What is a quick way to estimate C to F?

Double the Celsius and add 30 for a rough figure: 20°C ≈ 70°F (actual 68°F). It's close enough for weather in the everyday range.

The Kelvin scale starts at absolute zero (−273.15°C), the coldest possible temperature. It uses the same degree size as Celsius, so K = °C + 273.15. It's used in science.

Temperature Converter

Q: How do I convert Celsius to Fahrenheit?

Multiply the Celsius temperature by 9/5 and add 32. So 20°C is 20 × 1.8 + 32 = 68°F. To reverse it, subtract 32 then multiply by 5/9.

Convert instantly between Fahrenheit, Celsius, and Kelvin — enter a temperature in any scale and see the other two.

Celsius

—

Fahrenheit

—

Kelvin

—

The conversion formulas

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

Quick estimate

For a fast head-math conversion of Celsius to Fahrenheit, double it and add 30: 20°C ≈ 70°F (exact is 68°F). Good enough for checking the weather.

Reference temperatures

	°C	°F	K
Absolute zero	−273.15	−459.67	0
Water freezes	0	32	273.15
Room temperature	21	70	294
Body temperature	37	98.6	310
Water boils	100	212	373.15

The two scales cross at −40° (−40°C = −40°F). Celsius and Kelvin share the same degree size — a change of 1°C equals a change of 1 K — while Fahrenheit degrees are smaller (1.8 per Celsius degree). Handy when following recipes, checking overseas weather, or reading science. For lengths and weights, see the length and weight converters.

Frequently asked questions

How do I convert Celsius to Fahrenheit?

× 9/5 + 32. 20°C = 68°F.

Quick way to estimate C to F?

Double and add 30. 20°C ≈ 70°F.

What is Kelvin?

A scale starting at absolute zero; K = °C + 273.15.

Related calculators

Last reviewed: July 2026.