Temperature Converter: How to Convert Celsius to Fahrenheit, Kelvin and Why the World Uses Different Scales

Here's something that trips up almost everyone at some point: you're reading a recipe from an American website and it says "preheat your oven to 350 degrees." If you're in the UK, your oven is in Celsius. Is 350°F the same as 350°C? Absolutely not — 350°C would literally melt aluminium. What you actually need is about 175°C.

Temperature conversion is one of those things you don't think about until you urgently need it. Whether you're cooking, travelling, checking weather forecasts, or studying science, our free temperature converter handles Celsius, Fahrenheit, and Kelvin conversions instantly.

But this guide goes beyond just giving you a converter. We'll cover the formulas, the history, common reference points you can memorise, and why the world can't agree on a single temperature scale.

The Three Temperature Scales

Celsius (°C)

Used by virtually every country except the United States. Invented by Swedish astronomer Anders Celsius in 1742. The scale is based on water:

• 0°C = water freezes
• 100°C = water boils (at sea level)

Fun fact: Celsius originally designed his scale upside down — 0° was boiling and 100° was freezing. It was reversed after his death by fellow scientist Carl Linnaeus.

Fahrenheit (°F)

Used primarily in the United States, and occasionally in a few Caribbean nations. Created by German physicist Daniel Gabriel Fahrenheit in 1724. His reference points were:

• 0°F = the temperature of a mixture of ice, water, and ammonium chloride (the coldest thing he could reliably create in his lab)
• 32°F = water freezes
• 212°F = water boils
• 96°F = approximately human body temperature (he was slightly off — it's actually 98.6°F)

Kelvin (K)

The scientific standard, used in physics, chemistry, and engineering worldwide. Named after Lord Kelvin (William Thomson), a Belfast-born physicist. Kelvin uses the same increment size as Celsius but starts at absolute zero — the theoretical lowest possible temperature where all molecular motion stops.

• 0 K = absolute zero (-273.15°C)
• 273.15 K = water freezes
• 373.15 K = water boils

Note: Kelvin doesn't use a degree symbol. It's just "K," not "°K."

Conversion Formulas

Here are all the formulas you need. Or just use our temperature converter and skip the maths entirely.

Quick Mental Maths Trick: Celsius to Fahrenheit

The exact formula (multiply by 1.8, add 32) is hard to do in your head. Here's a faster approximation:

Double the Celsius, subtract 10%, add 32

Example: 25°C

• Double: 50
• Subtract 10%: 50 - 5 = 45
• Add 32: 45 + 32 = 77°F

Actual answer: 77°F. Perfect.

Even quicker (less accurate): double it and add 30. For 25°C: 50 + 30 = 80°F. Close enough for weather and cooking.

Common Temperature Reference Points

Memorise these and you'll rarely need a converter for everyday situations:

Cooking Temperature Conversions

This is probably the most common reason people need temperature conversion. American recipes use Fahrenheit; British and European recipes use Celsius. And then there are gas marks.

Fan oven tip: If a recipe gives a conventional oven temperature, reduce by 20°C for a fan oven. So 180°C conventional = 160°C fan.

Body Temperature: When to Worry

Normal body temperature is approximately 37°C (98.6°F), though it varies throughout the day and between individuals. According to the NHS:

• Normal: 36.1-37.2°C (97-99°F)
• Low-grade fever: 37.3-38°C (99.1-100.4°F)
• Fever: 38°C+ (100.4°F+)
• High fever: 39.5°C+ (103.1°F+) — seek medical advice
• Dangerous: 41°C+ (105.8°F+) — medical emergency

If you're using an American thermometer in the UK (or vice versa), our converter can help you quickly check whether a reading is concerning. Also check your health metrics with our BMI calculator or BMR calculator for a fuller picture of your wellbeing.

Weather Temperature Conversion

If you're travelling to the US or reading American weather forecasts, here's a quick guide:

Why Does the US Still Use Fahrenheit?

Almost every country in the world uses Celsius. The US is the notable exception, along with a handful of territories. The reason is largely historical inertia — the US attempted to switch to metric in the 1970s with the Metric Conversion Act of 1975, but it was voluntary rather than mandatory, and the public largely ignored it.

The cost of switching road signs, updating textbooks, retraining workers, and replacing equipment was deemed too high for too little practical benefit. So Americans continue to measure temperature in Fahrenheit, distance in miles, and weight in pounds — while the rest of the world quietly uses metric.

Interestingly, American scientists DO use Celsius and Kelvin. It's only everyday life that clings to Fahrenheit.

Temperature in Science

If you're studying science, you'll encounter all three scales:

• Chemistry: Kelvin for gas laws and thermodynamics. Celsius for lab work
• Physics: Kelvin almost exclusively
• Medicine: Celsius in the UK and most of the world. Fahrenheit in the US
• Engineering: Depends on the country and industry
• Meteorology: Celsius globally (except US weather forecasts)

For science students, our scientific calculator and percentage calculator are also useful companions for lab work and calculations.

Other Useful Converters

If you're converting temperatures, you might also need:

• Length Converter — convert between cm, inches, feet, metres and more
• Electricity Cost Calculator — work out how much your heating costs

Try Our Free Temperature Converter

Whether you're converting a recipe, checking a weather forecast, monitoring a fever, or doing science homework, our free temperature converter handles Celsius, Fahrenheit, and Kelvin conversions instantly.

Frequently Asked Questions

What is 0 degrees Celsius in Fahrenheit?

0°C = 32°F. This is the freezing point of water.

What is 100 degrees Fahrenheit in Celsius?

100°F = 37.78°C. This is slightly above normal body temperature.

What temperature is a fever?

A fever is generally defined as 38°C (100.4°F) or above. According to the NHS, a high fever of 39.5°C (103.1°F) or above warrants medical advice.

What is 350°F in Celsius?

350°F = 176.7°C, typically rounded to 180°C (or 160°C for fan ovens). This is the most common baking temperature in American recipes.

What is absolute zero?

Absolute zero is 0 Kelvin, -273.15°C, or -459.67°F. It's the theoretical lowest possible temperature where all molecular motion ceases. It has never been achieved in a laboratory, though scientists have come within billionths of a degree.

Why doesn't Kelvin use a degree symbol?

Kelvin is an absolute scale — it measures thermodynamic temperature from a true zero point. The degree symbol was dropped in 1967 by the General Conference on Weights and Measures to reflect this distinction.