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Temperature Converter

Convert between Celsius, Fahrenheit, and Kelvin.

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Unlike length or weight, temperature scales don't convert by simple multiplication — they need a formula with both a scaling factor and an offset, because Celsius, Fahrenheit, and Kelvin don't all start counting from the same zero point.

Celsius to Fahrenheit is °F = °C × 9/5 + 32, and the reverse is °C = (°F − 32) × 5/9. Celsius to Kelvin is simply °C + 273.15, since Kelvin uses the same size "degree" as Celsius but starts at absolute zero instead of water's freezing point.

For example, a comfortable room temperature of 21°C converts to (21 × 9/5) + 32 = 69.8°F, and to 21 + 273.15 = 294.15 K.

Celsius was designed in 1742 around water's properties: 0° is water's freezing point and 100° is its boiling point at standard atmospheric pressure, making it intuitive for weather and everyday science. Fahrenheit, proposed around 1724, is based on a different reference — reportedly an early brine (salt water) mixture's freezing point near 0°F and human body temperature near 96°F in the original scale — which is why its numbers for everyday temperatures don't line up with the round numbers Celsius uses.

Kelvin, introduced in 1848, exists specifically for science and engineering: it starts at absolute zero (−273.15°C), the coldest theoretically possible temperature where molecular motion stops entirely, which is why Kelvin has no negative values and is the standard unit in physics and chemistry.

Practically, Celsius dominates weather reporting and everyday use in most of the world, Fahrenheit persists mainly in the US for everyday temperatures, and Kelvin shows up specifically in scientific contexts — oven temperatures, cooking, and most household use stick to whichever of Celsius or Fahrenheit is standard where you are.

Fahrenheit's finer-grained scale (each Fahrenheit degree is smaller than each Celsius degree) is sometimes cited as an advantage for describing everyday weather without needing decimals — a comfortable range of outdoor temperatures spans roughly 0–100°F, which some find more intuitive than the equivalent roughly −18–38°C range, though this is largely a matter of familiarity rather than one scale being objectively better.

What's the formula from Celsius to Fahrenheit?

°F = °C × 9/5 + 32.

Why does 1 degree Fahrenheit represent a smaller temperature change than 1 degree Celsius?

Because the Fahrenheit scale spans 180 degrees between water's freezing and boiling points, versus 100 degrees on the Celsius scale for the same range — so each individual Fahrenheit degree is a smaller unit of temperature change.

What's the formula from Fahrenheit to Celsius?

°C = (°F − 32) × 5/9.

Why isn't temperature conversion just a simple multiplication?

Because Celsius, Fahrenheit, and Kelvin have different zero points, not just different-sized degrees — converting requires shifting the scale (adding or subtracting) in addition to scaling it.

What is absolute zero?

Absolute zero (0 K, or −273.15°C, or −459.67°F) is the coldest theoretically possible temperature, where molecular motion stops entirely — it's the fixed zero point Kelvin is built around.

Why does the US use Fahrenheit while most of the world uses Celsius?

It's largely historical — Fahrenheit was the dominant scale in English-speaking science and commerce when it was developed, and the US never switched when most other countries later adopted the metric system and Celsius.

At what temperature are Celsius and Fahrenheit equal?

At −40 degrees, both scales read the same value: −40°C = −40°F, the one point where the two scales intersect.

Why is Kelvin used in science instead of Celsius?

Kelvin has no negative values and starts at true zero (no molecular motion), which makes physical formulas involving temperature (like gas laws) work correctly without needing to add offset constants.

What is normal human body temperature in each scale?

Approximately 37°C, 98.6°F, or 310.15 K — though normal body temperature varies somewhat by individual and time of day.

Why did Celsius originally use different reference points than 0° and 100°?

It didn't — the modern Celsius scale has always used water's freezing point (0°) and boiling point (100°) at standard atmospheric pressure as its defining references, which is what makes it convenient for weather and everyday science.