Celsius, Fahrenheit & Kelvin: Formulas, Reference Points & Memory Tricks

Three scales cover 99% of daily life: Celsius, Fahrenheit and Kelvin. Two more (Rankine, Réaumur) show up in engineering and old cookbooks. The formulas are short and there are a few tricks that make mental conversion fast enough.

The Five Scales

• Celsius (°C) — water freezes at 0, boils at 100. Worldwide standard.
• Fahrenheit (°F) — water freezes at 32, boils at 212. US daily use.
• Kelvin (K) — same step size as Celsius but starts at absolute zero (−273.15 °C). Science standard. No degree symbol.
• Rankine (°R) — same as Kelvin but with Fahrenheit-sized steps. US engineering.
• Réaumur (°Ré) — water freezes at 0, boils at 80. Historic; still found in French and Swiss cheese-making.

The Formulas

• °F = (°C × 9/5) + 32
• °C = (°F − 32) × 5/9
• K = °C + 273.15
• °R = (°C + 273.15) × 9/5 = °F + 459.67
• °Ré = °C × 4/5

Mental Math Shortcuts

• °C → °F (rough): double, add 30. 20 °C → ~70 °F (exact: 68).
• °F → °C (rough): subtract 30, halve. 70 °F → ~20 °C.
• The −40 trick: −40 °C = −40 °F exactly. Useful sanity check.

Reference Points Worth Memorising

• Absolute zero: 0 K = −273.15 °C = −459.67 °F
• Water freezes: 0 °C = 32 °F
• Comfortable room: 20 °C = 68 °F
• Body temperature: 37 °C = 98.6 °F
• Hot summer day: 35 °C = 95 °F
• Water boils: 100 °C = 212 °F
• Oven (moderate): 180 °C = 356 °F = gas mark 4

Frequently Asked Questions