Temperature Converter
Convert between Celsius, Fahrenheit, and Kelvin. Rejects values below absolute zero (no physical temperature is colder than 0 K).
How to use this converter
Pick the unit you're starting from on the left, the unit you want on the right, and type a value. The converted figure updates as you type. The swap button (⇄) flips the direction instantly. Enter a value below absolute zero in any unit and the calculator will tell you why no valid conversion exists — useful for catching unit mistakes before they propagate.
The three scales
Celsius (°C) sets 0° at the freezing point of water and 100° at its boiling point — used everywhere outside the United States. Fahrenheit (°F) sets 32° at freezing and 212° at boiling, with a finer-grained step (180 degrees across the same range) — used in the US for everyday weather and cooking. Kelvin (K)uses the same step size as Celsius but anchors zero at absolute zero (−273.15 °C). It's the scientific standard for any work involving thermodynamics.
The conversion formulas
°C → °F: F = (C × 9/5) + 32 °F → °C: C = (F − 32) × 5/9 °C → K: K = C + 273.15 K → °C: C = K − 273.15 °F → K and backgo via Celsius — the calculator does this internally, so you don't need to chain anything by hand.
Useful reference points
Absolute zero: 0 K = −273.15 °C = −459.67 °F. Water freezes: 0 °C = 32 °F = 273.15 K. Comfortable room temperature: 20–22 °C = 68–72 °F. Body temperature: 37 °C = 98.6 °F. Water boils (sea level): 100 °C = 212 °F = 373.15 K. Sun's surface: ~5,500 °C ≈ 5,773 K (Fahrenheit becomes useless at scales this big).
Why the −40° coincidence is useful
−40 °C = −40 °F exactly. There's only one temperature where the two scales agree, and this is it — a consequence of their different slopes and offsets crossing once. Beyond being good trivia, it's a quick mental sanity check: a freezer calibrated to −40 should read the same number whichever scale you check it in.
Why absolute zero is a hard limit
Temperature is a measure of the average kinetic energy of the particles in a substance. At absolute zero, particles have the minimum possible energy — they don't literally stop (quantum mechanics forbids that), but they can't go slower. Negative Kelvin values have no physical meaning, and the same is true for Celsius below −273.15 or Fahrenheit below −459.67. The calculator enforces this — entering an unphysical value surfaces an explanatory error rather than quietly returning a meaningless conversion.
Frequently asked questions
Why does the calculator reject very cold values?▾
Below absolute zero (0 K, −273.15 °C, −459.67 °F) is not a valid physical temperature — atoms can't move slower than complete stillness. Rather than silently returning a wrong-but-finite number, the calculator surfaces a clear error so you know the input is unphysical (typically a unit mistake — e.g., entering −300 thinking it was Fahrenheit when the field is set to Celsius).
What's the formula for Celsius to Fahrenheit?▾
F = (C × 9/5) + 32. Going the other way: C = (F − 32) × 5/9. The 9/5 factor (or 1.8) reflects that a one-degree change in Celsius equals 1.8 degrees in Fahrenheit. The +32 offset is because Fahrenheit's zero point isn't freezing — it was historically a brine mixture used in the 1700s.
Why is there a temperature where Celsius and Fahrenheit are equal?▾
At −40°, both scales read the same. The two scales are linearly related but with a different slope and offset; their lines cross exactly once, at −40. It's a useful piece of trivia and an instant sanity check — if you ever convert and end up at −40 in both, you know either the input or the conversion is right.
When do scientists use Kelvin instead of Celsius?▾
Anything to do with thermodynamics, gas laws, or absolute energy levels — Kelvin's zero point is absolute zero, so ratios of Kelvin temperatures are physically meaningful. Doubling Kelvin doubles the average kinetic energy of the atoms; doubling Celsius doesn't mean anything physical. For day-to-day weather, science uses Celsius (which differs from Kelvin only by an offset).
What's the difference between °C and a Celsius degree?▾
"°C" (with the symbol) refers to a specific point on the scale: 25°C. A "Celsius degree" (or "degree Celsius" without a value) refers to a temperature interval: a 5-degree drop. Same with Fahrenheit. Kelvin omits the degree symbol entirely — "273 K", not "273°K" — because Kelvin is treated as a fundamental unit, like metres or seconds.
How accurate are the conversions?▾
Exact for the formula. The calculator does no rounding internally — display rounds to 4 decimal places, which is sufficient for any practical use. Conversions between Celsius, Fahrenheit, and Kelvin involve only addition, multiplication by simple ratios (9/5 or 5/9), and exact constants (32, 273.15, 459.67), so there's no accumulated error.
Related calculators
Length & Distance Converter
Convert between metric and imperial length units — millimetres, centimetres, metres, kilometres, inches, feet, yards, and miles.
Weight & Mass Converter
Convert between metric and imperial weight units — milligrams, grams, kilograms, ounces, pounds, stone, metric tons, and short tons.
Unit Converter
Convert length, weight, temperature, volume, area, and speed between metric and imperial units — fast and precise.