Understanding Temperature Scales
Celsius, Fahrenheit, and Kelvin each measure the same physical property — thermal energy — but with different origins, zero points, and degree sizes. Here's every formula, the reasoning behind each scale, and when to use which.
The Conversion Formulas
Unlike length or weight, temperature conversions are not simple ratios. The scales have different zero points and different degree sizes, so every conversion involves both multiplication and addition.
Celsius → Fahrenheit
°F = (°C × 9/5) + 32
Multiply by 9/5 (or 1.8) to account for the larger Fahrenheit degree, then add 32 to shift the zero point. Water freezes at 0°C but 32°F — that 32-degree offset is the key difference.
Example: 25°C → (25 × 1.8) + 32 = 77°F
Fahrenheit → Celsius
°C = (°F − 32) × 5/9
Reverse the process: subtract the 32-degree offset first, then multiply by 5/9 to shrink the degree size from Fahrenheit to Celsius.
Example: 98.6°F → (98.6 − 32) × 5/9 = 37°C (body temperature)
Celsius → Kelvin
K = °C + 273.15
Celsius and Kelvin have the same degree size — a 1-degree change in Celsius equals a 1-degree change in Kelvin. The only difference is the zero point: 0 K is absolute zero (−273.15°C), so you just add 273.15.
Example: 100°C → 100 + 273.15 = 373.15 K (boiling point of water)
Kelvin → Celsius
°C = K − 273.15
Simply subtract 273.15. Since the degree sizes are identical, there is no multiplication step — just a shift of the zero point.
Example: 310 K → 310 − 273.15 = 36.85°C
Fahrenheit → Kelvin
K = (°F − 32) × 5/9 + 273.15
Convert to Celsius first (subtract 32, multiply by 5/9), then add 273.15 to shift into the Kelvin scale. There is no shortcut — both the degree size and zero point differ.
Example: 212°F → (212 − 32) × 5/9 + 273.15 = 373.15 K
Kelvin → Fahrenheit
°F = (K − 273.15) × 9/5 + 32
Convert to Celsius (subtract 273.15), then to Fahrenheit (multiply by 9/5, add 32). This two-step conversion is necessary because Kelvin and Fahrenheit differ in both zero point and degree size.
Example: 0 K → (0 − 273.15) × 1.8 + 32 = −459.67°F (absolute zero)
Key Reference Points
These fixed points anchor the three temperature scales and illustrate how they relate to each other.
| Reference Point | Celsius | Fahrenheit | Kelvin |
|---|---|---|---|
| Absolute Zero | −273.15°C | −459.67°F | 0 K |
| Water Freezes | 0°C | 32°F | 273.15 K |
| Human Body | 37°C | 98.6°F | 310.15 K |
| Water Boils | 100°C | 212°F | 373.15 K |
| −40° Crossover | −40°C | −40°F | 233.15 K |
The History Behind Each Scale
Fahrenheit (1724)
Daniel Gabriel Fahrenheit, a Polish-born Dutch physicist, created his scale using three reference points: a brine solution as 0°F (the coldest temperature he could reliably reproduce), the freezing point of water as 32°F, and average human body temperature as 96°F (later revised to 98.6°F). The seemingly arbitrary numbers were chosen to avoid negative readings in the northern European climates where he worked, and to subdivide the range into fine degrees for precision glassblowing thermometers.
Celsius (1742)
Anders Celsius, a Swedish astronomer, originally proposed an inverted scale: 100° for freezing and 0° for boiling. A year after his death, fellow Swede Carl Linnaeus flipped it to the intuitive version we use today. The 0–100 range between water's phase transitions makes Celsius clean for everyday weather, cooking, and science. It is the default temperature unit in every country except the United States, the Bahamas, Palau, the Cayman Islands, and the Federated States of Micronesia.
Kelvin (1848)
William Thomson (later Lord Kelvin) proposed an absolute temperature scale anchored at the theoretical lowest possible temperature — the point at which all molecular motion ceases. By setting 0 K at absolute zero and using the same degree size as Celsius, Kelvin created a scale where temperature is directly proportional to thermal energy. This makes it indispensable in physics (the ideal gas law, blackbody radiation) and chemistry (reaction kinetics). Kelvin temperatures are written without a degree symbol: 300 K, not 300°K.
When to Use Each Scale
Celsius
- Weather reports (most countries)
- Cooking and food safety
- Medical thermometers (outside the US)
- HVAC and building standards
- General science and education
Fahrenheit
- US weather forecasts
- US cooking and oven settings
- US medical context (fever = 100.4°F)
- HVAC in North America
- Pool and spa temperature in the US
Kelvin
- Physics and thermodynamics
- Color temperature (lighting, photography)
- Astrophysics (stellar classification)
- Cryogenics and low-temp research
- Ideal gas law calculations
Temperature Conversion FAQ
? How do you convert Celsius to Fahrenheit?
Multiply the Celsius temperature by 9/5 (or 1.8), then add 32. The formula is: °F = (°C × 9/5) + 32. For example, 25°C = (25 × 1.8) + 32 = 77°F.
? How do you convert Fahrenheit to Celsius?
Subtract 32 from the Fahrenheit temperature, then multiply by 5/9. The formula is: °C = (°F − 32) × 5/9. For example, 98.6°F = (98.6 − 32) × 5/9 = 37°C.
? What is absolute zero in each temperature scale?
Absolute zero — the theoretical lowest possible temperature where all molecular motion stops — is 0 Kelvin, −273.15°C, and −459.67°F. It has never been reached in a laboratory, though scientists have come within billionths of a degree.
? When should I use Kelvin instead of Celsius or Fahrenheit?
Kelvin is the SI unit used in scientific contexts — physics, chemistry, and engineering. It starts at absolute zero, which makes it essential for thermodynamic calculations. Celsius is standard for everyday use globally, and Fahrenheit is used primarily in the United States for weather and cooking.