Whether you’re following a recipe from a foreign cookbook, checking a weather forecast abroad, or working in a lab, temperature conversions come up constantly. Celsius, Fahrenheit, and Kelvin each serve different purposes — and switching between them is easier than you think.
This guide covers all three scales with formulas, mental shortcuts, reference tables, and practical examples you’ll actually use.
The Three Temperature Scales
| Scale | Abbreviation | Used Where |
|---|---|---|
| Celsius | °C | Most countries worldwide (scientific + everyday) |
| Fahrenheit | °F | US, Bahamas, Cayman Islands, Liberia |
| Kelvin | K | Science, physics, astronomy, engineering |
Celsius to Fahrenheit: The Most Common Conversion
The Formula
°F = (°C × 9/5) + 32
°C = (°F - 32) × 5/9
Simple Mental Math
For quick estimates without a calculator:
-
Celsius to Fahrenheit: Double it and add 30
- 20°C → 20 × 2 + 30 = 70°F (actual: 68°F — close enough)
- 30°C → 30 × 2 + 30 = 90°F (actual: 86°F)
- 0°C → 0 × 2 + 30 = 30°F (actual: 32°F)
-
Fahrenheit to Celsius: Subtract 30, then halve it
- 80°F → (80 - 30) ÷ 2 = 25°C (actual: 26.7°C)
- 100°F → (100 - 30) ÷ 2 = 35°C (actual: 37.8°C)
Exact Conversion Trick
For precise conversion, memorise a few anchor points and interpolate:
- 0°C = 32°F (water freezes)
- 16°C = 61°F (mild day)
- 28°C = 82°F (hot day)
- 37°C = 98.6°F (body temperature)
- 100°C = 212°F (water boils)
Every 5°C change equals roughly 9°F change.
Common Temperature Reference Table
| °C | °F | What It Feels Like |
|---|---|---|
| -40 | -40 | Extreme cold — same on both scales! |
| -20 | -4 | Bitter cold, frostbite risk |
| -10 | 14 | Freezing cold |
| 0 | 32 | Water freezes, ice on roads |
| 5 | 41 | Cold, need a jacket |
| 10 | 50 | Cool, light jacket weather |
| 15 | 59 | Mild, comfortable |
| 20 | 68 | Room temperature, pleasant |
| 25 | 77 | Warm, shorts weather |
| 30 | 86 | Hot, beach day |
| 35 | 95 | Very hot, heat warning |
| 37 | 98.6 | Normal body temperature |
| 40 | 104 | Dangerous heat, heatstroke risk |
| 45 | 113 | Extreme heat, stay indoors |
| 50 | 122 | Record-breaking heat |
Use our Temperature Converter to convert any temperature instantly.
Weather Temperature Comparisons Around the World
How Different Countries Describe the Same Temperature
| Actual Temp | In the US (°F) | In Europe (°C) | In the UK |
|---|---|---|---|
| 0°C / 32°F | Freezing | Frigorífico / Gelé | Brass monkeys |
| 10°C / 50°F | Chilly | Fresco / Kühl | Bracing |
| 20°C / 68°F | Perfect | Agradable / Angenehm | Warm for the time of year |
| 30°C / 86°F | Hot | Calor / Heiss | Scorching, absolute scorcher |
| 40°C / 104°F | Dangerous | Ola de calor / Hitzewelle | Bloody boiling |
City Temperature Averages
| City | Winter (°C) | Winter (°F) | Summer (°C) | Summer (°F) |
|---|---|---|---|---|
| London | 2-8 | 36-46 | 15-24 | 59-75 |
| New York | -3-4 | 27-39 | 22-30 | 72-86 |
| Dubai | 14-24 | 57-75 | 32-43 | 90-109 |
| Tokyo | 0-10 | 32-50 | 22-31 | 72-88 |
| Sydney | 8-17 | 46-63 | 18-26 | 64-79 |
| Moscow | -10-(-3) | 14-27 | 15-25 | 59-77 |
| Doha | 14-22 | 57-72 | 33-44 | 91-111 |
| Singapore | 24-31 | 75-88 | 25-33 | 77-91 |
Kelvin: The Scientist’s Scale
Kelvin is the SI base unit for temperature. It starts at absolute zero (-273.15°C) — the point where all molecular motion stops.
Why Kelvin?
Kelvin uses the same step size as Celsius (1 K = 1°C), but with no negative numbers. This makes it essential for:
- Physics calculations (ideal gas law, thermodynamics)
- Astronomy (star temperatures)
- Engineering (cryogenics, materials science)
Conversion Formulas
K = °C + 273.15
°C = K - 273.15
K = (°F + 459.67) × 5/9
°F = K × 9/5 - 459.67
Kelvin Reference Points
| Temperature | Kelvin | Celsius | What’s Happening |
|---|---|---|---|
| Absolute zero | 0 K | -273.15°C | All molecular motion stops |
| Coldest lab temperature | 0.0001 K | -273.15°C | Achieved in laser cooling experiments |
| Liquid nitrogen | 77 K | -196°C | Cryogenic freezing, food storage |
| Dry ice (sublimation) | 195 K | -78°C | CO₂ solid to gas |
| Water freezes | 273.15 K | 0°C | Ice formation |
| Room temperature | 293-298 K | 20-25°C | Comfortable indoor temp |
| Average human body | 310 K | 37°C | Normal body temperature |
| Water boils | 373.15 K | 100°C | Sea level boiling point |
| Lead melts | 600 K | 327°C | Soldering, battery recycling |
| Surface of the Sun | 5,778 K | 5,505°C | Nuclear fusion at work |
| Core of the Sun | 15,700,000 K | ~15.7 million °C | Fusion reactor |
Cooking Temperature Conversions
Oven temperatures are one of the most common reasons people need temperature conversion — especially when following recipes from different countries.
Oven Temperature Gas Marks (UK)
| Gas Mark | °C | °F | Description |
|---|---|---|---|
| 1/4 | 110 | 225 | Very cool |
| 1/2 | 130 | 250 | Very cool |
| 1 | 140 | 275 | Cool |
| 2 | 150 | 300 | Cool |
| 3 | 160 | 325 | Warm |
| 4 | 175 | 350 | Moderate |
| 5 | 190 | 375 | Moderate hot |
| 6 | 200 | 400 | Hot |
| 7 | 220 | 425 | Hot |
| 8 | 230 | 450 | Very hot |
| 9 | 240 | 475 | Very hot |
| 10 | 260 | 500 | Extremely hot |
Meat Temperature Guide (Internal)
| Meat | Doneness | °C | °F |
|---|---|---|---|
| Beef / Lamb | Rare | 52 | 125 |
| Beef / Lamb | Medium rare | 57 | 135 |
| Beef / Lamb | Medium | 63 | 145 |
| Beef / Lamb | Well done | 71 | 160 |
| Chicken / Turkey | Fully cooked | 74 | 165 |
| Pork | Medium | 63 | 145 |
| Pork | Well done | 71 | 160 |
| Fish | Flakes easily | 63 | 145 |
Baking Temperatures
| Recipe | °C (fan) | °C (conventional) | °F |
|---|---|---|---|
| Sponge cake | 160 | 175 | 350 |
| Bread | 200 | 220 | 425 |
| Pizza | 220 | 240 | 475 |
| Cookies | 170 | 185 | 365 |
| Roast vegetables | 190 | 210 | 410 |
Body Temperature & Fever
Body temperature is another everyday conversion need, especially when using thermometers from different countries.
| Condition | °C | °F |
|---|---|---|
| Hypothermia | <35 | <95 |
| Normal (average) | 36.5-37.5 | 97.7-99.5 |
| Low-grade fever | 37.6-38.2 | 99.7-100.8 |
| Fever | 38.3-39.4 | 101-103 |
| High fever | 39.5+ | 103+ |
| Medical emergency | 41+ | 106+ |
Note: Body temperature varies by time of day, age, and measurement method (oral, armpit, ear, rectal). Morning temperatures are typically 0.5°C / 1°F lower.
Practical Scenarios
Scenario 1: You’re Following an American Recipe
The recipe says “preheat oven to 350°F.” Your oven is in Celsius.
350°F → (350 - 32) × 5/9 = 177°C — set it to 175°C or Gas Mark 4.
Scenario 2: Checking a Weather Forecast
You’re travelling to New York and the forecast says 85°F. Is that jacket weather?
85°F → (85 - 30) ÷ 2 = 27°C (approximate) — no jacket needed, it’s T-shirt weather.
Scenario 3: Science Class
You need room temperature in Kelvin for a gas law calculation.
22°C → 22 + 273.15 = 295.15 K
Scenario 4: A Child Has a Fever
Your digital thermometer reads 38.5°C. What’s that in Fahrenheit?
38.5°C → 38.5 × 9/5 + 32 = 101.3°F — yes, that’s a fever.
FAQ
What’s the easiest way to convert Celsius to Fahrenheit in my head? Double the Celsius and add 30. For 20°C: 20 × 2 + 30 = 70°F. It’s approximate (off by ~2°F) but fine for weather and general use.
Why does the US still use Fahrenheit? Historical reasons. Fahrenheit was the standard when thermometers became common, and the US never made the switch to metric. It’s deeply embedded in American infrastructure, weather reporting, and cooking.
At what temperature are Celsius and Fahrenheit the same? -40°. Both scales read the same at -40 degrees. This is the only point where they intersect.
Do I need Kelvin for everyday use? No. Kelvin is primarily used in science, engineering, and astronomy. For cooking, weather, and general life, Celsius and Fahrenheit are sufficient.
What’s the highest possible temperature? Theoretically, the Planck temperature (1.416 × 10³² K) — beyond that, our current physics breaks down. For comparison, the core of the Sun is only 15.7 million K.
Can water boil at a lower temperature? Yes. At higher altitudes, atmospheric pressure is lower, so water boils at a lower temperature. In Denver (1,600m elevation), water boils at about 95°C (203°F) instead of 100°C (212°F). This affects cooking times.
What’s absolute zero in Fahrenheit? Absolute zero is -459.67°F. It’s the point where atoms stop moving entirely.
How do I convert oven temperatures for fan-assisted vs conventional ovens? Fan ovens circulate hot air more efficiently, so you typically reduce the temperature by 20°C (or about 35°F). If a recipe says 200°C conventional, set a fan oven to 180°C.
Convert any temperature instantly with FreeToolz’s free Temperature Converter — Celsius, Fahrenheit, and Kelvin. No signup required.