Temperature Converter (°C, °F, K)

Celsius to Fahrenheit: °F = °C × 9/5 + 32. Fahrenheit to Celsius: °C = (°F − 32) × 5/9. Kelvin is Celsius plus 273.15. So 100 °C = 212 °F = 373.15 K (water boils), and 0 °C = 32 °F = 273.15 K (water freezes).

Most unit conversions are a single multiplication. Temperature needs two operations because the two scales disagree about two separate things: where zero sits, and how big a degree is. Celsius puts 0 and 100 at the freezing and boiling points of water at sea level, so the interval between them is 100 degrees. Fahrenheit was built on a different set of reference points — none of them the freezing point of pure water — and those same two events land at 32 and 212, an interval of 180 degrees. Everything follows from that pair of numbers. 180 ÷ 100 = 9/5, the size ratio: one Celsius degree covers as much temperature as 1.8 Fahrenheit degrees, so a Fahrenheit degree is the finer of the two. And the freezing point sitting at 32 rather than 0 is the offset that has to be added after the scaling. Skip the +32 and you have converted a temperature *difference* instead of a temperature.

Because the scales run at different rates from different starting points, they cross exactly once. That point is −40: −40 °C is −40 °F, precisely, with no rounding. The algebra is quick. Set the two equal and solve: x = x × 9/5 + 32 gives −4x/5 = 32, so x = −40. There is exactly one solution, because two straight lines with different slopes meet at exactly one point. It is more than a curiosity. It is a free sanity check: if a conversion returns a number on the wrong side of −40 relative to its input, the arithmetic went wrong somewhere. Above −40 the Fahrenheit value is always the larger of the two; below it, always the smaller.

Kelvin is the SI base unit for temperature and the odd one out in notation: you write 273.15 K, never "273.15 °K". The degree symbol was dropped in 1967, because a kelvin is not a degree on a scale — it is an absolute unit of temperature, like the meter is of length. Its zero is not a convention but a physical floor. Absolute zero, 0 K, is −273.15 °C or −459.67 °F, the point at which a system holds no thermal energy left to remove. Nothing is colder, so Kelvin never goes negative. Converting is pure addition: K = °C + 273.15. One kelvin and one Celsius degree are the same size, which creates a distinction worth being careful about: a reading of 20 °C is 293.15 K, but a change of 20 °C is a change of exactly 20 K. The same applies to Fahrenheit — to convert a temperature *difference* to Fahrenheit you multiply by 9/5 and stop. A rise of 10 °C is a rise of 18 °F, not 50 °F. Weather reports, gauge tolerances and cooking instructions all quote readings, so the offset applies; specification sheets often quote spans, so it does not.

The workhorse shortcut is double it and add 30: 20 °C → 70 °F, against a true 68 °F. It drifts at the extremes but stays within a few degrees across the whole range of ordinary weather, which is all you need for deciding on a jacket. Going the other way, subtract 30 and halve: 70 °F → 20 °C. Weather landmarks worth memorizing outright: 0 °C = 32 °F (freezing); 10 °C = 50 °F (cool); 20 °C = 68 °F (mild); 25 °C = 77 °F (warm); 30 °C = 86 °F (hot); 35 °C = 95 °F; 40 °C = 104 °F. Ovens are the other frequent lookup, and they are almost always rounded to the nearest 25 °F mark on the dial: 160 °C = 320 °F (dial 325); 180 °C = 356 °F (dial 350); 200 °C = 392 °F (dial 400); 220 °C = 428 °F (dial 425). Body temperature is a small lesson in false precision. The famous 98.6 °F is nothing more than 37 °C converted exactly — the extra decimal is an artifact of the arithmetic, not evidence that anyone measured to a tenth of a degree.