Temperature is the unit conversion people meet most often and the one with the least forgiving arithmetic, because unlike metres or kilograms, the scales do not share a zero. This converter handles all three scales in everyday and scientific use — Celsius, Fahrenheit and Kelvin — in every direction, so a weather report from abroad, an American recipe or a physics problem in absolute temperature becomes readable in the scale you think in.
Quick Answer
The three formulas are: °F = °C × 9/5 + 32, °C = (°F − 32) × 5/9 and K = °C + 273.15. Anchor points that never change: water freezes at 0 °C = 32 °F = 273.15 K and boils at 100 °C = 212 °F = 373.15 K. Body temperature, 37 °C, is 98.6 °F or 310.15 K.
Three Scales, Three Different Zeros
Celsius anchors its scale to water: 0° at freezing, 100° at boiling, a hundred even steps between. Fahrenheit, designed earlier with the instruments of its day, puts freezing water at 32° and boiling at 212° — 180 steps for the same interval — which is why the conversion needs both a multiplication (the steps are different sizes: 180 Fahrenheit degrees cover the same span as 100 Celsius degrees, hence 9/5) and a shift (the zeros are 32 degrees apart, hence +32). Every confusion in temperature conversion traces back to forgetting one of those two adjustments.
Kelvin sidesteps the shift problem by starting at the only zero that physics insists on: absolute zero, the temperature at which particles have minimal thermal motion, −273.15 °C. A kelvin is exactly the same size as a Celsius degree, so converting between Celsius and Kelvin is a pure offset — add or subtract 273.15, nothing to multiply. Kelvin values are written without the degree symbol (just "273.15 K") and can never be negative in ordinary physical terms, which is precisely why scientists prefer the scale for laws involving ratios: doubling from 100 K to 200 K genuinely doubles thermal energy in a way that doubling 100 °C to 200 °C does not.
The three scales meet in one famous place: −40. Solve °C × 9/5 + 32 = °C and the answer is −40, the single point where Celsius and Fahrenheit read the same number — a handy landmark on very cold days in Canada and the northern United States, where both scales are quoted.
How to Use the Temperature Conversion Tool
- Enter the temperature you have, as a plain number, including a minus sign for below-zero values.
- Select or note the scale it is measured in — entering an American forecast's 77 as if it were Celsius is the most common reason for absurd results.
- Read the equivalent in the other scales; all directions use the exact factors, so 25 °C comes out as exactly 77 °F and 298.15 K.
- For oven temperatures, remember many recipes quote gas marks or fan settings instead; convert the stated figure first, then apply your oven's conventions.
- When converting a temperature difference rather than a reading — "10 degrees warmer" — use only the multiplication (× 9/5), never the +32, because differences have no zero point to shift.
- Round-trip an important figure back through the converter to catch slips: an answer that returns your starting number is verified.
Worked Example: One Day in Three Scales
A weather app set to Celsius reports a summer afternoon of 37 °C — a heatwave in most of Europe. In Fahrenheit: 37 × 9/5 = 66.6, then + 32 = 98.6 °F, a number Americans recognise as body temperature, which is exactly why the day feels oppressive. In Kelvin: 37 + 273.15 = 310.15 K, the figure a scientist would use. The same moment, three costumes. Reference points that calibrate your intuition:
- −40 °C = −40 °F = 233.15 K (the crossover point)
- 0 °C = 32 °F = 273.15 K (water freezes)
- 20 °C = 68 °F = 293.15 K (a mild room)
- 25 °C = 77 °F = 298.15 K (a warm day)
- 100 °C = 212 °F = 373.15 K (water boils)
- 180 °C = 356 °F = 453.15 K (a standard oven)
Conversion Errors That Actually Happen
Applying the +32 to a temperature difference tops the list: a recipe that says "increase by 20 degrees" in Fahrenheit terms means 36 degrees of Celsius change, not 52 °C. Second is direction mix-ups with the 9/5 factor — going from Fahrenheit to Celsius uses 5/9 after subtracting 32, in that order; subtracting after multiplying garbles everything. Third, the shorthand "double it and add 30" for Celsius to Fahrenheit is a weather-only approximation: at 25 °C it gives 80 instead of 77, fine for deciding on a jacket, wrong for an oven or an incubator.
Kelvin mistakes are subtler. Writing "degrees Kelvin" or a degree symbol marks a value as carelessly transcribed in scientific contexts, and treating a Kelvin difference like an absolute reading (or vice versa) matters in engineering: a coolant rated to 400 K is rated to 126.85 °C, not to a dangerously misread figure. When the stakes are a chemical process, a medical device or a specification, convert exactly with the formulas here rather than by feel.
One more scale completes the map, though daily life rarely visits it. Engineers in the United States sometimes use Rankine, the Fahrenheit-sized twin of Kelvin: it starts at absolute zero and counts in Fahrenheit degrees, so °R = °F + 459.67, and water freezes at 491.67 °R. Its existence underlines the pattern behind all four scales — two are relative rulers laid against water (Celsius and Fahrenheit) and two are absolute measures laid against physics itself (Kelvin and Rankine). The absolute zero shared by both, 0 K, equals −273.15 °C and −459.67 °F, and this converter's three everyday scales are all anchored to that same unreachable floor.
Temperature Tools for Specific Directions
If your work is mostly one pair of scales, the dedicated pages skip a step: Celsius to Fahrenheit for the everyday pair, and Kelvin to Celsius for coming back from the absolute scale into daily terms. Every other unit family on the site — mass, length, volume, number systems — is gathered in the converters section.