The Four Temperature Scales
Temperature can be expressed on four different scales. Understanding which scale to use — and how to convert between them — matters for daily life, cooking, travel, medicine, and scientific work.
Celsius (°C)
Celsius is the standard temperature scale in science and in everyday life for 95% of the world's population. It's defined with reference to water: 0°C is the freezing point of water at standard atmospheric pressure (101.325 kPa), and 100°C is the boiling point. The scale was originally designed by Anders Celsius in 1742, though he actually defined it in reverse (100° as freezing, 0° as boiling) — it was reversed by Carl Linnaeus shortly after.
Fahrenheit (°F)
Fahrenheit is used primarily in the United States, its territories, and a handful of other countries for everyday weather, cooking, and body temperature. On this scale, water freezes at 32°F and boils at 212°F. Body temperature is 98.6°F (37°C). The scale was created by Daniel Fahrenheit in 1724; he set 0°F as the temperature of a brine solution and 96°F as human body temperature (since adjusted to 98.6°F).
Kelvin (K)
Kelvin is the SI base unit of thermodynamic temperature and the standard scale in science. Unlike Celsius and Fahrenheit, Kelvin has no negative values — it starts at absolute zero (0 K = −273.15°C = −459.67°F), the coldest theoretically possible temperature where all molecular motion ceases. Kelvin intervals are identical to Celsius intervals; K = °C + 273.15. Scientists use Kelvin for thermodynamics, astrophysics, and chemistry.
Rankine (°R)
Rankine is an absolute temperature scale like Kelvin, but based on Fahrenheit degrees rather than Celsius. Absolute zero is 0°R = 0 K = −459.67°F. Rankine intervals are the same size as Fahrenheit intervals. It's used in some American engineering applications (e.g., thermodynamics in aerospace), but rarely elsewhere.
Exact Conversion Formulas
| From → To | Formula |
|---|---|
| Celsius → Fahrenheit | °F = (°C × 9/5) + 32 |
| Fahrenheit → Celsius | °C = (°F − 32) × 5/9 |
| Celsius → Kelvin | K = °C + 273.15 |
| Kelvin → Celsius | °C = K − 273.15 |
| Fahrenheit → Kelvin | K = (°F − 32) × 5/9 + 273.15 |
| Kelvin → Fahrenheit | °F = (K − 273.15) × 9/5 + 32 |
| Celsius → Rankine | °R = (°C + 273.15) × 9/5 |
| Rankine → Kelvin | K = °R × 5/9 |
Key Temperature Reference Points
| Reference | Celsius | Fahrenheit | Kelvin |
|---|---|---|---|
| Absolute zero | −273.15°C | −459.67°F | 0 K |
| Water freezes | 0°C | 32°F | 273.15 K |
| Room temperature | 20–22°C | 68–72°F | 293–295 K |
| Body temperature | 37°C | 98.6°F | 310.15 K |
| Water boils | 100°C | 212°F | 373.15 K |
| Oven (moderate) | 180°C | 356°F | 453.15 K |
Common Use Cases
Weather & Travel
Traveling between the US and the rest of the world means constant temperature scale switching. A quick mental trick: to estimate Celsius from Fahrenheit, subtract 30 then divide by 2 (e.g., 86°F → 56 ÷ 2 = 28°C, exact is 30°C). For precision, use the converter. Useful reference: 0°C = 32°F (cold), 10°C = 50°F (cool), 20°C = 68°F (mild), 30°C = 86°F (warm), 40°C = 104°F (very hot).
Cooking & Baking
Oven temperatures are a constant source of confusion between metric and imperial cookbooks. Common conversions: 160°C = 320°F (low), 180°C = 356°F (moderate), 200°C = 392°F (hot), 220°C = 428°F (very hot). Fan-assisted ovens typically run 10–20°C hotter than conventional, so reduce by that amount when adapting recipes.
Medical
Normal human body temperature is 37.0°C (98.6°F). A temperature above 38°C (100.4°F) is generally considered a fever. Hypothermia begins below 35°C (95°F). Most of the world uses Celsius for medical measurements; the US uses Fahrenheit on patient-facing devices but Celsius in scientific and pharmaceutical contexts.
Frequently Asked Questions
What is the formula to convert Celsius to Fahrenheit?
°F = (°C × 9/5) + 32. Example: 25°C × 1.8 + 32 = 45 + 32 = 77°F. To reverse: °C = (°F − 32) × 5/9. Example: (77°F − 32) × 5/9 = 45 × 0.5556 = 25°C.
At what temperature are Celsius and Fahrenheit the same?
Celsius and Fahrenheit are equal at −40°. That is, −40°C = −40°F. This is the only temperature where the two scales intersect. You can verify: (−40 × 9/5) + 32 = −72 + 32 = −40°F. ✓
What is absolute zero in Celsius?
Absolute zero is 0 Kelvin = −273.15°C = −459.67°F. At this temperature, all classical molecular motion would cease. It has never been reached in practice; the coldest recorded lab temperatures are about 100 picokelvin (1 × 10⁻¹⁰ K) above absolute zero.
Related Articles
Temperature formulas are exact per SI and NIST standards. Accuracy note.