Celsius, Fahrenheit, and Kelvin: Converting Between Temperature Scales
Temperature is the awkward one, because the scales do not share a zero. Every other conversion is multiplication; this one needs an offset as well. The Temperature Converter handles all three scales.
Why multiplying alone does not work
Celsius puts zero at water's freezing point. Fahrenheit puts zero somewhere near the coldest temperature Daniel Fahrenheit could reliably reproduce with a salt and ice mixture, and freezing at 32. So the two scales disagree about where zero is and about how big a degree is - a Fahrenheit degree is five ninths of a Celsius one.
That is why the formula has two steps: Celsius to Fahrenheit is multiply by 1.8 and add 32, and back the other way is subtract 32 first, then divide by 1.8. Subtracting before dividing is the part people get wrong, and doing it in the wrong order on a hot day turns 30 degrees into 22 rather than 86.
The two scales cross at minus 40, which is the same temperature in both. It is a useful sanity check: if your conversion does not agree at minus 40, the arithmetic is wrong.
The shortcut worth knowing
Double the Celsius and add 30. Twenty degrees becomes 70 against a true 68; 25 becomes 80 against 77; 30 becomes 90 against 86. Always a few degrees high, and close enough to know whether to take a coat.
Backwards: subtract 30 and halve. Seventy becomes 20 against a true 21.
Kelvin has no degrees, and that is deliberate
Kelvin starts at absolute zero, the point where there is no thermal energy left, so there are no negative kelvins. A kelvin is exactly the same size as a Celsius degree, which makes the conversion pleasantly simple: add 273.15 to Celsius.
You write 300 K, not 300 degrees K. The kelvin is an absolute unit rather than a position on a scale, so the word degree does not belong - a rule that looks pedantic and is the one thing physicists will correct you on.
Where the distinction matters
A temperature difference and a temperature are not the same conversion. A rise of 10 degrees Celsius is a rise of 18 degrees Fahrenheit - you apply the factor but not the offset, because an offset applied to a difference counts the gap from zero twice. Heating and cooling figures, and anything quoted per degree, are differences.
Common questions
What is normal body temperature?
Around 37 Celsius, 98.6 Fahrenheit - though that figure comes from 19th-century averages and healthy people vary by a degree either way.
What is a fever?
Generally 38 Celsius or 100.4 Fahrenheit and above, measured orally.
Which countries use Fahrenheit?
The United States and a handful of territories. Everywhere else is Celsius, with the UK using Celsius officially and Fahrenheit in some newspaper headlines about hot weather.
Is there a Rankine scale?
Yes - absolute zero with Fahrenheit-sized degrees. It appears in some American engineering work and almost nowhere else.
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