Temperature Converter: Celsius, Fahrenheit and Kelvin

The temperature converter moves a value between the four temperature scales in common use: Celsius (°C), Fahrenheit (°F), Kelvin (K) and Rankine (°R). Type on either side and the opposite field follows, the All Units list shows all four scales, and every conversion runs in the page.

Updated 2026-09-30Reviewed by CalcX Editorial Team5 sources5 min read

What the temperature converter does

The scales do not share one zero point: Fahrenheit and Celsius anchor zero to reference temperatures, Kelvin and Rankine to absolute zero. Offsets are applied exactly, so 0 °C reads 273.15 K and 491.67 °R.

For other quantities, see the length converter and the weight converter; temperatures inside larger calculations belong to the scientific calculator.

How to use it

Choose the units.The page opens with Celsius left and Fahrenheit right; both dropdowns list the four scales.
Type the temperature.A number on either side recalculates the other immediately; a leading minus sign works, so -40 is valid.
Read every scale at once.The All Units list shows the same value in all four units with their symbols.
Swap directions.The arrow button exchanges the units and carries the value you were reading into the input.
Use an expression when it helps.6*6 evaluates as you type with a small = 36 hint; Enter settles it into a number, and the keypad button opens an on-screen pad.

Worked examples

Every readout below comes from this converter; type the same numbers to reproduce it.

Water's fixed points

Entry	°C	°F	K	°R
0 °C	0	32	273.15	491.67
100 °C	100	212	373.15	671.67
37 °C	37	98.6	310.15	558.27
−40 °C	−40	−40	233.15	419.67

Absolute zero, and below it

With Kelvin on the left and Celsius on the right, 0 K reads -273.15 °C, -459.67 °F and 5.684342e-14 °R in the list. At −1 K, Celsius reads -274.15 and Fahrenheit -461.47 — no error. Enter −300 on the Celsius side and Kelvin reads -26.85 K.

Where rounding shows

80 °F → °C reads 26.6666666667; press swap and Fahrenheit reads 80.0000000001. It repeats elsewhere: 350 °F → °C reads 176.666666667, and 1 °F → °C reads -17.2222222222.

Where the scales come from

Daniel Gabriel Fahrenheit proposed his scale in 1724; water's freezing and boiling points were later fixed at 32 °F and 212 °F, 180 degrees apart. Anders Celsius proposed a hundred-degree scale in 1742, and the modern orientation — 0 at the ice point — was in use within a few years; the unit was renamed from centigrade in 1948. The scales cross at −40, which the converter shows directly.

Kelvin begins at absolute zero, −273.15 °C, with Celsius-sized degrees, so 0 °C is exactly 273.15 K. Rankine uses Fahrenheit-sized degrees from the same zero, which is why 0 °C is 491.67 °R. Since 2019 the kelvin has been defined by fixing the Boltzmann constant; the triple point of water keeps its assigned 273.16 K (0.01 °C), and ITS-90 preserves the practical fixed points — water, tin, gold — that thermometers are calibrated against.

Reading the results, and their limits

Display.: Results keep up to 12 significant digits and at most 10 decimals, with thousands separators: one million degrees Celsius reads 1,800,032 °F. Magnitudes at or above 10¹⁵ or below 10⁻¹⁰ switch to exponential form with six decimals, so 10¹⁵ °C reads 1.800000e+15 °F. Repeating decimals stop where the display stops: 80 °F is 26.6666666667 °C and 350 °F is 176.666666667 °C.
No absolute-zero guard.: The converter is arithmetic, not a physics check: any number can go into any field. −1 K reads -274.15 °C and -461.47 °F without a warning, and −300 °C reads -26.85 K. At 0 K, Celsius reads -273.15 and Fahrenheit -459.67, but Rankine shows 5.684342e-14 °R instead of 0 — floating-point residue, not physics.
Points, not intervals.: Entries are converted as specific temperatures; there is no difference mode. A 10 °C interval is 18 °F, but the converter reads 10 °C as 50 °F.
Rounded values carry forward.: Swap takes the value on screen: an 80 °F entry converted to Celsius and swapped back reads 80.0000000001 °F.
Temperatures only.: It does not handle other quantities: lengths live on the length converter, masses on the weight converter, and mixed calculations on the scientific calculator.

Common mistakes

Typing a value below zero into Kelvin and trusting the result. The converter has no absolute-zero check: −1 K reads −274.15 °C and −300 °C reads −26.85 K, though neither is possible. For a temperature, 0 K is the floor; for a difference, convert the ratio instead — 1 K steps equal 1 °C steps.
Converting a difference as a point: a 10 °C rise is 18 °F, not 50 °F.
Reading the last displayed digit as exact: the 80 °F round trip returns 80.0000000001 °F.
Confusing °F with °R: Rankine starts at absolute zero, so 32 °F is 491.67 °R.

Where it is used

Oven and recipe settings. A 350 °F recipe step is 176.666666667 °C here, so a chart that lists fan or gas settings in Celsius can follow the same number.
Weather and travel. A forecast in Fahrenheit converts to Celsius for a traveller, and −40 is the single point where the two scales read the same.
Science and cold work. Kelvin and Rankine start at absolute zero, so 0 K reads −273.15 °C and 0 °C reads 273.15 K exactly.

Sources

BIPM, The International System of Units (SI Brochure), 9th edition, bipm.org — the kelvin and degree Celsius definitions and the Boltzmann constant （访问日期：2026-09-30）.
NIST, Special Publication 811: Guide for the Use of the International System of Units (SI), nist.gov — conversion rules and unit conventions （访问日期：2026-09-30）.
NIST, Kelvin: ITS-90 — fixed points of the International Temperature Scale of 1990 （访问日期：2026-09-30）.
Wikipedia, Fahrenheit — the 1724 scale and its fixed points （访问日期：2026-09-30）.
Wikipedia, Celsius — the 1742 scale and the centigrade rename （访问日期：2026-09-30）.