Temperature Converter — Fahrenheit, Celsius, Kelvin and Rankine
Fahrenheit to Celsius is subtract 32, then divide by 1.8. Celsius to Fahrenheit is multiply by 1.8, then add 32. Enter a temperature here and you get all four scales at once, along with the jobsite reference points — concrete placement, heat stress, cold stress — and the sourced tools that work with each of them.
That converts to
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Results are estimates for planning purposes. Verify with your supplier, engineer, or a qualified professional before ordering material or performing work.
Fahrenheit to Celsius at a glance
| °F | °C | What it is |
|---|---|---|
| −40 | −40.0 | The one reading the two scales share. |
| 0 | −17.8 | Zero on the Fahrenheit scale — nothing physical happens here. |
| 32 | 0.0 | Water freezes. Zero on the Celsius scale. |
| 50 | 10.0 | — |
| 68 | 20.0 | Room temperature, and the reference for many material properties. |
| 86 | 30.0 | — |
| 98.6 | 37.0 | Normal core body temperature. |
| 100 | 37.8 | — |
| 212 | 100.0 | Water boils at one standard atmosphere. Lower at altitude. |
| −459.67 | −273.15 | Absolute zero — 0 K and 0 °R. |
Only three of these are physical constants: the freezing point, the boiling point at one atmosphere, and absolute zero. The rest are round numbers on one scale that land awkwardly on the other, which is the whole reason the conversion has to be done rather than remembered.
A difference is not a temperature
This is the error that survives into professional work, because it looks like a conversion and is not one. When the number in front of you is a change — a rise overnight, a tolerance on a specification, a gradient across a slab, a rate per hour — you convert with the ratio alone:
- A change of 1 °F is a change of 5/9 °C, which is 0.556.
- A change of 1 °C is a change of 1.8 °F.
- The 32 never appears. A change of 1 K equals a change of 1 °C exactly.
Run a 10 °F rise through the temperature formula and you get −12.2, which is not a rise at all. Run it through the ratio and you get 5.6 °C, which is the answer. NIST publishes the interval factor separately from the temperature relation for exactly this reason.
When the temperature is the decision
Converting a reading is the easy part. Most of the time the reading is an input to something else, and the something else does not run on air temperature alone.
- Heat stress. Air temperature on its own does not tell you whether work is safe. Humidity, radiant heat, air movement, workload, clothing and acclimatization all move the answer, which is why the recognized methods use a wet bulb globe temperature or a heat index instead of a thermometer. Take a converted reading to the WBGT calculator or the heat index work/rest calculator, and to the water intake calculator for hydration.
- Cold stress. Wind turns a tolerable temperature into a dangerous one, and the exposure time that matters is the time to frostbite, not the reading. The wind chill calculator uses the published National Weather Service chart.
- Concrete. Hot and cold weather concreting are both governed by the temperature of the concrete and of the surfaces it touches, and by the specification for the job — not by the air reading and not by a rule of thumb from a converter page. Convert the number, then go to the mix design, the specification and the supplier.
- Equipment. Operating ranges, hydraulic fluid viscosity, battery capacity and torque values on nameplates are given at stated temperatures. Convert the units on the plate; do not convert the rating into a permission.
How it's calculated
- Fahrenheit and Celsius
°C = (°F − 32) / 1.8 °F = °C × 1.8 + 32Exact. The 1.8 is 9/5, the ratio of the degree sizes; the 32 is the offset between the two zero points. Both operations are needed in both directions, and getting them the wrong way round is the classic error.- Kelvin
K = °C + 273.15 K = (°F + 459.67) / 1.8Kelvin uses the Celsius degree and starts at absolute zero. It takes no degree sign: 300 K, not 300 °K.- Rankine
°R = °F + 459.67 K = °R / 1.8Rankine is to Fahrenheit what kelvin is to Celsius — the same degree size, counted from absolute zero. It survives in US thermodynamics and combustion work.- The one where the scales cross
−40 °F = −40 °CThe only temperature at which the two everyday scales read the same number. A useful check that a conversion has been done in the right direction.- A temperature interval is not a temperature
a change of 1 °F = a change of 5/9 °C = a change of 5/9 KConverting a difference — a rise, a tolerance, a gradient — uses the ratio only. The 32 does not appear. A 10 °F rise is a 5.6 °C rise, not −12.2.
Frequently asked questions
Subtract 32, then divide by 1.8. So 90 °F is (90 − 32) / 1.8 = 32.2 °C, and 100 °F is 37.8 °C. Going the other way, multiply by 1.8 and add 32: 20 °C is 68 °F. Both steps are needed in both directions, and doing them in the wrong order is the mistake this page exists to prevent.
Double the Celsius and add 30. It is not exact, but it is within a few degrees over the range of ordinary weather: 20 °C gives an estimated 70 against a true 68, and 30 °C gives 90 against 86. For anything that governs a decision — concrete placement, a heat stress threshold, an equipment rating — do the real arithmetic.
Minus 40. It is the one point where the two scales cross, and it makes a useful sanity check: if a conversion result is on the wrong side of −40 relative to the input, the operations were done in the wrong order.
Kelvin is the SI temperature scale. It uses the same degree size as Celsius but starts at absolute zero, so K = °C + 273.15. Water freezes at 273.15 K. You need it for anything thermodynamic — gas law calculations, combustion, radiant heat — and for equipment specified in SI. It takes no degree symbol: write 300 K, never 300 °K.
The absolute scale built on the Fahrenheit degree, °R = °F + 459.67. It is to Fahrenheit what kelvin is to Celsius, and it survives in US thermodynamics, HVAC and combustion engineering, where an absolute temperature is required but the rest of the calculation is in customary units.
Use the ratio only and leave the offset out. A change of 1 °F is a change of 5/9 °C, so a 10-degree Fahrenheit rise is a 5.6-degree Celsius rise. Running a difference through the full formula gives nonsense — it would report a 10 °F rise as −12.2 °C. This trips people up on tolerances, gradients and rates of change far more often than on plain temperatures.
There is no single number, and any page that gives you one is guessing. Heat stress depends on humidity, radiant heat, air movement, workload, clothing and acclimatization, not on air temperature alone — which is why the assessment methods use a wet bulb globe temperature or a heat index rather than a thermometer reading. Convert the reading here, then take it to the WBGT tool or the heat index work/rest tool, which use the published OSHA and NIOSH methods.
Cold-weather concreting is governed by the temperature of the concrete and of the surfaces it touches, not by the reading on the truck thermometer, and the requirements come from the specification for the job. What is universally true is that hydration slows sharply as the mix cools, that freezing fresh concrete before it has gained strength does permanent damage, and that frozen subgrade and frost-covered forms and rebar are all placement stoppers. Convert the temperature here and go to the specification, the mix design and the ready-mix supplier for the thresholds.
Sources & references
- NIST Special Publication 811 (2008 edition) — Guide for the Use of the International System of Units (SI), Appendix B.8: degree Fahrenheit to degree Celsius t/°C = (t/°F − 32)/1.8, to kelvin T/K = (t/°F + 459.67)/1.8, degree Rankine to kelvin T/K = (T/°R)/1.8, and the temperature-interval factor 5.555 556 E−01
- OSHA — Heat: overview of heat injury and illness prevention, the agency's own page on how heat hazards are assessed on the job
- NIST — U.S. Survey Foot: the reference page for how US customary units are defined through SI, the same framework that fixes the temperature relations above
Content checked against these sources — last reviewed August 30, 2026.