Temperature Conversion Formula

The three main temperature scales–Celsius, Fahrenheit, and Kelvin–measure the same physical property but use different starting points and increments. Converting between them requires specific formulas that work consistently across all ranges.

Temperature Conversion Formula Overview

Temperature conversion relies on linear relationships between scales. Each scale has a different reference point and degree size, so conversions involve both multiplication and addition.

Celsius (°C) – used worldwide, freezes at 0° and boils at 100° Fahrenheit (°F) – primarily used in the United States, freezes at 32° and boils at 212° Kelvin (K) – the absolute temperature scale used in science, starts at 0 (absolute zero)

Celsius to Fahrenheit Formula

The formula converts Celsius degrees to Fahrenheit:

(°C × 9/5) + 32 = °F

The ratio 9/5 (or 1.8) comes from the different degree sizes: a Celsius degree is smaller than a Fahrenheit degree. Adding 32 accounts for the offset between freezing points.

Example: Convert 25°C to Fahrenheit

• (25 × 9/5) + 32
• (25 × 1.8) + 32
• 45 + 32
• 25°C = 77°F

Example: Convert −10°C to Fahrenheit

• (−10 × 9/5) + 32
• (−10 × 1.8) + 32
• −18 + 32
• −10°C = 14°F

Fahrenheit to Celsius Formula

To reverse the conversion, subtract the offset first, then apply the inverse ratio:

(°F − 32) × 5/9 = °C

The 5/9 ratio is the inverse of 9/5, converting Fahrenheit’s smaller degrees back to Celsius’s larger ones.

Example: Convert 98.6°F (human body temperature) to Celsius

• (98.6 − 32) × 5/9
• 66.6 × 5/9
• 333 ÷ 9
• 98.6°F = 37°C

Example: Convert 32°F to Celsius

• (32 − 32) × 5/9
• 0 × 5/9
• 32°F = 0°C

Kelvin Conversion Formula

Kelvin uses the same degree size as Celsius but starts at absolute zero. Convert between these two directly:

°C + 273.15 = K

K − 273.15 = °C

Unlike Celsius and Fahrenheit, Kelvin requires no multiplication–only a fixed offset. The value 273.15 represents the Celsius temperature of absolute zero.

Example: Convert 20°C to Kelvin

• 20 + 273.15
• 20°C = 293.15 K

Example: Convert 0 K to Celsius

• 0 − 273.15
• 0 K = −273.15°C

For Fahrenheit to Kelvin conversions, combine both formulas:

(°F − 32) × 5/9 + 273.15 = K

Example: Convert 77°F to Kelvin

• (77 − 32) × 5/9 + 273.15
• 45 × 5/9 + 273.15
• 25 + 273.15
• 77°F = 298.15 K

Common Temperature Conversions

Which Formula Should You Use?

For everyday use: Celsius and Fahrenheit conversions are most common. Use (°C × 9/5) + 32 or (°F − 32) × 5/9 depending on your direction.

For scientific work: Convert to Kelvin. The formula K = °C + 273.15 is simpler and avoids the multiplication step.

For temperature differences: The degree size differs between scales. A change of 1°C equals 1.8°F (or 9/5°F) in magnitude, but for absolute temperatures, always use the full conversion formulas above.

Frequently Asked Questions

What is the formula to convert Celsius to Fahrenheit?

Multiply Celsius by 9/5, then add 32. Formula: (°C × 9/5) + 32 = °F. For example, 0°C converts to 32°F, and 100°C converts to 212°F.

How do I convert Fahrenheit to Celsius?

Subtract 32 from Fahrenheit, then multiply by 5/9. Formula: (°F − 32) × 5/9 = °C. Room temperature of 68°F equals 20°C.

What is absolute zero in Celsius?

Absolute zero is −273.15°C, which equals 0 K (Kelvin). No temperature can go below this point in physics.

Why use Kelvin instead of Celsius?

Kelvin is the SI unit of temperature used in science and engineering. It starts at absolute zero, making calculations in thermodynamics and chemistry more straightforward.

What is the boiling point of water in Fahrenheit?

Water boils at 212°F, which equals 100°C or 373.15 K at sea level atmospheric pressure.

These formulas are standard definitions of temperature scales; no assumptions are made about climate, weather, or measurement conditions.