What is the fastest way to convert Celsius to Fahrenheit in my head?

Double the Celsius value and add 30. 25 °C → 50 + 30 = 80 °F (exact: 77 °F). For a better answer, double it, subtract a tenth of that, then add 32: 25 → 50 − 5 + 32 = 77 °F.

Is there a temperature where Celsius and Fahrenheit are the same?

Yes, −40. Setting C = C × 9/5 + 32 gives C = −40, so −40 °C = −40 °F. On the Kelvin and Celsius scales no such point exists; they differ by 273.15 everywhere.

Why does Kelvin have no degree sign?

Since 1967 the SI unit is simply the kelvin (symbol K), not the “degree Kelvin”, because it is an absolute unit counted from absolute zero rather than a scale with an arbitrary zero. Write 300 K, not 300 °K.

What is 350 °F in Celsius for baking?

(350 − 32) × 5/9 = 176.7 °C. Most ovens are set in 5–10 degree steps, so use 175 or 180 °C; 400 °F is about 200 °C and 450 °F about 230 °C.

Temperature Converter – Celsius, Fahrenheit, Kelvin, Rankine

How to use it

Type the temperature – negative values and decimals are accepted.
Choose the scale you have under From and the one you want under To; Swap scales reverses them.
The headline shows the converted value with the formula and the numbers filled in; the rows below give all four scales at once.
Use the reference chips (−40 °C, 37 °C, 98.6 °F, 350 °F, 0 K …) to check a familiar value, and copy the result or the link.

Example

Convert 100 °C to Fahrenheit: 100 × 9/5 = 180, plus 32 = 212 °F. In kelvin it is 100 + 273.15 = 373.15 K, and in Rankine 373.15 × 9/5 = 671.67 °R. The other way, a recipe at 350 °F is (350 − 32) × 5/9 = 176.67 °C, so set a metric oven to 175–180 °C.

Formulas

°F = °C × 9/5 + 32          °C = (°F − 32) × 5/9
K  = °C + 273.15            °C = K − 273.15
K  = (°F + 459.67) × 5/9    °F = K × 9/5 − 459.67
°R = °F + 459.67            °R = K × 9/5
°R = (°C + 273.15) × 9/5    °C = (°R − 491.67) × 5/9

The kelvin is the SI base unit of thermodynamic temperature; since the 2019 SI revision it is fixed through the Boltzmann constant, and the Celsius scale is defined by 0 °C = 273.15 K with degrees of the same size. The Fahrenheit degree is exactly 5/9 of a kelvin, with 32 °F at 0 °C and 212 °F at 100 °C. Rankine, used in some US engineering work, is the absolute scale with Fahrenheit-sized degrees. Readings below absolute zero are flagged but still computed, because the same arithmetic applies to temperature differences (a change of 18 °F is a change of 10 °C – the +32 offset does not apply to differences).

Reference table

°C	°F	K	Note
-40	-40	233.15	where °C and °F coincide
-30	-22	243.15
-20	-4	253.15
-18	-0.4	255.15	typical freezer
-10	14	263.15
0	32	273.15	water freezes
4	39.2	277.15	typical fridge
10	50	283.15
20	68	293.15	room temperature
25	77	298.15
30	86	303.15	hot summer day
37	98.6	310.15	body temperature
40	104	313.15
50	122	323.15
60	140	333.15
70	158	343.15
80	176	353.15
90	194	363.15
100	212	373.15	water boils at sea level
150	302	423.15
180	356	453.15	common oven setting
200	392	473.15
250	482	523.15

Sources: BIPM – The International System of Units (SI brochure, 9th edition), section 2.3.1 kelvin, NIST – SI Units: Temperature.

Body temperature and fever

98.6 °F is the 19th-century average that became the textbook “normal” (37 °C); modern measurements put the average nearer 36.6 °C (97.9 °F), and normal readings range from about 36.1 to 37.2 °C (97 to 99 °F). 38 °C (100.4 °F) is the common threshold for a fever.

Questions

What is the fastest way to convert Celsius to Fahrenheit in my head?: Double the Celsius value and add 30. 25 °C → 50 + 30 = 80 °F (exact: 77 °F). For a better answer, double it, subtract a tenth of that, then add 32: 25 → 50 − 5 + 32 = 77 °F.
Is there a temperature where Celsius and Fahrenheit are the same?: Yes, −40. Setting C = C × 9/5 + 32 gives C = −40, so −40 °C = −40 °F. On the Kelvin and Celsius scales no such point exists; they differ by 273.15 everywhere.
Why does Kelvin have no degree sign?: Since 1967 the SI unit is simply the kelvin (symbol K), not the “degree Kelvin”, because it is an absolute unit counted from absolute zero rather than a scale with an arbitrary zero. Write 300 K, not 300 °K.
What is 350 °F in Celsius for baking?: (350 − 32) × 5/9 = 176.7 °C. Most ovens are set in 5–10 degree steps, so use 175 or 180 °C; 400 °F is about 200 °C and 450 °F about 230 °C.

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