Celsius to Fahrenheit to Kelvin: the correct formulae
Temperature is not a simple scale factor. Zero on one scale is not zero on another — so “just multiply by 1.8” without the ±32 (or Kelvin offset) is wrong.
The three scales
| Point | °C | °F | K |
|---|---|---|---|
| Absolute zero | −273.15 | −459.67 | 0 |
| Water freezes (std) | 0 | 32 | 273.15 |
| Water boils (std) | 100 | 212 | 373.15 |
| Body-ish | 37 | 98.6 | 310.15 |
Formulae that work
K = C + 273.15
C = K - 273.15
F = C * 9/5 + 32
C = (F - 32) * 5/9
F = K * 9/5 - 459.67
K = (F + 459.67) * 5/9Implement via Kelvin as a hub if you support many scales — one place for absolute-zero checks.
The classic bug
// Wrong: treats 0 °C like 0 °F
f = c * 1.8; // 0 → 0 (should be 32); 100 → 180 (should be 212)
// Right
f = c * 1.8 + 32;Unit tests with 0 °C → 32 °F and 100 °C → 212 °F catch this immediately. Also test a negative: −40 °C = −40 °F (the scales cross).
API check
curl -s 'https://api.oddlathe.com/v1/units?value=20&from=C&to=F'
# → roughly 68
curl -s 'https://api.oddlathe.com/v1/units?value=-300&from=C&to=K'
# → error: below absolute zeroLive docs: /docs/units. Length/mass/data sizes live on the same endpoint.
Sensor and weather gotchas
- Integer °C truncated then converted — convert with floats (or fixed-point) before rounding for display.
- “Room temperature” defaults — 20 °C ≠ 20 °F; seed configs in Kelvin or always label the unit.
- Delta vs absolute — a change of 1 °C equals a change of 1 K and 1.8 °F; the ±32 applies only to absolute readings.
Related
Unit converter · KB vs KiB / MB vs MiB · Metric tonne vs ton · Unix seconds vs milliseconds · Anvil /v1/units · Anvil date/diff
Questions
What is the Celsius to Fahrenheit formula?
°F = °C × 9/5 + 32. The +32 is required; multiplying alone is wrong.
How do I convert Fahrenheit to Celsius?
°C = (°F − 32) × 5/9.
How do I convert Celsius to Kelvin?
K = °C + 273.15. Kelvin has no degree sign in modern SI writing.
Why did my converter say 0 °C is 0 °F?
It skipped the offset. 0 °C is 32 °F.
Can temperature be below 0 K?
No. Physical absolute zero is 0 K (−273.15 °C). A correct API rejects values below that.
Is a 5 °C rise the same as a 5 K rise?
Yes for differences. A 5 °C rise is a 9 °F rise, not 5 °F.
Published .