Temperature Converter

What is temperature conversion?

Temperature conversion translates values between Celsius (used by most countries), Fahrenheit (used in the United States), and Kelvin (used in science). Unlike most unit conversions that involve simple multiplication, temperature conversions require both scaling and offset adjustments because the three scales have different zero points and different degree sizes.

Celsius sets 0 at the freezing point of water and 100 at the boiling point. Fahrenheit sets 32 at freezing and 212 at boiling. Kelvin uses the same degree size as Celsius but starts at absolute zero (-273.15 C), the lowest physically possible temperature where all molecular motion stops.

A brief history of temperature scales

The Fahrenheit scale was invented in 1724 by German physicist Daniel Gabriel Fahrenheit. He chose 0 °F as the coldest temperature he could reproduce in his laboratory — a mixture of ice, water, and ammonium chloride — and 96 °F as normal human body temperature (later recalibrated to 98.6 °F). The scale quickly became the scientific and everyday standard across Europe and the British Empire.

The Celsius scale was proposed in 1742 by Swedish astronomer Anders Celsius, who originally set 0 at boiling and 100 at freezing — the reverse of what we use today. The scale was inverted shortly after his death and became the international scientific standard following the adoption of the metric system. Today every country except the United States, the Cayman Islands, and Liberia uses Celsius for everyday temperatures.

The Kelvin scale was proposed in 1848 by British mathematician and physicist William Thomson, later Lord Kelvin. He defined absolute zero — 0 K, equal to −273.15 °C — as the point at which all molecular motion ceases. Because it starts at the lowest physically possible temperature, Kelvin has no negative values and is the standard in thermodynamics, chemistry, and astrophysics. The degree size in Kelvin is identical to that of Celsius, so converting between them requires only adding or subtracting 273.15.

Conversion formulas

Each scale has a different zero point and a different degree size, which is why conversions require both multiplication and addition rather than just one operation. The exact formulas are:

• Celsius to Fahrenheit: F = C * 9/5 + 32.
• Fahrenheit to Celsius: C = (F - 32) * 5/9.
• Celsius to Kelvin: K = C + 273.15.
• Kelvin to Celsius: C = K - 273.15.

The Rankine scale, sometimes used in engineering thermodynamics in the United States, works like Kelvin but uses Fahrenheit-sized degrees. Rankine zero is the same absolute zero as Kelvin; 0 °R = 0 K = −273.15 °C. Convert from Kelvin with R = K × 9/5, or from Celsius with R = (C + 273.15) × 9/5.

Worked conversion examples

The following examples walk through the formulas step by step with exact arithmetic so you can verify any conversion you perform by hand:

• Body temperature: 37 °C to Fahrenheit — apply °F = °C × 9/5 + 32 → 37 × 9/5 = 66.6, then 66.6 + 32 = 98.6 °F. To Kelvin: 37 + 273.15 = 310.15 K.
• Boiling water: 100 °C to Fahrenheit — 100 × 9/5 = 180, then 180 + 32 = 212 °F. To Kelvin: 100 + 273.15 = 373.15 K.
• Freezing water: 32 °F to Celsius — apply °C = (°F − 32) × 5/9 → (32 − 32) × 5/9 = 0 × 5/9 = 0 °C exactly.
• A warm summer day: 86 °F to Celsius — (86 − 32) × 5/9 = 54 × 5/9 = 270/9 = 30 °C.
• Absolute zero: 0 K to Celsius — 0 − 273.15 = −273.15 °C. To Fahrenheit: −273.15 × 9/5 + 32 = −491.67 + 32 = −459.67 °F.

How to use this tool

Enter a temperature value in any of the three scales and the equivalent values in the other two scales are calculated instantly. The tool also shows common reference points for context (body temperature, room temperature, water freezing/boiling).

Common temperature reference points

The table below lists the most frequently referenced temperatures in both everyday life and science. Use these as a mental calibration when you need a quick sanity check on a conversion:

• Absolute zero: −273.15 °C / −459.67 °F / 0 K — the lowest possible temperature; all molecular motion stops.
• Water freezes: 0 °C / 32 °F / 273.15 K — the ice point at standard atmospheric pressure.
• Room temperature: 20–22 °C / 68–72 °F / 293–295 K — the standard range for comfortable indoor environments.
• Normal body temperature: 37 °C / 98.6 °F / 310.15 K — average core human temperature; a fever begins at roughly 38 °C / 100.4 °F.
• Water boils: 100 °C / 212 °F / 373.15 K — the boiling point at standard atmospheric pressure (101.325 kPa).
• Oven (moderate baking): 180 °C / 356 °F / 453.15 K — a typical recipe temperature for cakes and roasts.

Absolute zero: -273.15 C / -459.67 F / 0 K. Water freezes: 0 C / 32 F / 273.15 K. Room temperature: 20-22 C / 68-72 F. Body temperature: 37 C / 98.6 F. Water boils: 100 C / 212 F / 373.15 K. A quick mental approximation: double the Celsius value and add 30 to get a rough Fahrenheit estimate.

Common mistakes to avoid

Temperature conversion errors are surprisingly common, even among professionals. Here are the mistakes that come up most often:

• Forgetting the ×9/5 scaling factor when converting Celsius to Fahrenheit. A common shortcut — doubling the Celsius value and adding 30 — is only a rough approximation (it gives 104 °F for 37 °C instead of the correct 98.6 °F). Always use the exact formula for accurate results.
• Reversing the direction of the formula. To convert °C to °F you multiply first, then add 32. To convert °F to °C you subtract 32 first, then multiply. Doing the operations in the wrong order gives a completely different answer.
• Treating Kelvin like Celsius. Because the degree size is the same, it is tempting to skip the 273.15 offset. But 37 °C is 310.15 K, not 37 K — the difference matters enormously in scientific calculations.
• Writing the degree symbol with Kelvin. Kelvin values are written as a plain number followed by K (e.g., 373.15 K), not 373.15 °K. The degree symbol is reserved for Celsius, Fahrenheit, and Rankine.

Frequently asked questions

Why does the United States use Fahrenheit?

Fahrenheit was the standard temperature scale when the US became independent. While most countries adopted Celsius as part of the metric system in the 19th and 20th centuries, the US never mandated the switch for everyday use. Fahrenheit arguably provides finer granularity for weather temperatures (0-100 F roughly covers the range of outdoor temperatures experienced in most of the US).

What is the temperature where Celsius and Fahrenheit are equal?

Celsius and Fahrenheit are equal at -40 degrees. You can verify: (-40) * 9/5 + 32 = -72 + 32 = -40. This is the only point where the two scales intersect. It is also extremely cold — about the temperature of winter in northern Siberia or central Antarctica.

Why does science use Kelvin instead of Celsius?

Many physical laws — including the ideal gas law (PV = nRT), Wien's displacement law for blackbody radiation, and the Boltzmann distribution in statistical mechanics — require an absolute temperature scale where zero means zero thermal energy. Using Celsius would produce meaningless negative values in those equations. Kelvin provides the absolute reference point that thermodynamics requires, while its degree size (identical to Celsius) makes converting between the two trivial.

What is a quick mental trick for estimating Celsius to Fahrenheit?

A useful approximation is to double the Celsius value and add 30. For example, 20 °C → 40 + 30 = 70 °F (actual: 68 °F) and 35 °C → 70 + 30 = 100 °F (actual: 95 °F). The estimate is usually within 3–5 °F for typical weather temperatures. For precision — medical, scientific, or engineering use — always apply the exact formula: °F = °C × 9/5 + 32.