Celsius, Fahrenheit, and Kelvin

Three scales describe the same physical quantity — thermodynamic temperature — but anchor their zero points differently. Celsius and Kelvin share the same size degree; Fahrenheit uses smaller degrees and a different offset. Converting between them is linear algebra once you know the fixed formulas.

Core formulas

Exact relationships used in our Temperature converter:

• °F = (°C × 9/5) + 32
• °C = (°F − 32) × 5/9
• K = °C + 273.15
• °C = K − 273.15

Kelvin is the SI base unit. It starts at absolute zero, where molecular motion theoretically stops. You never write “degrees Kelvin” — just kelvin (K).

Reference points

• Water freezes: 0 °C = 32 °F = 273.15 K
• Water boils (sea level): 100 °C = 212 °F = 373.15 K
• Comfortable room: ~20 °C = 68 °F
• Human body (oral): ~37 °C = 98.6 °F

Worked example

A recipe calls for 350 °F. Convert to Celsius: (350 − 32) × 5/9 = 318 × 5/9 ≈ 176.7 °C. For oven settings see also our Oven Temperature converter, which includes gas mark.

Why two everyday scales persist

Fahrenheit’s smaller degrees give finer steps without decimals in weather bands where humans live. Celsius aligns with the metric system and scientific work. Neither is “more accurate” — they are notation choices.

FAQs

Do Celsius and Kelvin degrees change size?

No — a 1 °C change equals a 1 K change; only the zero point differs.

Is −40 special?

Yes — −40 °C equals −40 °F exactly.

Why 273.15 and not 273?

The modern definition ties the kelvin to the Boltzmann constant; 273.15 is the defined offset from the Celsius scale.

Sources

BIPM SI brochure (kelvin definition); NIST unit conversion factors.