About Temperature Converter
To convert Celsius to Fahrenheit, multiply by 9/5 and add 32: 25 C x 1.8 + 32 = 77 F. To go the other way, subtract 32 then multiply by 5/9: (98.6 - 32) x 5/9 = 37 C, normal body temperature. The two scales meet at exactly -40 degrees, and Kelvin starts at absolute zero, -273.15 C.
By the ToolFluency team · Updated June 2026
Convert between Celsius, Fahrenheit, and Kelvin instantly. Free, no sign-up required.
How to use
- Type a temperature into any of the three fields — Celsius, Fahrenheit, or Kelvin. The other two update automatically as you type, so there is no Convert button to press and no From/To selectors to set.
- Use the °Celsius field for everyday weather, cooking (most non-US recipes), and scientific work outside the US. The freezing point of water is 0 °C, body temperature is 37 °C, and water boils at 100 °C at sea level.
- Use the °Fahrenheit field for US weather and US cooking recipes. Water freezes at 32 °F, body temperature is 98.6 °F, and water boils at 212 °F. Each Fahrenheit degree is 5/9 of a Celsius degree, which is why Fahrenheit feels finer-grained for weather.
- Use the Kelvin field for scientific work — thermodynamics, gas laws, cryogenics, and astrophysics. Kelvin starts at absolute zero (0 K = -273.15 °C) and uses the same degree size as Celsius, so K = C + 273.15.
- Check the Reference cards at the bottom for quick anchor points: 0 °C (freezing), 37 °C (body), 100 °C (boiling), and the curiosity that -40 °C equals exactly -40 °F (the only crossover between the two scales).
- Decimals work in every field — type 36.8 in the Celsius box for a precise body temp reading, or 98.6 in Fahrenheit for the equivalent. Negative temperatures work too; type -10 °F to see -23.33 °C and 249.82 K.
Examples
Body temperature
Normal body temperature is 37 °C = 98.6 °F = 310.15 K. A fever starts around 38 °C (100.4 °F). The classic 98.6 °F figure traces back to Carl Wunderlich's 1868 study; modern data suggests average is closer to 98.2 °F (36.8 °C), with healthy daytime range 97-99 °F.
Oven temperatures
A US recipe calling for 350 °F equals 176.67 °C — round to 175 °C or 180 °C on a metric oven. 425 °F (a hot pizza oven) is 218.33 °C, typically dialed to 220 °C. The 5/9 conversion factor makes round-Fahrenheit numbers always become awkward Celsius values.
Cryogenic and stellar
Liquid nitrogen boils at 77.36 K = -195.79 °C = -320.42 °F. The Sun's surface is roughly 5,778 K = 5,505 °C = 9,941 °F. The cosmic microwave background is 2.725 K — just above absolute zero — explaining why deep-space telescopes need extreme cooling.
Frequently asked questions
How do I convert Celsius to Fahrenheit?
F = C × 9/5 + 32. So 25 °C × 1.8 + 32 = 77 °F. Quick mental approximation: double the Celsius value and add 30 (close within 1 °F over the everyday weather range). Going the other way, C = (F - 32) × 5/9, or for mental math, subtract 30 and halve. The two scales meet at exactly -40°: -40 °C = -40 °F.
How do I convert Fahrenheit to Celsius?
C = (F - 32) × 5/9. So (98.6 - 32) × 5/9 = 37 °C (normal body temperature). Subtract 32 first because the two scales have different zero points: 0 °C is 32 °F, not 0 °F. Then scale by 5/9 because 100 Celsius degrees span the same range as 180 Fahrenheit degrees (the freezing-to-boiling interval), and 100/180 reduces to 5/9.
What is absolute zero in Fahrenheit and Celsius?
0 K = -273.15 °C = -459.67 °F. Absolute zero is the theoretical temperature where all classical molecular motion stops; in quantum terms, it is where a system reaches its ground-state energy. It cannot be physically reached (third law of thermodynamics), but lab experiments have come within billionths of a kelvin. Kelvin uses absolute zero as its 0 point and never goes negative, which is why scientific equations of state always use kelvin.
Why is Kelvin used in science?
Kelvin is an absolute scale — its zero is a physical minimum, not a chosen reference. Many physics equations (ideal gas law PV = nRT, Stefan-Boltzmann radiation, blackbody spectra) only work with absolute temperatures. Doubling kelvin doubles the average kinetic energy; doubling Celsius is meaningless. Kelvin uses the same degree size as Celsius, so conversion is straightforward: K = C + 273.15. Since 2019, the kelvin has been defined by fixing the Boltzmann constant exactly.
What is the Rankine scale?
Rankine is the absolute scale that uses Fahrenheit-sized degrees: 0 °R = absolute zero = -459.67 °F, and water freezes at 491.67 °R, boils at 671.67 °R. To convert: °R = °F + 459.67, or °R = K × 1.8. It is used in some US engineering disciplines (combustion, HVAC, aerospace) where calculations need an absolute scale but the field has historically used Fahrenheit. This converter does not include Rankine; multiply your Kelvin value by 1.8 to get it.
Why does body temperature vary throughout the day?
Core body temperature follows a circadian rhythm: lowest in the early morning (around 4 AM, often 36.4 °C / 97.5 °F), highest in the late afternoon (around 6 PM, around 37.0-37.2 °C / 98.6-99.0 °F). It also rises with exercise, eating, ovulation (women), and infection. The classic 98.6 °F average is from a 19th-century study; modern measurements with digital thermometers find healthy adults average closer to 98.0-98.2 °F.
How do weather temperatures translate between scales?
0 °F (US bitter cold) = -17.78 °C. 32 °F (US freezing) = 0 °C. 50 °F (cool spring) = 10 °C. 70 °F (mild) = 21.1 °C. 90 °F (hot summer) = 32.2 °C. 100 °F (heat wave) = 37.8 °C. A useful Canadian-vs-US shortcut: same single-digit number in Celsius and Fahrenheit feels very different — 30 °C is hot summer (86 °F), but 30 °F is freezing winter (-1.1 °C).
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