Temperature Unit Converter

How to use this calculator

Three inputs. One result that shows all 4 scales simultaneously.

Temperature is the value you’re converting. Type it in. Negatives work fine; temperatures below zero are common in Celsius and Fahrenheit and are exactly what Kelvin was designed to avoid.

From is your source scale. The dropdown has 4 options: Celsius (°C), Fahrenheit (°F), Kelvin (K), and Rankine (°R).

To is your target scale. Same 4 options.

Click Calculate and the blue results panel shows your converted value plus all 4 scales side by side: Celsius, Fahrenheit, Kelvin, and Rankine. So if you enter 100°C, you immediately see 212°F, 373.15 K, and 671.67°R without running 3 separate conversions.

The Thermometer section below the results panel visualises where your temperature sits on a scale with common reference markers: absolute zero, water freezing, room temperature, body temperature, water boiling. It’s a fast sanity check that the converted number lands where you expect.

Why temperature conversion isn’t just multiplication

Every other unit conversion is proportional. 2 kilometres is twice 1 kilometre. Double the number, double the quantity.

Temperature doesn’t work that way. 20°C is not twice as hot as 10°C. 40°F is not twice as hot as 20°F. The scales have arbitrary zero points: 0°C is where water freezes (a useful reference, not a physical absolute), and 0°F was set by Fahrenheit as the lowest temperature he could reproduce in his lab in 1724 using a brine solution.

Because the zeros are in different places, you can’t convert by ratio alone. You have to shift the scale first (account for the offset between zeros), then scale it (account for the different degree sizes).

Kelvin fixes the zero problem. 0 K is absolute zero: the coldest temperature physically possible, where all molecular motion stops. On the Kelvin scale, 200 K really is twice as hot as 100 K in a thermodynamic sense. Rankine does the same for Fahrenheit: same degree size as Fahrenheit, but starting at absolute zero instead of Fahrenheit’s arbitrary 0.

Celsius and Fahrenheit are convenient scales built around human reference points. Kelvin and Rankine are thermodynamic scales built around physics. The calculator converts between all four, but only Kelvin and Rankine support true ratio comparisons.

The formulas

Celsius to Fahrenheit:

Fahrenheit to Celsius:

Celsius to Kelvin:

Kelvin to Celsius:

Fahrenheit to Rankine:

Rankine to Kelvin:

The 9/5 factor (1.8) is the degree size ratio: one Fahrenheit degree is 5/9 the size of one Celsius degree. The +32 and -32 handle the offset between where the two scales set their zeros. The 273.15 is the offset between Celsius zero (water’s freezing point) and absolute zero.

Celsius to Fahrenheit: full reference table

The conversion table everyone actually needs. Covers the full practical range from extreme cold to extreme heat.

°C	°F	K	What it is
-273.15	-459.67	0	Absolute zero
-200	-328	73.15	Liquid nitrogen range
-100	-148	173.15	Extremely cold industrial processes
-78.5	-109.3	194.65	Dry ice (solid CO₂) sublimation
-40	-40	233.15	Where °C and °F are equal
-30	-22	243.15	Extreme winter, Siberia/Canada
-20	-4	253.15	Very cold winter
-10	14	263.15	Cold winter day
0	32	273.15	Water freezing point
4	39.2	277.15	Water maximum density
10	50	283.15	Cold spring day
15	59	288.15	Mild day, standard atmosphere reference
20	68	293.15	Room temperature (typical)
21	69.8	294.15	Comfortable room temperature
25	77	298.15	Warm room, standard lab temperature
30	86	303.15	Hot day
35	95	308.15	Very hot day, body discomfort threshold
37	98.6	310.15	Normal human body temperature
40	104	313.15	Fever territory
50	122	323.15	Extreme heat wave
56.7	134	329.85	Highest recorded air temperature (Death Valley, 1913)
60	140	333.15	Hot enough to damage skin quickly
100	212	373.15	Water boiling point at sea level
180	356	453.15	Typical oven temperature (moderate)
200	392	473.15	Hot oven
232	450	505.15	Tin melting point
327	621	600.15	Lead melting point
660	1,220	933.15	Aluminium melting point
1,085	1,985	1,358.15	Copper melting point
1,538	2,800	1,811.15	Iron melting point
5,500	9,932	5,773.15	Surface of the Sun (approx.)

Fahrenheit to Celsius: cooking and weather range

The range most people actually need day to day. Covers weather, cooking, and food safety.

°F	°C	Context
-40	-40	Extreme cold (same in both scales)
0	-17.8	Very cold winter day (US)
14	-10	Cold winter day
32	0	Freezing point of water
40	4.4	Refrigerator temperature (safe food storage)
50	10	Cold spring/autumn day
60	15.6	Mild day
70	21.1	Comfortable room temperature
72	22.2	Standard US thermostat setting
80	26.7	Warm summer day
90	32.2	Hot summer day
98.6	37	Normal body temperature
100	37.8	Fever threshold
104	40	High fever
140	60	Food safety: minimum pasteurisation
165	73.9	Safe internal temp for poultry (USDA)
212	100	Boiling point of water
250	121.1	Low oven (slow cook)
325	162.8	Moderate oven
375	190.6	Standard baking temperature
400	204.4	Hot oven
450	232.2	Very hot oven, pizza
500	260	Maximum most home ovens reach
550	287.8	Self-cleaning oven cycle

The -40 coincidence and other fixed points

-40°F = -40°C. The two scales cross exactly once. Below -40, Celsius numbers are warmer-sounding (less negative) than Fahrenheit. Above -40, Fahrenheit numbers are warmer-sounding (larger) than Celsius. This crossing point is a useful sanity check: if you’re working near -40 and your Celsius and Fahrenheit values are roughly equal, you’re probably in the right range.

0°F is not arbitrary. Daniel Gabriel Fahrenheit set 0°F as the lowest temperature he could reproduce with a salt-water-ice mixture in his Danzig lab in 1724. It wasn’t random; it was the practical lower limit of his measurement environment. He then set 96°F as body temperature (later adjusted to 98.6°F after the scale was redefined around water’s freezing and boiling points).

0°C was chosen for utility. Anders Celsius originally set 0°C as the boiling point and 100°C as the freezing point. Carolus Linnaeus reversed it to the version we use today. The choice of water’s phase change points made the scale reproducible anywhere with a thermometer and a boiling pot.

Absolute zero is -273.15°C = -459.67°F = 0 K. This isn’t a definition. It’s a physical limit derived from the behaviour of ideal gases as they approach zero volume at zero pressure. No object has ever been cooled to exactly 0 K, though labs have reached within billionths of a degree.

Kelvin and Rankine: when you need them

Most people never use Kelvin in daily life. Scientists and engineers use it constantly.

Kelvin is the SI unit of thermodynamic temperature. Any formula involving the laws of thermodynamics (gas laws, blackbody radiation, entropy calculations) requires Kelvin because the relationships only hold true when temperature starts at absolute zero. The ideal gas law PV = nRT requires T in Kelvin. Stefan-Boltzmann radiation law requires T in Kelvin. Boltzmann’s equation requires T in Kelvin.

Rankine is Kelvin’s imperial counterpart: the same absolute zero reference point, but with Fahrenheit-sized degrees. It’s used in some US engineering contexts, particularly aerospace and thermodynamics, where engineers prefer Fahrenheit-based units but need an absolute scale. 0°R = 0 K = absolute zero. 491.67°R = 32°F = 0°C.

Fixed point	K	°R	°C	°F
Absolute zero	0	0	-273.15	-459.67
Water freezes	273.15	491.67	0	32
Room temp (20°C)	293.15	527.67	20	68
Body temp	310.15	558.27	37	98.6
Water boils	373.15	671.67	100	212

Common mistakes people make

Doing the arithmetic in the wrong order. For Fahrenheit to Celsius: subtract 32 first, then multiply by 5/9. Many people multiply first and get a wildly wrong answer. 72°F: correct is (72-32) × 5/9 = 40 × 0.556 = 22.2°C. Wrong order: 72 × 5/9 = 40, then 40-32 = 8°C. That’s a 14-degree error.

Confusing body temperature between scales. 37°C and 98.6°F are normal body temperature. A fever of 38.5°C sounds mild but is 101.3°F. Parents in Fahrenheit-using countries sometimes worry less at “38” than they should because they’re pattern-matching against the Fahrenheit fever threshold of 100, not realising they’re reading Celsius.

Using approximate rules for precision work. “Double and add 30” is a rough estimate for °C to °F. It works around 20°C (gives 70°F vs actual 68°F). It fails badly at extremes: 100°C by that rule gives 230°F, not 212°F. Use the exact formula or the calculator for anything that matters.

Assuming Kelvin uses the degree symbol. Kelvin is written as 300 K, not 300°K. Using °K marks you as unfamiliar with SI conventions in any technical document. The calculator displays it correctly; just match that format when writing your results.

Forgetting altitude affects boiling point. The water boiling point at sea level is 100°C / 212°F. At altitude, it drops: at 2,000 m it’s about 93°C / 199°F, at 5,000 m about 83°C / 181°F. This matters for cooking at altitude and for engineering systems involving steam. The converter gives the standard sea-level value.

Temperature in specific fields

Different fields have strong conventions about which scale they use. Knowing this saves you from unnecessary conversions.

Weather and everyday life: Celsius in most of the world, Fahrenheit in the US and a few other countries. Meteorologists globally use Celsius for surface temperatures.

Cooking: Celsius in most countries, Fahrenheit in the US. Recipes from American sources are almost always in °F. Oven temperatures between 150-260°C (300-500°F) are typical baking range.

Medicine and body temperature: Celsius in clinical settings worldwide except the US. Rectal temperature (most accurate) runs about 0.5°C higher than oral.

Chemistry and laboratory science: Celsius for practical work, Kelvin for thermodynamic calculations. A reaction described as occurring “at room temperature” means roughly 20-25°C (293-298 K).

Physics and thermodynamics: Kelvin exclusively for any formula involving temperature. Celsius only for describing conditions informally.

Cryogenics: Kelvin. Liquid nitrogen boils at 77 K (-196°C). Liquid helium boils at 4.2 K (-268.95°C). These temperatures don’t have intuitive Celsius equivalents for most people, which is why the field uses Kelvin throughout.

Aerospace engineering (US): often Rankine for thermodynamic calculations when working in the imperial system. Fahrenheit for operating condition descriptions.

Materials science and metallurgy: Celsius for most work. Metal melting and processing temperatures are often hundreds to thousands of degrees Celsius where the difference between scales is obvious from context.

The bottom line

Temperature conversion requires two steps: shift, then scale (or scale, then shift, depending on direction). It’s the one unit conversion where mental arithmetic reliably fails people because the zero points don’t align.

The calculator does the arithmetic and shows all 4 scales at once. Enter your temperature, pick your scale, and read the result. The thermometer visual below tells you immediately if the converted number is in sensible territory.

If you only remember one thing: subtract 32 before multiplying when going from Fahrenheit to Celsius. That’s where most hand calculations go wrong.