🌡️ Temperature Converter

What is the Temperature Converter?

Three temperature scales cover almost everything you will run into. Celsius (C) is the everyday metric scale used by most of the world: it pins 0 to the freezing point of water and 100 to its boiling point at sea level, which makes it easy to reason about weather and cooking. Fahrenheit (F) is the scale used mainly in the United States, where water freezes at 32 and boils at 212, giving 180 degrees between the two fixed points instead of 100. Kelvin (K) is the scientific (SI) scale: it uses the same size step as Celsius but starts from absolute zero rather than the freezing point of water, so there are no negative values.

Absolute zero is the coldest temperature physically possible, the point where molecular motion is at its minimum. It sits at 0 K, which is -273.15 C and -459.67 F. Because Kelvin counts up from that floor, converting Celsius to Kelvin is just a shift: add 273.15. There is no multiplication because a one-degree change in Celsius is exactly a one-kelvin change, which is why scientists can swap between the two without rescaling.

The formulas differ because the scales disagree on two things: where zero sits and how big one degree is. Celsius and Kelvin share a degree size, so they differ only by an offset (273.15). Celsius and Fahrenheit share neither, so converting between them needs both a scale factor (9/5, because 180 Fahrenheit degrees span the same range as 100 Celsius degrees) and an offset (32, because Fahrenheit places water freezing at 32 rather than 0). To go from Fahrenheit straight to Kelvin you simply chain the two steps: convert to Celsius first, then add 273.15.

A neat feature of the Celsius and Fahrenheit scales is that they cross at exactly -40, the one temperature where both read the same number. That happens because the two lines have different slopes and meet at a single point. Knowing this gives you a quick sanity check: if your converted value drifts apart from a known anchor (water freezing at 0 C and 32 F, body temperature at 37 C and 98.6 F), you have probably dropped the offset or the 9/5 factor somewhere.

You may also see two older scales in textbooks. Rankine (R) is to Fahrenheit what Kelvin is to Celsius: an absolute scale that starts at absolute zero but keeps the Fahrenheit-sized degree, so 0 R = -459.67 F. Réaumur, now largely historical, set water freezing at 0 and boiling at 80. For practical purposes Celsius, Fahrenheit and Kelvin are the three you need, and this tool keeps all three in sync as you type.

A few mental shortcuts help when you have no calculator handy. To approximate Celsius to Fahrenheit, double the Celsius value and add 30 (so 20 C is roughly 70 F, close to the exact 68 F). Going the other way, subtract 30 from Fahrenheit and halve it. These are rough, drifting by a few degrees at the extremes, so use the exact formulas above when precision matters, such as a fever reading or an oven setting.

When to use it

• Reading a weather forecast or recipe written in the other unit while travelling.
• Setting an oven from a recipe that lists Fahrenheit when your dial is in Celsius.
• Converting a body-temperature reading between scales to check for a fever.
• Doing science or engineering homework that requires Kelvin for gas-law calculations.
• Checking a thermostat, fridge or freezer setting against a recommended figure in the other scale.
• Interpreting industrial or lab equipment that reports in Kelvin or Rankine.

How to use the Temperature Converter

1. Type a number into the Celsius, Fahrenheit or Kelvin field.
2. The other two scales update automatically as you type.
3. Use a minus sign for sub-zero values (for example -5 in Celsius).
4. Read off the converted values, or copy the result you need.

Formula & method

Worked examples

Common temperatures in Celsius and Fahrenheit

Body temperature and fever guide

Common oven temperatures (Fahrenheit to Celsius, rounded)

Common mistakes to avoid

• Forgetting the +32 offset. When converting Celsius to Fahrenheit, multiplying by 9/5 alone is not enough. You must add 32 afterwards, otherwise every answer comes out far too low.
• Subtracting 32 in the wrong order. Going from Fahrenheit to Celsius, subtract 32 first, then multiply by 5/9. Multiplying before subtracting gives the wrong result.
• Adding 273 instead of 273.15. For everyday work the rounding is harmless, but for precise science use the full 273.15 when converting Celsius to Kelvin.
• Writing degrees with Kelvin. Kelvin values are written as "300 K", not "300 °K". Kelvin is not a degree scale, so it takes no degree symbol.
• Using a rough shortcut where precision matters. The "double and add 30" trick is fine for guessing the weather but can be off by several degrees. For a fever reading or an oven, use the exact 9/5 and 32 formula.
• Mixing up a temperature with a temperature difference. A change of 10 Celsius degrees equals 18 Fahrenheit degrees (the offset of 32 does not apply to differences). Only add or subtract 32 when converting an actual reading, not a gap between two readings.

Glossary

Celsius

The metric temperature scale where water freezes at 0 and boils at 100 at sea level. Also called centigrade.

Fahrenheit

The scale used mainly in the United States, where water freezes at 32 and boils at 212.

Kelvin

The SI temperature scale, with the same degree size as Celsius but starting from absolute zero, so 0 K = -273.15 °C.

Absolute zero

The lowest possible temperature, where molecular motion is minimal: 0 K, -273.15 °C, or -459.67 °F.

Rankine (R)

An absolute temperature scale that starts at absolute zero but uses Fahrenheit-sized degrees, so 0 R = -459.67 °F. Used in some engineering fields.

Réaumur

A largely historical scale on which water freezes at 0 and boils at 80, once common in parts of Europe.

Centigrade

An older name for the Celsius scale, named for its 100 degrees between water freezing and boiling.

Offset

The fixed number added or subtracted because two scales place their zero at different points, such as the 32 between Celsius and Fahrenheit.

Frequently asked questions