolute zero cannot be reached by any finite number of thermodynamic processes. We can approach it arbitrarily closely, but never actually reach it.

Modern experimental physics has come remarkably close to absolute zero:

• Laser cooling of atoms achieves temperatures in the microkelvin (10⁻⁶ K) to nanokelvin (10⁻⁹ K) range
• Dilution refrigerators used in condensed matter physics reach below 0.002 K (2 millikelvin)
• Adiabatic nuclear demagnetization has achieved temperatures below 100 picokelvin (10⁻¹⁰ K)
• The current record is approximately 38 picokelvin, achieved at NIST using magnetically trapped sodium atoms

These ultra-low temperatures enable observation of exotic quantum phenomena including Bose-Einstein condensates (predicted by Einstein and Bose, first created in 1995), superfluidity, superconductivity, and quantum phase transitions. Research at these temperatures has applications in quantum computing, precision measurement, and fundamental physics.

The value -273.15°C comes from the definition of the Kelvin scale starting at absolute zero (0 K), combined with the relationship that 0°C = 273.15 K. This makes -273.15°C the coldest conceivable temperature, a fundamental limit set by the laws of thermodynamics and quantum mechanics, not just a practical engineering challenge.

Why is Kelvin used in science instead of Celsius?

Kelvin is preferred in scientific calculations because it's an absolute temperature scale starting at absolute zero (0 K), making thermodynamic equations simpler and more intuitive. Unlike Celsius (which has an arbitrary zero point at water's freezing point), Kelvin directly relates to the fundamental physical property of thermal energy and molecular kinetic motion.

There are several key advantages of using Kelvin in scientific work:

1. Proportional Relationships: In the ideal gas law (PV = nRT), doubling the Kelvin temperature doubles the pressure (at constant volume) or volume (at constant pressure). This direct proportionality only works with absolute temperature. If you tried using Celsius, doubling from 10°C to 20°C wouldn't double the pressure—you'd need to go from 283.15 K to 566.3 K (which is 293.15°C), a non-intuitive relationship.

2. Thermodynamic Calculations: Entropy change (ΔS = Q/T), Carnot efficiency (η = 1 - T_cold/T_hot), and Stefan-Boltzmann radiation law (P = σT⁴) all require absolute temperature. Using Celsius would give physically meaningless or incorrect results. For example, a Carnot heat engine operating between 0°C and 100°C would show 100% efficiency if you incorrectly used Celsius (100/0 = undefined), but the correct calculation using Kelvin (273.15 K to 373.15 K) gives the proper efficiency of 26.8%.

3. Statistical Mechanics: The Boltzmann distribution, which describes particle energy distributions, uses absolute temperature: P(E) ∝ exp(-E/kT), where k is Boltzmann's constant. Temperature here directly represents average thermal energy per degree of freedom: ½kT for each translational, rotational, or vibrational mode. This connection only makes sense with absolute temperature starting at zero.

4. Rate Processes: Chemical reaction rates follow the Arrhenius equation: k = A·exp(-Ea/RT), where R is the gas constant and T must be in Kelvin. Activation energies and temperature-dependent phenomena naturally express themselves in terms of absolute temperature.

5. SI Base Unit: The kelvin is one of the seven SI base units, placing it on equal footing with meters, kilograms, seconds, etc. The Celsius scale is technically a derived scale defined in terms of kelvin. For international standardization and consistency, SI base units are preferred in scientific publications and measurements.

That said, Celsius remains valuable for practical, everyday measurements where its reference points (water freezing and boiling) provide intuitive context. Many scientists use Celsius for recording ambient conditions, material processing temperatures, or biological experiments where the absolute zero point is irrelevant, then convert to Kelvin only when performing theoretical calculations. The BIPM SI Brochure explicitly allows both units but recommends Kelvin for expressing thermodynamic temperature in scientific contexts.

What is 100 degrees Celsius in Kelvin?

100 degrees Celsius equals 373.15 Kelvin. This is the boiling point of water at standard atmospheric pressure (101.325 kPa or 1 atmosphere), one of the most important reference temperatures in science and everyday life.

The conversion is straightforward: K = °C + 273.15, so 100 + 273.15 = 373.15 K. This temperature marks the phase transition where liquid water rapidly converts to water vapor (steam) under normal atmospheric conditions at sea level.

This boiling point was historically significant in defining temperature scales. Anders Celsius originally set up his scale with 0° at water's boiling point and 100° at the freezing point (opposite of today's convention!). The scale was later inverted to its current form: 0°C = freezing, 100°C = boiling. These two fixed points—273.15 K and 373.15 K in Kelvin—provided an easily reproducible 100-degree interval for calibrating thermometers.

It's important to note that water's boiling point varies with pressure according to the Clausius-Clapeyron equation. At higher altitudes where atmospheric pressure is lower, water boils at temperatures below 100°C (373.15 K). For example:

• At sea level (101.325 kPa): 100°C = 373.15 K
• At Denver, Colorado (~84 kPa, 1 mile elevation): ~95°C = 368.15 K
• At Mount Everest summit (~34 kPa): ~72°C = 345.15 K
• In a pressure cooker (~200 kPa): ~120°C = 393.15 K

The standard boiling point of 100°C/373.15 K specifically refers to a pressure of exactly 101.325 kPa (1 atmosphere). This is codified in the International Temperature Scale of 1990 (ITS-90) maintained by BIPM as a practical realization of thermodynamic temperature. For high-precision thermometry, the triple point of water (273.16 K) is preferred over the boiling point because it's less sensitive to impurities and doesn't require precise pressure control—it occurs at a unique temperature-pressure combination.

Can Kelvin be negative?

No, Kelvin cannot be negative in normal circumstances. The Kelvin scale is an absolute temperature scale that starts at absolute zero (0 K = -273.15°C), the lowest theoretically possible temperature. By definition, there are no temperatures below absolute zero on the Kelvin scale in classical and conventional quantum thermodynamics.

Absolute zero represents the state where a system has minimal thermal energy—specifically, only zero-point energy remains due to quantum mechanical uncertainty. The third law of thermodynamics states that absolute zero cannot be reached by any finite series of processes, but more fundamentally, it defines the lower bound of temperature. Going below 0 K would imply extracting energy from a system that already has the minimum possible energy, which violates energy conservation.

However, there's a fascinating quantum mechanical exception: In certain exotic quantum systems with population inversion (where higher energy states are more populated than lower ones), physicists have mathematically defined "negative absolute temperatures." But these are deeply counterintuitive and don't represent cold temperatures at all.

Negative absolute temperatures in specialized quantum systems:

• Are actually hotter than infinite temperature: The temperature scale conceptually goes: 0 K → +∞ K → -∞ K → 0 K (forming a loop, not a line)
• Require systems with bounded energy: Population inversion can only occur when there's a maximum energy level, unlike typical kinetic energy which is unbounded
• Have been created in laboratories: Researchers have achieved negative temperatures in nuclear spin systems and ultracold quantum gases (Braun et al., 2013, Science)
• Heat flows backwards: Energy spontaneously flows from negative temperature systems to positive temperature systems, even if the positive temperature is extremely high

For practical purposes in virtually all real-world applications—chemistry, engineering, meteorology, medicine, materials science, and most physics—Kelvin is always positive or zero. The temperature scale ranges from 0 K (absolute zero) upward to arbitrarily high values: room temperature (~293 K), the Sun's surface (~5778 K), stellar cores (millions of kelvin), or even higher in fusion reactors and particle collisions.

According to NIST and BIPM standards, thermodynamic temperature is defined as a positive quantity. The mathematical formalism allowing negative temperatures in population-inverted systems is a specialized theoretical construct that, while scientifically fascinating, doesn't affect standard temperature measurements or the practical use of the Kelvin scale. For all everyday and most scientific purposes, remember: Kelvin temperatures range from 0 K to positive infinity, never negative.

What is room temperature in Kelvin?

Room temperature typically ranges from 293.15 to 298.15 Kelvin (20-25°C or 68-77°F), depending on the standard being referenced and regional comfort preferences. Different organizations define slightly different values for "standard room temperature" based on their specific needs.

Official Standards:

• NIST (National Institute of Standards and Technology): 293.15 K (20°C) is commonly used as the reference temperature for physical constants and engineering calculations
• IUPAC (International Union of Pure and Applied Chemistry): 298.15 K (25°C) is the standard temperature for reporting thermochemical data, paired with a pressure of 1 bar (100 kPa) for "standard ambient temperature and pressure" (SATP)
• ISO (International Organization for Standardization): ISO 7730 recommends 293-296 K (20-23°C) for sedentary activity in offices
• ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers): Recommends 294.15-297.15 K (21-24°C) in summer and 293.15-295.15 K (20-22°C) in winter for thermal comfort

Why Different Standards? The choice of room temperature standard depends on the application:

For scientific experiments: Climate-controlled laboratories maintain strict temperature control, typically at 293±1 K (20±1°C), to ensure reproducible results. Some precision measurements require even tighter control: ±0.1 K or better. This stability is crucial for experiments sensitive to thermal expansion, reaction rates, or electronic properties.

For thermochemical data: Chemists prefer 298.15 K (25°C) because it's close to typical laboratory conditions in many parts of the world and provides a convenient standard for tabulating enthalpies of formation, Gibbs free energies, and equilibrium constants. Having a universal standard allows comparison of data from different laboratories worldwide.

For human comfort: Thermal comfort standards vary by climate, clothing, activity level, and cultural norms. The "comfort zone" typically spans 293-300 K (20-27°C) with the preferred temperature depending on humidity and air movement. Offices in hot climates often maintain lower temperatures (around 295 K or 22°C) while those in colder regions might use slightly higher setpoints.

For electronics and computing: Data centers and electronics testing facilities often maintain temperatures around 291-295 K (18-22°C) to prevent overheating of equipment while balancing energy efficiency. Modern data centers increasingly use higher temperatures (up to 300 K or 27°C) with improved cooling designs to reduce energy consumption.

In practice, when someone refers to "room temperature" without further specification in a scientific paper, it typically means approximately 293-298 K (20-25°C). If precise temperature control was important for the experiment, the actual value should be explicitly stated. The NIST and BIPM recommend always specifying the exact temperature rather than using ambiguous terms like "room temperature" for reproducible scientific work.

Is there a degree symbol for Kelvin?

No, Kelvin does not use a degree symbol. The correct notation is simply 'K' without the degree symbol (°). For example, write "273.15 K" not "273.15°K". This is an official convention established by the 13th General Conference on Weights and Measures (CGPM) in 1968 and is strictly followed in all scientific literature.

Why No Degree Symbol? The absence of the degree symbol emphasizes a fundamental distinction between absolute and relative temperature scales:

Absolute scale (Kelvin): Measures thermodynamic temperature directly from absolute zero. Like other SI base units (meter, kilogram, second), it represents a fundamental quantity without reference to arbitrary fixed points. We say "5 meters" not "5 degrees of meter," and similarly we say "300 kelvin" or "300 K," not "300 degrees Kelvin."

Relative scales (Celsius, Fahrenheit): Measure temperature relative to arbitrarily chosen fixed points (like water's freezing and boiling points). The degree symbol (°) indicates these are "degrees" above or below those reference points, not absolute measurements of thermal energy.

Historical Context: Prior to 1968, you would occasionally see "°K" (degrees Kelvin) in older scientific literature, similar to "°C" for degrees Celsius. However, Lord Kelvin (William Thomson) originally conceived his scale as an absolute measure, and the modern SI system formalized this by removing the degree symbol and changing the unit name from "degrees Kelvin" to simply "kelvin" (lowercase when spelled out, uppercase K as the symbol).

Correct Usage Examples:

• ✓ 273.15 K (correct)
• ✗ 273.15°K (incorrect—outdated notation, not used since 1968)
• ✓ The temperature is 300 kelvin (correct—lowercase when spelled out)
• ✓ 300 K (correct—uppercase symbol)
• ✓ A temperature change of 5 K or 5 kelvins (both correct)
• ✗ A temperature change of 5 degrees Kelvin (incorrect—don't say "degrees")

Temperature Differences: Interestingly, for temperature intervals or differences (not absolute temperatures), you can correctly say "5 degrees Celsius" or simply "5 kelvins" or "5 K"—they're numerically identical since one degree Celsius equals one kelvin in magnitude. But for absolute temperature, always use kelvin without the degree symbol.

According to the BIPM SI Brochure (9th edition), Section 2.3.1: "The unit of thermodynamic temperature, the kelvin, is the SI base unit with unit symbol K." The brochure explicitly shows examples using K without the degree symbol throughout. Following this convention is important for scientific writing, publications, and technical communications to maintain consistency with international standards.

What is the Kelvin scale based on?

The Kelvin scale is based on absolute zero (0 K) and, since May 20, 2019, is defined by fixing the Boltzmann constant (k) to an exact numerical value. This modern definition represents a fundamental shift in how we define temperature, linking it directly to energy at the molecular level rather than to material properties.

Current Definition (Post-2019 SI Redefinition): The kelvin is defined by taking the fixed numerical value of the Boltzmann constant k to be exactly 1.380649×10⁻²³ J/K (joules per kelvin), where the joule is defined in terms of the kilogram, meter, and second. This definition is expressed mathematically as:

k = 1.380649×10⁻²³ J K⁻¹ = 1.380649×10⁻²³ kg m² s⁻² K⁻¹

The Boltzmann constant relates the average kinetic energy of particles in a gas to temperature: E = (3/2)kT for monatomic ideal gases. By fixing k, temperature becomes fundamentally linked to energy, making it a truly physical (rather than artifact-based) measurement.

Historical Definition (Pre-2019): Before the SI redefinition, the kelvin was defined using the triple point of water—the unique temperature and pressure (273.16 K and 611.657 Pa) at which ice, liquid water, and water vapor coexist in thermodynamic equilibrium. Specifically, the kelvin was defined as 1/273.16 of the thermodynamic temperature of the triple point of water.

This historical definition had practical advantages: the triple point can be easily reproduced in specialized cells (triple point cells) for calibrating thermometers with high precision (±0.0001 K). However, it had a fundamental limitation—temperature was defined by a property of a specific substance (water) rather than by universal physical constants.

Absolute Zero Foundation: Regardless of the formal definition, the Kelvin scale's conceptual foundation remains absolute zero (0 K), the lowest theoretically possible temperature. At absolute zero:

• All classical thermal motion ceases (though quantum zero-point motion persists due to Heisenberg uncertainty)
• Entropy of a perfect crystal approaches zero (third law of thermodynamics)
• No thermal energy can be extracted from the system
• The system exists in its quantum mechanical ground state

Why Change the Definition? The 2019 SI redefinition (affecting not just the kelvin but also the kilogram, ampere, and mole) aimed to base all SI units on invariant constants of nature rather than physical artifacts or material properties. Benefits include:

• Universal reproducibility: Any laboratory with sufficiently precise equipment can realize the kelvin without needing a reference artifact
• Improved consistency: All SI units now derive from fundamental constants (speed of light, Planck constant, elementary charge, Boltzmann constant, Avogadro constant)
• Better scalability: The new definition works equally well at all temperatures, from nanokelvin to millions of kelvin, without needing different reference points for different ranges
• Philosophical consistency: Temperature now has a clear physical meaning—average thermal energy per degree of freedom—rather than being defined by water's properties

Practical Impact: For everyday users, nothing changed. The numerical value of temperatures remained identical: water still freezes at approximately 273.15 K and boils at approximately 373.15 K at standard pressure. The change only affects high-precision metrology laboratories that realize the kelvin from first principles. According to the BIPM, the new definition maintains continuity with previous measurements while providing a more robust foundation for future advances in thermometry.

The International Temperature Scale of 1990 (ITS-90) remains the practical guide for realizing thermodynamic temperature with various fixed points and interpolation methods across different temperature ranges, but it's now understood as an approximation to the fundamental Boltzmann-constant-based definition rather than the definition itself.