How this instrument works
All three scales measure the same physical thing — how vigorously matter's molecules are jostling — and differ only in where they put zero and how big they make one step. Celsius and kelvin use steps of identical size, so converting between them is pure addition: K = °C + 273.15. Fahrenheit uses a smaller step, five-ninths the size of a Celsius degree, and plants its zero elsewhere, which is why its formula needs both a multiplication and a shift: °F = °C × 9⁄5 + 32.
The constants in those formulas are definitions, not measurements. Since the SI redefinition of the kelvin in 2019, the unit is fixed through the Boltzmann constant, and the Celsius scale is defined from it by the exact relation t = T − 273.15. Nothing in this instrument is empirical: 9⁄5, 32 and 273.15 are agreed numbers, so a conversion done carefully is exact to as many decimals as you care to carry.
Two landmarks are worth memorising. At −40 the Celsius and Fahrenheit scales cross — the one reading where both thermometers agree — and at −273.15 °C sits absolute zero, the floor below which no temperature exists. This instrument refuses figures below that floor, because they do not describe anything physical.
- Type your reading into the Temperature field — the default is 20, a comfortable room.
- Pick the scale you measured in from the unit menu: °C, °F or K.
- Read the Fahrenheit (°F) and Kelvin (K) lines below; both update as you type, to two decimals.
- If the guard trips, your figure is below −273.15 °C — check the sign, or check which unit is selected.
Worked example — the 20 °C room
A thermostat set to 20 °C, the textbook comfortable room. Fahrenheit: multiply by 9⁄5 to get 36, add 32, and read 68 °F. Kelvin: add the offset directly — 20 + 273.15 = 293.15 K. Both results are exact; no rounding happened at any step.
Run it backwards as a check: (68 − 32) × 5⁄9 = 36 × 5⁄9 = 20 °C, landing precisely on the original figure. That clean round trip is the fingerprint of a linear conversion built from defined constants — nothing here is measured, so nothing drifts.
Questions
Why does −40 °C equal −40 °F?
Because the two scales cross there. Fahrenheit climbs faster from a lower zero, so somewhere the lines must intersect; solve x = 9x⁄5 + 32 and the algebra gives x = −40, the one reading where both thermometers agree. It doubles as a sanity check for any converter — enter −40 and the two fields should match exactly.
Why is there no degree symbol on the kelvin?
Because the kelvin is an absolute SI unit, not a position on a relative scale. Kelvin readings count up from absolute zero itself, so 293.15 K is a genuine quantity of temperature rather than a distance from an arbitrary reference. The convention since 1967 is to write K alone — 293.15 K, never °K — the same way one writes metres or seconds without decoration.
Is the 273.15 offset exact, or rounded?
Exact, by definition. The Celsius scale is defined from the kelvin by t = T − 273.15, so the offset carries no uncertainty at all — you may append as many zeros as you like. What was once measured (the ice point of water) is now the defined anchor; the 2019 SI redefinition moved the physics into the Boltzmann constant and left this relation untouched.
What is absolute zero, and why does the instrument stop there?
Absolute zero is −273.15 °C, equal to 0 K and −459.67 °F — the theoretical floor where molecular motion reaches its quantum minimum. No system can be cooled below it, so a figure under −273.15 °C describes nothing physical; the instrument flags it as a sign error or a unit mix-up rather than converting nonsense politely.
How do I convert Celsius to Fahrenheit in my head?
Double the Celsius figure and add 30. That approximation replaces × 9⁄5 with × 2 and 32 with 30, and it lands close through the everyday range: 20 °C estimates as 70 °F against a true 68 °F. The shortcut is exact at 10 °C and drifts by about two Fahrenheit degrees for every ten Celsius you move away — fine for weather, not for laboratory work.