What each Temperature unit is
Celsius °C
Celsius puts zero at the freezing point of water and 100 at its boiling point, at sea level. Anders Celsius originally had the scale the other way up, with zero as boiling.
Fahrenheit °F
Fahrenheit puts freezing at 32 and boiling at 212, so a Fahrenheit degree is five ninths the size of a Celsius one. Converting therefore takes two steps, not one: a multiplication for the difference in degree size, and a shift for the different zero points.
Kelvin K
Kelvin starts at absolute zero, the point where there is no thermal energy left to remove, and its degrees are the same size as Celsius ones. It takes no degree sign — it is 300 K, not 300 °K.
Notes on the common Temperature conversions
celsius to fahrenheit
Reading a metric forecast without converting every number
The Celsius weather range is short enough to learn outright, which beats converting each figure. 0 °C is freezing, 10 °C wants a jacket, 20 °C is a pleasant day, 30 °C is hot, 40 °C is a heat emergency. In Fahrenheit those are 32, 50, 68, 86 and 104 — five landmarks that cover almost every forecast you will read on a trip.
The compression is what takes getting used to. One Celsius degree is 1.8 Fahrenheit degrees, so a forecast moving from 22 to 26 has moved from 72 to 79, and a range that looks narrow is not. It is also why metric forecasts quote fewer decimals: a single Celsius degree carries nearly as much information as two Fahrenheit ones, and the extra digit would be noise.
fahrenheit to celsius
What the Fahrenheit weather numbers actually feel like
Fahrenheit spreads the range that weather occupies over roughly twice as many numbers as Celsius, which is the honest argument in its favour and the reason its figures look inflated. 0 °F is −18 °C, a hard northern winter day. 32 °F is freezing. 50 °F is 10 °C and means a coat. 68 °F is 20 °C, a comfortable room. 86 °F is 30 °C and is hot. 100 °F is 37.8 °C, which is why an American heatwave headline reads "triple digits".
Three landmarks carry most of it: 0, 50 and 100 map to −18 °C, 10 °C and 38 °C. Below 50 is jacket weather, above 90 is uncomfortable, and the span between covers nearly every day an American describes out loud. Celsius compresses the same range into roughly 0 to 35, which is why a single degree carries more information there and why metric forecasts quote fewer of them.
celsius to kelvin
Adding 273.15 is the entire operation
Every other conversion on this site rescales a quantity. This one moves an origin. A kelvin is defined to be exactly the same size as a degree Celsius, so the two scales differ only in where zero sits, and the conversion is 273.15 added to the Celsius figure and nothing more. 20 °C is 293.15 K, 0 °C is 273.15 K, and −273.15 °C is 0 K.
That makes this the only conversion in the temperature category with no multiplication in it, and the easiest one to check. The two figures must always differ by exactly 273.15, so an answer out by 273 or by 0.15 has lost a digit rather than made a rounding error. If a result looks approximately right, subtract and confirm it exactly — approximately right is not a state this conversion has.
fahrenheit to kelvin
One constant does the whole job: 459.67
The two-step route works — take 32 off, multiply by 5/9, add 273.15 — but it carries a rounding hazard in the middle and an extra opportunity to misremember the order. Folding the constants together gives K = (°F + 459.67) × 5/9, which is one addition and one multiplication. 68 °F becomes (68 + 459.67) × 5/9, or 293.15 K.
The two forms are algebraically identical: 273.15 K is 491.67 Fahrenheit degrees above absolute zero, and moving that constant across the multiplication turns the 32 into 459.67. The single-step version is the one worth committing to memory, because its constant means something — it is absolute zero in Fahrenheit with the sign flipped — where the 32 and the 273.15 of the long route are two unrelated facts held in the same head.
kelvin to celsius
Subtract 273.15, then check the answer against 300
Take 273.15 off and the conversion is finished. A kelvin and a degree Celsius are the same size, so nothing is scaled: 300 K is 26.85 °C, 273.15 K is 0 °C, and 0 K is −273.15 °C. There is no multiplication anywhere and no order of operations to get wrong, which makes this the least error-prone conversion in the category.
The check worth running is against 300. Almost every kelvin figure describing something a person could stand beside sits between 250 and 320 K, so an answer far from that band should read as far from ordinary — 200 K is −73 °C and belongs to a laboratory freezer, 400 K is 127 °C and is past the boiling point of water. A kelvin figure in the low thousands is almost certainly a colour temperature rather than heat, which is a different problem entirely.
kelvin to fahrenheit
Multiply by 1.8, then take off 459.67
The formula is °F = K × 9/5 − 459.67, and the multiplication comes first. The two scales disagree about the size of a degree as well as about where zero falls, so the rescaling has to happen before the origin is moved. 300 K becomes 300 × 1.8 = 540, minus 459.67, which is 80.33 °F.
The constant is absolute zero expressed in Fahrenheit, and it is exact rather than measured. Doing the subtraction first produces −159.67 × 1.8 = −287.4 °F, which is a long way from 80 and is the failure worth watching for. The two-step route through Celsius — subtract 273.15, multiply by 9/5, add 32 — reaches the same answer through more places to lose a digit.