Unit Converter

Free Temperature Converter

Convert between Celsius, Fahrenheit, Kelvin, Rankine and Réaumur instantly. Shows the formula used and the value in all five scales. Includes common reference temperatures.

Live conversion Formula shown 100% offline 5 scales

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Common temperatures

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Converted value

100 °C

100 °C = 100 °C

Same scale

All scales for 100 °C

Scale	Value

Tip: Type in the From field or change either scale. The formula used is shown below the result. Absolute zero is −273.15 °C, the coldest temperature physically possible.

About Temperature Conversion

Temperature conversion is different from length or weight conversion because the scales have different zero points. Celsius and Fahrenheit both place zero somewhere other than the coldest possible temperature, so converting between them requires both a multiplier and an offset. This is why °C to °F is not simply a multiplication — you have to add 32 after scaling.

The Kelvin scale is different: it starts at absolute zero (−273.15 °C), the theoretical minimum temperature where all molecular motion stops. Because Kelvin starts at the true zero of thermal energy, it has no negative values and is the standard unit for scientific work.

The five scales supported

Scale	Symbol	Zero point	Used by
Celsius	°C	Water freezing point	Most of the world, science
Fahrenheit	°F	Brine freezing point	United States, some Caribbean
Kelvin	K	Absolute zero	Science, physics, astronomy
Rankine	°R	Absolute zero (Fahrenheit sized)	US engineering (thermodynamics)
Réaumur	°Ré	Water freezing point	Historical, some European cheese-making

The conversion formulas

From → To	Formula
Celsius → Fahrenheit	°F = °C × 9/5 + 32
Fahrenheit → Celsius	°C = (°F − 32) × 5/9
Celsius → Kelvin	K = °C + 273.15
Kelvin → Celsius	°C = K − 273.15
Fahrenheit → Kelvin	K = (°F − 32) × 5/9 + 273.15
Celsius → Rankine	°R = (°C + 273.15) × 9/5
Celsius → Réaumur	°Ré = °C × 4/5

Common reference temperatures

Reference	°C	°F	K
Absolute zero	−273.15	−459.67	0
Water freezes	0	32	273.15
Room temperature	20	68	293.15
Human body	37	98.6	310.15
Water boils (sea level)	100	212	373.15

Common use cases

Travel

Convert °F weather forecasts to °C for international trips.

Cooking

Convert oven temperatures between °C and °F for recipes.

Science

Convert to Kelvin for chemistry, physics and thermodynamics.

Medicine

Convert body temperatures between °C and °F.

HVAC & Engineering

Convert between °C, °F and Rankine for building systems.

Weather

Compare temperature forecasts and climate data across regions.

Privacy and accuracy

This temperature converter runs entirely in your browser. All formulas are the standard definitional equations, so the results are exact to the precision you choose. Nothing is uploaded, stored or logged.

Frequently Asked Questions

How do I convert Celsius to Fahrenheit?

Multiply the Celsius value by 9/5, then add 32. For example, 100 °C → 100 × 9/5 + 32 = 180 + 32 = 212 °F. The formula is °F = °C × 9/5 + 32. You can also think of it as doubling the Celsius value, subtracting 10% of the doubled value, then adding 32 — but the exact formula is the safe one.

How do I convert Fahrenheit to Celsius?

Subtract 32 from the Fahrenheit value, then multiply by 5/9. For example, 212 °F → (212 − 32) × 5/9 = 180 × 5/9 = 100 °C. The formula is °C = (°F − 32) × 5/9.

At what temperature are Celsius and Fahrenheit the same?

At −40 degrees. Because the two scales cross at that point, −40 °C equals −40 °F exactly. It is the only temperature where the two numbers are identical.

What is absolute zero?

Absolute zero is the theoretical lowest possible temperature, defined as −273.15 °C or −459.67 °F or 0 K. At this temperature, particles have the minimum quantum-mechanical vibrational energy allowed by physics. No cooling method has ever reached it exactly, though labs have gotten within a few billionths of a Kelvin.

Why do scientists use Kelvin instead of Celsius?

Because Kelvin starts at absolute zero, it has no negative values and scales linearly with thermal energy. This makes formulas in thermodynamics, gas laws and quantum mechanics simpler. For example, Charles's law says gas volume is proportional to temperature — but only if the temperature is measured in Kelvin, not Celsius.

What is the Rankine scale?

The Rankine scale is to Fahrenheit what Kelvin is to Celsius: it starts at absolute zero but uses the same degree size as Fahrenheit. 0 K = 0 °R, and 491.67 °R = 32 °F (water's freezing point). Rankine is used in some US engineering fields, particularly thermodynamics.

Is this temperature converter free?

Yes. It is 100% free, requires no sign-up, and runs entirely in your browser.

Is my input sent to a server?

No. All conversions happen locally in JavaScript. Nothing is uploaded, stored or logged.

How accurate is the conversion?

The formulas used here are the exact definitional equations. The only source of rounding is the precision setting — increase decimal places for more digits.

Can I enter a temperature below absolute zero?

Mathematically, yes — the tool will convert any number. Physically, negative Kelvin is impossible, and any conversion resulting in a Kelvin value below zero will trigger a warning. The math is provided for completeness, but the value has no physical meaning.