Temperature Converter — Celsius to Fahrenheit, Kelvin | Instant
Convert between Celsius, Fahrenheit, and Kelvin instantly. See the formula and worked examples. The most common temperature conversions explained.
Common reference temperatures
| Reference Point | °C | °F | K |
|---|---|---|---|
| Absolute zero | −273.15 | −459.67 | 0 |
| Water freezing point | 0 | 32 | 273.15 |
| Room temperature | 20–22 | 68–72 | 293–295 |
| Human body temperature | 37 | 98.6 | 310.15 |
| Water boiling point (sea level) | 100 | 212 | 373.15 |
| Oven — moderate | 175 | 350 | 448 |
Celsius to Fahrenheit — The Formula Explained
The conversion formula is: °F = (°C × 9/5) + 32
The 9/5 factor accounts for the different scale sizes — a Fahrenheit degree is smaller than a Celsius degree. The +32 offset accounts for the different zero points. Water freezes at 0°C but at 32°F, because Fahrenheit's zero was originally calibrated to a specific brine solution, not pure water.
For a quick mental approximation: double the Celsius value and add 30. This gives a result within 1–2°F for common temperatures. At 20°C: (20×2)+30 = 70°F. The real answer is 68°F — close enough for everyday use.
Fahrenheit to Celsius — The Reverse Formula
°C = (°F − 32) × 5/9
Subtract 32 first (to remove the offset), then multiply by 5/9 (to adjust the scale). Mental math shortcut: subtract 30, then divide by 2. At 98°F: (98−30)/2 = 34°C. Real answer is 36.7°C — reasonable approximation for general use.
Kelvin — The Scientific Temperature Scale
Kelvin is the SI unit of temperature used in science. It starts at absolute zero — the theoretical point where all molecular motion stops, at −273.15°C. There are no negative Kelvin temperatures; 0K is the absolute minimum.
The Kelvin scale uses the same degree size as Celsius — just shifted by 273.15. So room temperature (22°C) is 295.15K. The surface of the sun is approximately 5,778K. The cosmic microwave background radiation is 2.725K — the "temperature" of deep space.
Converting Celsius to Kelvin: add 273.15. Converting Kelvin to Celsius: subtract 273.15. Fahrenheit to Kelvin requires an intermediate Celsius step.
Frequently Asked Questions
At what temperature are Celsius and Fahrenheit the same?
At exactly −40 degrees. Both scales converge at −40: −40°C = −40°F. This is the one temperature where you don't need a formula — just remember −40 is the crossover point.
Why does the US still use Fahrenheit?
Historical inertia, primarily. The US adopted Fahrenheit in the colonial era and, unlike most countries, never transitioned to Celsius. The Metric Conversion Act of 1975 made the metric system the preferred US measurement system but didn't mandate it — resulting in continued dual use. Scientific and medical fields in the US use Celsius and Kelvin.
What is a "normal" body temperature in Celsius?
The classic value is 37°C (98.6°F), but this is an average. Normal body temperature ranges from approximately 36.1°C to 37.2°C (97°F to 99°F). Temperature also varies by measurement location — oral, axillary (armpit), and rectal temperatures differ by 0.3–0.6°C. A fever is generally considered to start at 38°C (100.4°F).
Why does water boil at different temperatures at altitude?
Atmospheric pressure decreases with altitude, which lowers the boiling point of water. At sea level, water boils at 100°C (212°F). At 2,000m altitude (like Denver, Colorado), water boils at approximately 93°C (199°F). At the summit of Everest (8,849m), water boils at about 70°C (158°F) — too cool to make a proper cup of tea.