About Temperature Converter — Celsius, Fahrenheit, Kelvin & Rankine
The temperature converter translates between the two everyday scales — Celsius and Fahrenheit — and the two absolute scales built on them: Kelvin (Celsius-sized degrees starting at absolute zero) and Rankine (Fahrenheit-sized degrees starting at absolute zero, still common in US thermodynamics). The relations are exact by definition: °F = °C × 1.8 + 32, K = °C + 273.15, and °R = °F + 459.67, so unlike the factor-based converters there is no rounded constant anywhere in this tool.
Temperature is the one quantity here that converts with an offset as well as a scale — which is why 0 °C is 32 °F, not 0 °F, and why the scales cross at exactly −40. It is also the one quantity with a hard floor: nothing is colder than absolute zero, so the tool rejects −500 °C or −1 K as physically meaningless instead of converting them, and when you switch the input scale it clamps the value to the new scale's floor rather than letting a valid entry become an error.
How It Works
- Enter the temperature, pick the scale it is in ("from"), and pick the scale you need ("to"). The headline shows the converted value; the table beneath shows the same temperature on all four scales at once.
- Every conversion pivots through Celsius with the exact relations — from-scale to °C, then °C to the target scale. Kelvin and Rankine are reached by adding the absolute-zero offset (273.15 in Celsius degrees, 459.67 in Fahrenheit degrees).
- Inputs below the from-scale's absolute zero (−273.15 °C, −459.67 °F, 0 K, 0 °R) are rejected with a clear message. Switching the from-scale clamps the current value up to the new floor when needed — a reading of −100 is fine in °C or °F but clamps to 0 when you switch to kelvin.
- Remember the difference between a temperature and a temperature DIFFERENCE: a 10 °C rise is an 18 °F rise (multiply by 1.8 only — no +32), and a 10 K difference IS a 10 °C difference. This tool converts temperature points; for differences, use the scale factor alone.
Worked Example
Water boils at 212 °F at standard atmospheric pressure, and a lab worksheet wants it in Celsius and Kelvin. Through the pivot: °C = (212 − 32) ÷ 1.8 = 180 ÷ 1.8 = 100 °C exactly, and K = 100 + 273.15 = 373.15 K — the chart's last row. On the Rankine scale the same point is 212 + 459.67 = 671.67 °R. The −40 row above it is the famous crossing point where Celsius and Fahrenheit agree.
Fahrenheit to Celsius chart (with Kelvin)
Landmark temperatures converted with the exact relations °C = (°F − 32) ÷ 1.8 and K = °C + 273.15, shown to 6 significant digits — exactly what the converter outputs for the same inputs.
| Temperature | °C | K |
|---|---|---|
| −40 °F (scales cross) | -40 | 233.15 |
| 0 °F (blizzard cold) | -17.7778 | 255.372 |
| 32 °F (water freezes) | 0 | 273.15 |
| 68 °F (room temperature) | 20 | 293.15 |
| 98.6 °F (body temperature) | 37 | 310.15 |
| 212 °F (water boils) | 100 | 373.15 |
Formulas
- Celsius ↔ Fahrenheit (exact)
°F = °C × 1.8 + 32; °C = (°F − 32) ÷ 1.8- Absolute scales (exact)
K = °C + 273.15; °R = °F + 459.67 = K × 1.8- Temperature differences (no offset!)
Δ°F = Δ°C × 1.8; ΔK = Δ°C
Standards & References
- Exact scale relations per the ITS-90 / SI definitions collected in NIST SP 811 App. B.8: °F = °C × 1.8 + 32; K = °C + 273.15; °R = °F + 459.67
- Absolute zero: −273.15 °C = 0 K = −459.67 °F = 0 °R — inputs below the from-scale's floor are rejected
- Kelvin is the SI base unit of thermodynamic temperature; Celsius is defined from it (°C = K − 273.15)
Frequently Asked Questions
How do I convert Fahrenheit to Celsius?
Subtract 32, then divide by 1.8: °C = (°F − 32) ÷ 1.8. So 68 °F is (68 − 32) ÷ 1.8 = 20 °C, and 98.6 °F is exactly 37 °C. Going the other way, multiply by 1.8 and add 32. The mental-math approximation "subtract 30, halve it" lands within a degree or two across the everyday weather range.
Why is −40 the same in Celsius and Fahrenheit?
Because the scales are two different lines that must cross somewhere: setting °C = °F in °F = °C × 1.8 + 32 gives x = 1.8x + 32, so x = −40. Below the crossing Fahrenheit reads colder-sounding numbers; above it, warmer-sounding ones. It is a favorite checkpoint for verifying any converter — this tool's chart pins it in the first row.
Why does the converter reject −1 K but accept 0.1 K?
Kelvin starts at absolute zero — the state of minimum thermal energy — so negative kelvin temperatures do not describe anything a thermometer can read (the exotic "negative temperature" of population-inverted systems is a statistical-mechanics notion, hotter than infinity, not colder than zero). 0.1 K is a perfectly real cryogenic temperature: it converts to −273.05 °C.
What is the Rankine scale for?
Rankine is to Fahrenheit what Kelvin is to Celsius: same degree size, zero moved to absolute zero (°R = °F + 459.67). US thermodynamic practice — gas-turbine cycles, compressible-flow tables, older HVAC psychrometrics — uses it because absolute temperatures are required whenever you multiply or divide by T, as in the ideal gas law. 537 °R (77 °F) is a common reference state.
Do I add 32 when converting a temperature difference?
No — that is the classic mistake. Offsets cancel when you subtract two temperatures, so a difference converts with the scale factor alone: Δ°F = Δ°C × 1.8, and a kelvin difference equals a Celsius difference. A U-value quoted per °C is numerically identical per kelvin, and an 18 °F approach temperature is a 10 °C approach. Only temperature POINTS need the +32.