Instant °F to °C Temperature Conversion | Medical Fever Chart | Based on NIST & WHO Standards
The Fahrenheit scale (°F) is a temperature scale based on one proposed in 1724 by German physicist Daniel Gabriel Fahrenheit. In this scale, water freezes at 32°F and boils at 212°F at standard atmospheric pressure—a 180-degree separation. The scale was originally defined using three reference points: 0°F was the freezing point of a brine solution (salt and water mixture), 32°F was the freezing point of pure water, and 96°F was approximately human body temperature (later refined to 98.6°F as medical measurement improved).
According to NIST Special Publication 811, the Fahrenheit scale is primarily used in the United States, Bahamas, Belize, Cayman Islands, and Palau for everyday temperature measurement, weather reporting, and cooking. The official conversion relationship to Celsius is: t/°C = (5/9)(t/°F - 32), where t represents the numerical value of temperature. This formula provides the exact mathematical relationship between the two scales.
The Celsius scale (°C), also known as centigrade, is the temperature scale used by the International System of Units (SI). It was developed by Swedish astronomer Anders Celsius in 1742 and is now the most widely used temperature scale globally. The scale is defined by two fixed points: 0°C is the freezing point of water at standard atmospheric pressure, and 100°C is the boiling point of water. This 100-degree separation makes the Celsius scale intuitive and decimal-friendly.
According to the BIPM SI Brochure (Section 2.1.1.5), the Celsius temperature (t) is defined in terms of thermodynamic temperature (T) as: t = T - T₀, where T₀ = 273.15 K (the ice point). This means that 0°C = 273.15 K exactly. The Celsius scale is used in virtually all countries for everyday temperature measurement, weather forecasts, medical thermometry, and scientific research.
The formula °C = (°F - 32) × 5/9 accounts for both the different zero points and the different degree sizes between the two scales. Breaking this down:
For the forward conversion (Celsius to Fahrenheit), we reverse the operations: °F = (°C × 9/5) + 32. First multiply by 9/5 to convert from Celsius degree size to Fahrenheit degree size, then add 32 to adjust for the different zero points. These formulas are mathematically exact and form the basis of all temperature conversion calculations in science, medicine, engineering, and everyday use.
NIST SP 811 Appendix B.7 provides guidance on rounding converted numerical values. The key principle is to maintain precision appropriate to the measurement's inherent accuracy and intended use. For medical temperature readings, rounding to one decimal place (e.g., 98.6°F or 37.0°C) is standard because digital thermometers typically measure to ±0.1° accuracy. For weather reporting, whole numbers often suffice (e.g., "25°C" or "77°F"). For scientific applications, maintain full precision through calculations and round only the final result.
Common mistake: over-rounding intermediate steps. For example, if converting 100°F, the precise result is 37.777...°C. If you round the intermediate calculation (100 - 32 = 68 → 70) before multiplying by 5/9, you'd get 38.9°C, creating a significant error. Always perform the full calculation with precision, then round the final answer appropriately: (100 - 32) × 5/9 = 68 × 5/9 = 37.78°C.
Quick reference for body temperature assessment based on WHO and CDC guidelines:
| Temp Range | Fahrenheit | Celsius | Medical Significance |
|---|---|---|---|
| Hypothermia | <95°F | <35°C | Medical emergency |
| Normal | 97-99°F | 36.1-37.2°C | Healthy body temperature |
| Low Fever | 99.1-100.4°F | 37.3-38.0°C | Mild fever, monitor symptoms |
| Moderate Fever | 100.5-102.2°F | 38.1-39.0°C | Fever present, consider medication |
| High Fever | 102.3-104°F | 39.1-40.0°C | Significant fever, consult doctor |
| Very High | >104°F | >40°C | URGENT: Seek immediate care |
| Fahrenheit | Celsius | Common Use |
|---|---|---|
| -40°F | -40°C | Extremely cold, same value |
| 0°F | -17.8°C | Very cold winter |
| 32°F | 0°C | Water freezes |
| 50°F | 10°C | Cool weather |
| 68°F | 20°C | Room temperature |
| 70°F | 21.1°C | Pleasant weather |
| 77°F | 25°C | Warm, comfortable |
| 86°F | 30°C | Hot summer day |
| 98.6°F | 37°C | Normal body temp |
| 100°F | 37.8°C | Low-grade fever |
| 100.4°F | 38°C | Fever threshold |
| 104°F | 40°C | High fever |
| 180°F | 82.2°C | Hot water |
| 212°F | 100°C | Water boils |
| 350°F | 177°C | Common baking temp |
| 400°F | 204°C | Hot oven |
Healthcare professionals and parents use °F to °C conversion for fever assessment. Example: A thermometer reads 101.3°F, converting to 38.5°C helps determine if it's in the fever range requiring medical attention.
Common values: 98.6°F (37°C), 100.4°F (38°C), 102.2°F (39°C), 104°F (40°C)
Pro tip: Fever is >100.4°F (38°C) for adults, >100.4°F for infants under 3 months requires immediate care.
Converting recipe temperatures between American (°F) and international (°C) standards. American recipe "bake at 350°F" converts to 177°C (often rounded to 175°C or 180°C) for European ovens.
Common values: 300°F (149°C), 350°F (177°C), 400°F (204°C), 425°F (218°C)
Pro tip: Round to nearest 5°C or 10°C for practical oven use.
International travelers from the US seeing "25°C" in weather forecasts need to know it equals 77°F—warm and comfortable weather.
Quick mental math: Subtract 30, divide by 2. Example: 86°F → 56 → 28°C (actual: 30°C - close enough!)
Scientists convert between Fahrenheit (US standard) and Celsius (SI unit) for international collaboration and publication consistency.
Pro tip: For precision, use exact formula (°F - 32) × 5/9.
98.6°F equals exactly 37°C, which is considered the average normal human body temperature. This conversion uses the standard formula: °C = (98.6 - 32) × 5/9 = 66.6 × 5/9 = 37°C.
According to the Mayo Clinic and modern medical research, normal body temperature actually ranges from 97°F to 99°F (36.1°C to 37.2°C), varying based on time of day, activity level, age, gender, and individual physiology. Body temperature typically runs lowest in the early morning and highest in late afternoon/evening, with fluctuations of up to 1°F being normal throughout the day.
The 98.6°F (37°C) standard originated from research conducted in the 1860s by German physician Carl Wunderlich, who analyzed over one million temperature readings. However, more recent studies suggest the average may be slightly lower (around 98.2°F or 36.8°C) in modern populations, possibly due to improved health, different measurement methods, or actual physiological changes over time.
Reference: NIST Special Publication 811 provides the exact conversion formula, while WHO and CDC use 37°C as the reference point for normal body temperature in clinical guidelines.
The official formula to convert Fahrenheit to Celsius is: °C = (°F - 32) × 5/9, which can also be written as °C = (°F - 32) / 1.8. Both forms are mathematically equivalent since 5/9 ≈ 0.556 and 1/1.8 ≈ 0.556.
Step-by-step conversion process:
Worked Example: Convert 68°F to Celsius
Step 1: Start with 68°F
Step 2: 68 - 32 = 36
Step 3: 36 × 5/9 = 36 / 1.8 = 20°C
This formula accounts for two fundamental differences between the scales: (1) the different zero points—water freezes at 32°F but 0°C, requiring the "-32" adjustment first, and (2) the different degree sizes—there are 180 Fahrenheit degrees between water's freezing and boiling points but only 100 Celsius degrees, requiring the "×5/9" scaling factor to compress the Fahrenheit range into the Celsius range.
According to NIST SP 811, this is the exact, authoritative conversion relationship used in all scientific, medical, and engineering applications. For the reverse conversion (Celsius to Fahrenheit), rearrange to: °F = (°C × 9/5) + 32.
100°F converts to 37.78°C (rounded to 37.8°C) using the formula: (100 - 32) × 5/9 = 68 × 5/9 = 37.777...°C. This temperature is considered a low-grade fever or elevated temperature, though it falls just below the official fever threshold.
According to WHO guidelines and CDC standards, fever is medically defined as a measured temperature of 100.4°F (38°C) or greater for adults. At 100°F (37.8°C), you're in what clinicians call the "borderline zone"—above the normal healthy range of 97-99°F (36.1-37.2°C) but below the standard fever threshold of 100.4°F (38°C).
Clinical significance: A temperature of 100°F often indicates your immune system is beginning to respond to infection or inflammation, but it's not yet classified as a full fever. This reading can be normal after exercise, in hot environments, or in the late afternoon when body temperature naturally peaks. However, if accompanied by symptoms like chills, fatigue, body aches, or if it persists or rises, it warrants monitoring.
What to do: Monitor your temperature every 4-6 hours. Watch for additional symptoms. Stay hydrated and rest. If the temperature rises above 100.4°F (38°C), persists for more than 24-48 hours, or you develop concerning symptoms, consult a healthcare provider. For infants under 3 months, any temperature above 100.4°F requires immediate medical evaluation.
100.4°F converts to exactly 38°C using the conversion formula: (100.4 - 32) × 5/9 = 68.4 × 5/9 = 38°C. This temperature represents the official medical threshold for fever as defined by major health organizations.
The CDC defines fever as 100.4°F (38°C) or greater, or feeling feverish/having chills. This is the same threshold used by WHO and medical professionals worldwide. At exactly 100.4°F, you have reached the fever boundary—your body temperature is officially elevated enough to be classified as fever.
What this means clinically: At 100.4°F (38°C), your body is actively mounting an immune response to fight infection. This is common during viral infections (colds, flu, COVID-19), bacterial infections (urinary tract infections, strep throat), or inflammatory conditions. The fever itself is a defense mechanism—it creates an environment less favorable to pathogens and activates immune system components.
Treatment guidance: Stay well-hydrated (8-10 glasses of water daily), get adequate rest, and monitor your temperature every 4-6 hours. Over-the-counter fever reducers like acetaminophen (Tylenol) or ibuprofen (Advil, Motrin) can be used if you're uncomfortable, though moderate fever (up to 102°F/38.9°C) that's well-tolerated doesn't necessarily need medication. Seek medical care if: fever persists more than 3 days, rises above 103°F (39.4°C), is accompanied by severe headache/stiff neck/confusion/difficulty breathing, or you have concerning underlying health conditions. For infants under 3 months, any fever ≥100.4°F requires immediate pediatric evaluation.
-40 degrees is the only temperature where Fahrenheit and Celsius scales intersect: -40°F = -40°C
Verification using the conversion formula: °C = (°F - 32) × 5/9 → °C = (-40 - 32) × 5/9 = -72 × 5/9 = -40 ✓
At -40°, exposed skin freezes in minutes, metal becomes brittle, and extreme cold weather gear is required for survival. This is an extremely cold temperature rarely experienced except in polar regions or extreme winter conditions.
Quick Method: "Subtract 30, then divide by 2"
Example with 68°F: 68 - 30 = 38 → 38 / 2 = 19°C (actual: 20°C) ✓ Within 1 degree!
More accurate mental method: Subtract 32, multiply by 5, divide by 9. Example: 77°F → 45 → 225 → 25°C ✓ Exact!
Adults: 104°F (40°C)+ requires urgent care; 105.8°F (41°C)+ is a medical emergency with organ damage risk.
Infants (0-3 months): 100.4°F (38°C)+ requires immediate medical care due to vulnerable immune system.
Warning signs at any fever: Lasting >3 days, severe headache, stiff neck, confusion, difficulty breathing, persistent vomiting, seizures, unresponsiveness.
Hyperpyrexia: Temperatures >106.7°F (41.5°C) can cause protein denaturation and brain damage, requiring emergency cooling measures and intensive care.
70°F equals 21.1°C using the conversion formula: (70 - 32) × 5/9 = 38 × 5/9 = 21.111...°C (rounded to 21.1°C).
This is considered pleasant, comfortable weather for most people—ideal for outdoor activities without being too hot or too cold. At 21°C (70°F), you typically don't need heavy layers or cooling equipment, making it a commonly cited "perfect" temperature for spring and fall weather.
Context: Many thermostats are set around 68-72°F (20-22°C) for indoor comfort. At 70°F, most people can wear light long sleeves or short sleeves comfortably.
350°F equals 176.7°C (often rounded to 175°C or 180°C for practical oven use) using the formula: (350 - 32) × 5/9 = 318 × 5/9 = 176.666...°C.
350°F is the most commonly used baking temperature in American recipes. When converting international recipes or using ovens with Celsius controls, you'll typically set the oven to 175°C or 180°C as the nearest practical setting—the 1-3°C difference is negligible for most baking applications.
Common uses: Cookies, cakes, muffins, roasted vegetables, casseroles. This moderate temperature allows even cooking without burning exteriors.
Pro tip: Most ovens adjust in 5°C or 10°C increments. Round 176.7°C to either 175°C or 180°C based on your oven's controls.