Three temperature scales run the modern world — Celsius for most of it, Fahrenheit for everyday life in the United States, and Kelvin for science — and this converter moves between all three, rejecting the one thing none of them may cross: absolute zero.
The Formulas
Temperature conversions are affine: they need both a rescaling and an offset, because the scales start at different zeros and size their degrees differently. A Celsius degree equals a kelvin exactly; a Fahrenheit degree is 5/9 of a Celsius degree.
This tool routes every conversion through Celsius internally (Fahrenheit → Celsius → Kelvin, for instance), and its all-scales view rounds each result to two decimals. It validates the physics too: any input that would land below −273.15 °C is refused with "Temperature cannot be below absolute zero," because no physical system can be colder — molecular kinetic energy has a floor, and 0 K is it.
Reference Points Worth Memorizing
| Phenomenon | °C | °F | K | | --- | --- | --- | --- | | Absolute zero | −273.15 | −459.67 | 0 | | Water freezes (1 atm) | 0 | 32 | 273.15 | | Human body (average) | 37 | 98.6 | 310.15 | | Water boils (1 atm) | 100 | 212 | 373.15 | | The crossover point | −40 | −40 | 233.15 |
The −40 row is the famous coincidence: solve and you get , the single temperature where the two everyday scales agree. The converter covers Celsius, Fahrenheit, and Kelvin only — Rankine, the Fahrenheit-sized absolute scale still used in some engineering, is not included.
Worked Example — Reading a Fever
A thermometer shows 102.2 °F. In Celsius: 39.0 °C. In kelvin: 312.15 K. (That is a genuine fever — normal body temperature centers near 37 °C / 98.6 °F, though healthy individuals vary by a few tenths through the day.)
Mental Shortcuts — and Their Limits
For weather, the quick approximation °F ≈ °C × 2 + 30 is within a couple of degrees across the comfortable range (−5 °C gives 20 vs the exact 23 °F) and fine for choosing a jacket. It is not fine for cooking, laboratories, medicine, or machinery: oven settings (180 °C = 356 °F, not 390), fever thresholds, and process temperatures deserve the exact formulas above. Doubling instead of multiplying by 1.8 is the shortcut's core error, and it grows with distance from the freezing point, where both methods coincide at 32 °F.
A second practical trap is differences versus values: a rise of 10 °C is a rise of 18 °F (multiply by 9/5, with no +32 — the offset cancels when subtracting). Climate reporting uses this constantly; a "2 °C warmer" world is 3.6 °F warmer, not 35.6 °F.
Limitations
Conversions use the exact defined offsets (273.15) per the international temperature scale; real measurements carry sensor error, and reference points like boiling shift with pressure and altitude. Kelvin takes no degree symbol and no plural debate: the unit is simply the kelvin, symbol K.
Frequently Asked Questions
Why does Kelvin have no degree symbol?
Because it is an absolute scale with a true zero, not a relative "degrees from an arbitrary point" scale like Celsius ("degrees" from freezing) or Fahrenheit. Since the 2019 SI redefinition, the kelvin is anchored to the Boltzmann constant itself.
What is the fastest exact way to convert Celsius to Fahrenheit mentally?
Multiply by 9, divide by 5, add 32 — or use the equivalent ×1.8 + 32. For round numbers, "double it, subtract a tenth of that double, add 32" gets you exact results with friendlier arithmetic: 20 °C → 40 − 4 + 32 = 68 °F.
Can a temperature be below 0 K if I enter it as Fahrenheit?
No. The floor is physical, not scale-specific: −500 °F is −295.6 °C, below absolute zero, so the converter rejects it exactly as it rejects −300 °C or −1 K.