Three temperature scales are in common use today. Celsius (°C) is the standard in science and most of the world. Fahrenheit (°F) is used primarily in the United States and a handful of other countries for everyday weather and cooking. Kelvin (K) is the SI base unit used in physics and chemistry — it starts at absolute zero, the coldest possible temperature.
Example: 20°C → (20 × 9/5) + 32 = 36 + 32 = 68°F
Example: 98.6°F → (98.6 − 32) × 5/9 = 66.6 × 0.5556 = 37°C (normal body temperature)
Example: 0°C (freezing point of water) = 273.15 K
| Description | Celsius (°C) | Fahrenheit (°F) | Kelvin (K) |
|---|---|---|---|
| Absolute zero | −273.15 | −459.67 | 0 |
| Freezing point of water | 0 | 32 | 273.15 |
| Cold winter day (UK) | −5 | 23 | 268.15 |
| Comfortable room temp | 21 | 70 | 294.15 |
| Human body temperature | 37 | 98.6 | 310.15 |
| Hot summer day | 35 | 95 | 308.15 |
| Boiling point of water | 100 | 212 | 373.15 |
| Oven (moderate) | 180 | 356 | 453.15 |
For quick estimates when you don't have a calculator:
These shortcuts are within 2–4°F accuracy for typical weather and body temperatures — not precise enough for cooking or science, but fine for "is it warm enough for a T-shirt?"
Gabriel Fahrenheit (1686–1736) was a German physicist who developed his scale in 1724. He set 0°F at the temperature of an ice/salt/water mixture (the coldest he could reliably create in a lab) and calibrated 96°F to body temperature. Later adjustments moved body temperature to 98.6°F.
Anders Celsius (1701–1744) proposed the centigrade scale in 1742 — 0° for the boiling point of water and 100° for freezing. Confusingly, this was inverted from today's convention; it was reversed after his death to the form we use today.
Lord Kelvin (William Thomson, 1824–1907) established the absolute scale based on thermodynamic principles — 0 K is absolute zero, the theoretical minimum temperature at which all molecular motion stops.
| Oven Setting | °C (Fan) | °C (Conventional) | °F | Gas Mark |
|---|---|---|---|---|
| Very low | 120 | 140 | 275 | 1 |
| Low | 140 | 160 | 325 | 3 |
| Moderate | 160 | 180 | 350 | 4 |
| Moderately hot | 170 | 190 | 375 | 5 |
| Hot | 190 | 210 | 425 | 7 |
| Very hot | 210 | 230 | 450 | 8 |
100°C is exactly 212°F — the boiling point of water at sea level (1 atm pressure). At altitude, where air pressure is lower, water boils at a lower temperature — at the summit of Everest (~8,849m), water boils at approximately 70°C (158°F), which is why cooking times must be extended significantly at high altitude.
Double the Celsius value and add 30. For 20°C: (20×2)+30 = 70°F (exact answer is 68°F). This estimate is accurate to within 3–4°F for temperatures between -10°C and 40°C — the range you encounter in everyday life. Not precise enough for medical or cooking use, but excellent for getting a feel for weather forecasts.
The US adopted Fahrenheit as its standard before the Metric Conversion Act of 1975 established a national policy of voluntary metrication. Unlike most countries, the US never mandated metric adoption. The Fahrenheit scale arguably has some practical appeal for human-scale weather — 0°F is bitterly cold and 100°F is dangerously hot, giving intuitive meaning to the range — but scientifically and globally, Celsius is far more practical.
−40°. At exactly −40°C = −40°F the two scales intersect. You can verify with the formula: (−40 × 9/5) + 32 = −72 + 32 = −40. This is also approximately the temperature of extremely cold winter days in interior Alaska, northern Canada, and Siberia.