Temperature is the one conversion in this category that cannot be done by multiplying by a fixed factor — each scale has its own zero point, so converting needs an offset as well.
Key terms
- Celsius (°C) — 0° is water’s freezing point, 100° is its boiling point at sea level.
- Fahrenheit (°F) — 32° is freezing, 212° is boiling. Its relationship to Celsius has been fixed by international agreement since 1954.
- Kelvin (K) — the SI base unit. 0 K is absolute zero, a genuine physical floor rather than an arbitrary reference point.
How it works
Conversions, via Celsius
°F = °C × 9/5 + 32 · °C = (°F − 32) × 5/9 · K = °C + 273.15
Both relationships are exact by international definition — nothing here is rounded.
Why 0° is not “none of the thing”
Unlike a length or a weight, 0 on the Celsius or Fahrenheit scale does not mean an absence — it is an arbitrary reference point. That is why temperature can go negative in ordinary use, and why “twice as hot” is meaningless on either scale: 20°C is not twice as hot as 10°C, because the zero point was chosen, not discovered. Kelvin is the exception. 0 K is absolute zero, so ratios of kelvin values are physically meaningful in a way ratios of Celsius or Fahrenheit values are not — this calculator declines a value below 0 K rather than converting it, because nothing can be colder.
How to use this calculator
- Type a value into any of the three fields.
- The other two update immediately.
- −40 is the one number where Celsius and Fahrenheit agree — worth trying, since it is the fastest way to check the arithmetic is right.
Frequently asked questions
Why do Celsius and Fahrenheit agree at −40?
Because the conversion is a straight line, and −40 happens to be the point where that line crosses itself on both scales — the algebra works out so that −40°C and −40°F are the identical temperature, the only point where the two numbers match.
Is the Fahrenheit-to-Celsius relationship exact or approximate?
Exact. Since 1954, Fahrenheit has been formally defined in terms of Celsius by international agreement, so this conversion carries no rounding error at all.
What is absolute zero, and why can’t this calculator go below it?
Absolute zero, 0 K (−273.15°C), is the coldest temperature theoretically possible — the point at which particles have minimal thermal motion. A calculator that converted −5 K would be converting a temperature that cannot physically exist, so this one declines instead.
Why does Kelvin have no degree symbol?
By SI convention, kelvin is written as a plain unit — “300 K”, not “300°K” — because it is defined as an absolute scale rather than a scale of degrees relative to an arbitrary zero.
Which scale should I use for cooking or weather?
Whichever your audience expects — Fahrenheit in the US, Celsius almost everywhere else. Neither is more “correct”; they are the same physical temperature expressed on two different rulers.
