Temperature Converter

One input, four temperature scales

Temperature questions rarely arrive in the scale you happen to need. A recipe says 180 C, the oven dial reads Fahrenheit, a weather app shows 41 F, and a chemistry problem wants kelvin. This converter takes one reading and returns Celsius, Fahrenheit, kelvin and degrees Rankine together, so the number you need is already on screen before you open a second tool.

Celsius and Fahrenheit are relative scales. Their zero points were picked for practical reference points, so a reading tells you how far you sit from that zero. Kelvin and Rankine are absolute: their zero is absolute zero, the coldest temperature there is. NIST puts that point at 0 K, -273.15 °C, -459.67 °F and 0 °R. Nothing in everyday life sits below it, which is why an absolute reading never goes negative.

There is one trap to know before the math. A temperature is a point on a scale, while a temperature difference is an interval between two points. 20 °C is a place on the thermometer. A 20 °C warm-up is a distance along it, and the two convert differently. The converter has a mode for each, and the sections below show where the paths split.

The formulas behind each arrow

Four formulas cover every pair of scales, and they share two constants: a ratio of 1.8 and an offset. The ratio says one Celsius degree is 1.8 Fahrenheit degrees, or 9/5 of one. The offsets say where each scale puts its zero, and they are the only part that changes when you switch direction.

NIST Special Publication 811, the United States guide to the International System of Units, defines Celsius temperature as t = T - T0 with T0 fixed at 273.15 K. In plain words, a Celsius reading is a kelvin reading shifted down by 273.15. The same guide says a temperature difference may be expressed in kelvins or in degrees Celsius, because the two units are the same size.

Rankine works like kelvin with Fahrenheit-sized degrees: same ratio, zero moved to absolute zero. That is why the step between the two absolute scales is a plain multiplication, with no offset to add.

Worked example: 37 C on all four scales

Take 37 °C, the number most people know as body temperature. To reach Fahrenheit, multiply by 1.8 and add 32: 37 x 1.8 = 66.6, then 66.6 + 32 = 98.6 °F. To reach kelvin, add 273.15: 37 + 273.15 = 310.15 K. To reach Rankine, multiply the kelvin value by 1.8: 310.15 x 1.8 = 558.27 °R, and adding 459.67 to 98.6 gives the same 558.27.

Run the result backwards as a check. (98.6 - 32) / 1.8 = 66.6 / 1.8 = 37 °C. 310.15 - 273.15 = 37 °C. 558.27 / 1.8 = 310.15 K. Four numbers, one temperature, and every route lands in the same place.

Notice how close the Fahrenheit and Rankine numbers sit. They use the same size degree, so 98.6 and 558.27 differ by exactly 459.67. The same happens between Celsius and kelvin, where the gap is always 273.15. Once you see the constant gap, the two absolute scales stop looking like separate systems.

Differences multiply by 1.8 and never add 32

A difference only has a size, so the offsets drop out. Multiply a Celsius interval by 1.8 to get the Fahrenheit interval, and divide to come back. A 10 °C warm front is an 18 °F warm front. A 5 °C drop is a 9 °F drop. Kelvin intervals match Celsius intervals exactly, and Rankine intervals match Fahrenheit intervals exactly, because each pair uses the same size degree.

Weather makes the error easy to see. An overnight low of 41 °F and an afternoon high of 77 °F differ by 36 °F. Divide by 1.8 and the day warmed by 20 °C, not 45 °C. Converting both readings first and subtracting gives the same 20 °C, but only after two full conversions and a subtraction. The difference mode does the whole job in one step.

Cooks hit the same wall. Moving an oven from 350 °F to 375 °F is a 25 °F bump, about 14 °C, so a Celsius dial goes from 177 °C to 191 °C. Adding 25 to the Celsius number instead would land near 202 °C and burn the crust before the middle sets.

Reference points from absolute zero to a hot oven

Some temperatures come up often enough to be worth carrying in your head. The table below lines them up across all four scales, with the cooking rows taken from the USDA safe minimum chart and the fever row from CDC guidance. The values are rounded to one decimal place, which is more precision than a kitchen thermometer can deliver anyway.

Two rows deserve a note. NIST describes room temperature as about 70 °F, 21 °C or 294 K, and the table keeps the unrounded conversions so the columns agree with each other. The boiling row assumes sea level pressure. Water boils below 100 °C at altitude, so a recipe that depends on boiling temperature needs a correction no converter can supply.

Where these constants and cooking numbers come from

The constants on this page are not folklore. NIST Special Publication 811 fixes the ice point at 273.15 K and defines the Celsius scale by that single offset. The redefinition of the kelvin that took effect in 2019, approved by the General Conference on Weights and Measures, fixed the Boltzmann constant at exactly 1.380649 x 10^-23 joule per kelvin, so the unit now rests on a physical constant rather than on a property of water. Before that, from 1954, the kelvin was defined as 1/273.16 of the temperature of the triple point of water, which sits at 273.16 K.

Cooking numbers have a different source. The USDA Food Safety and Inspection Service publishes a safe minimum internal temperature chart, last updated in April 2025, which lists 145 °F (62.8 °C) for whole cuts with a 3 minute rest, 160 °F (71.1 °C) for ground meats, and 165 °F (73.9 °C) for all poultry, leftovers and casseroles. Those are the numbers in the table above, converted to the other three scales.

Health numbers come from CDC. Its case definitions treat a measured temperature of 100.4 °F (38 °C) or higher as a fever. That is the row this page labels fever threshold. A single reading above it is a data point, not a diagnosis, and a thermometer's own accuracy is usually plus or minus 0.5 °F, so treat the second decimal place as decoration rather than measurement.

Rankine, the absolute scale built on Fahrenheit

Rankine is the scale most people have never met, and it exists for one reason: American engineering already ran on Fahrenheit, so an absolute version of Fahrenheit saved a conversion in steam tables and thermodynamics homework. The degree is the same size as a Fahrenheit degree, and zero sits at absolute zero, so 0 °R = -459.67 °F and 0 °F = 459.67 °R.

The unit survives in the Rankine cycle, the model behind most steam power plants, where temperatures are written in °R so that ratios behave correctly. It also shows up in some US heating and cooling work. Everywhere else, kelvin does that job, which is why a physics course outside the United States will look at a Rankine figure and ask what it means.

Converting between the two absolute scales is the cleanest operation on the page: multiply by 1.8 or divide by 1.8. A 300 K reading is 540 °R. An 80 °F room is 539.67 °R, and the same room reads 26.7 °C and 299.8 K. Four scales, one room, and no offsets to remember once the zero points are set.

Which page to use for a single pair

This site already carries four single-pair converters, and they do a narrower job with more depth. Use Celsius to Fahrenheit when you need the oven chart and the 9/5 arithmetic spelled out. Fahrenheit to Celsius is the one to open for a forecast or a thermostat set in the other system. Kelvin to Celsius suits lab and homework work, and Kelvin to Fahrenheit covers the astronomy and engineering cases where a Kelvin value has to become a Fahrenheit one.

The page you are on is the wide view: one input, four outputs, plus the difference mode and the reference table. Reach for it when you want to compare scales side by side, when you are converting a change rather than a reading, or when you are not sure yet which pair you need.

Two neighbors cover related ground. The food safety temperature chart lists doneness targets by meat, and the candy temperature chart maps sugar stages to degrees. Use them for the target, and this converter when the target needs to move between scales. If you are converting a longer list of mixed units, the conversion calculator handles that job.