Temperature Converter
Temperature Doesn't Convert Like Other Units
Every other converter on this site multiplies by a factor. Temperature can't — because Celsius and Fahrenheit both have arbitrary zero points that don't coincide with each other or with absolute zero. Converting temperature requires an offset as well as a scale change, which is exactly why 0°C is 32°F rather than 0°F, and why doubling the Celsius number does not double the Fahrenheit number. Kelvin and Rankine fix this by placing zero at absolute zero, making them the scales physics actually uses.
The Formulas
This converter normalises every input to Celsius first, then converts out to the target scale. The exact expressions it applies:
F = C × 9/5 + 32
C = K − 273.15
K = C + 273.15
C = (R − 491.67) × 5/9
R = (C + 273.15) × 9/5
Pairs not listed — Fahrenheit to Kelvin, say — are handled by chaining: source scale to Celsius, Celsius to target scale. The 5/9 and 9/5 ratios come from the fact that the Fahrenheit degree is 5/9 the size of the Celsius degree; there are 180 Fahrenheit degrees between water's freezing and boiling points against 100 Celsius degrees.
The Guard Rails
The calculator rejects physically impossible input rather than returning a nonsense number:
- A negative Kelvin value is refused — 0 K is absolute zero.
- A negative Rankine value is refused for the same reason.
- Any input that resolves below −273.15°C is refused, which catches things like −500°F.
Four Scales Side by Side
| Reference point | Celsius | Fahrenheit | Kelvin | Rankine |
|---|---|---|---|---|
| Absolute zero | −273.15 | −459.67 | 0 | 0 |
| Scales cross | −40 | −40 | 233.15 | 419.67 |
| Water freezes | 0 | 32 | 273.15 | 491.67 |
| Cool day | 10 | 50 | 283.15 | 509.67 |
| Room temperature | 20 | 68 | 293.15 | 527.67 |
| Warm room | 25 | 77 | 298.15 | 536.67 |
| Body temperature | 37 | 98.6 | 310.15 | 558.27 |
| Hot bath | 50 | 122 | 323.15 | 581.67 |
| Water boils | 100 | 212 | 373.15 | 671.67 |
| Moderate oven | 180 | 356 | 453.15 | 815.67 |
| Hot oven | 200 | 392 | 473.15 | 851.67 |
−40 is the only temperature where Celsius and Fahrenheit read the same number — it is the point where the two lines cross.
Why the Scales Diverge
The Fahrenheit line starts 32 units higher and climbs 1.8 times as fast — the offset and the slope are both required.
Fahrenheit to Celsius, Common Values
| Fahrenheit | Celsius |
|---|---|
| −40°F | −40°C |
| 0°F | −17.7778°C |
| 32°F | 0°C |
| 50°F | 10°C |
| 68°F | 20°C |
| 72°F | 22.2222°C |
| 98.6°F | 37°C |
| 212°F | 100°C |
Where This Comes Up
- Thermodynamics coursework — gas law and efficiency equations require absolute scales. Feeding Celsius into an equation that expects Kelvin produces answers that are wrong by hundreds of degrees.
- Engineering in US customary units — Rankine is the absolute counterpart of Fahrenheit and still appears in US thermodynamics and HVAC texts.
- Medical readings — a fever threshold quoted in one scale against a thermometer reading in the other.
- Weather and travel — forecasts published in the scale you don't think in.
How to Use This Calculator
- Enter the temperature reading in the Value field — negatives and decimals are both accepted.
- Choose its current scale in the first dropdown: Celsius, Fahrenheit, Kelvin or Rankine.
- Choose the target scale in the second dropdown.
- Select Calculate. The result includes the specific formula used for that pairing, not just the number.
Related Calculations
Cooking rather than physics? The Oven Temperature Converter adds UK gas marks to the Celsius/Fahrenheit pairing. For heat as a quantity rather than a level, see the Energy Conversion Calculator.