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Fahrenheit vs. Celsius

The exact formula, and the quick approximation that gets you within 2°.

C = (F − 32) × 5/9. Quick estimate: subtract 30 and halve, or double and add 30.

The temperature conversion formula is simple but awkward to do in your head. A close approximation lets you convert weather or body temperatures to within a degree or two without a calculator.

Part of: Unit Conversion Tools

Quick answer

C = (F − 32) × 5/9. Quick estimate: subtract 30 and halve, or double and add 30.

What you are trying to do
The exact formula, and the quick approximation that gets you within 2°.
Best next step
Temperature Converter
Limit to remember
Treat this as a practical aid for the task, not a replacement for professional judgment.

Key points

  • ▸ Exact: C = (F − 32) × 5/9. F = C × 9/5 + 32.
  • ▸ Quick F → C: subtract 30, halve. (68 − 30) ÷ 2 = 19 (actual: 20).
  • ▸ Quick C → F: double, add 30. 20 × 2 + 30 = 70 (actual: 68).
  • ▸ Approximation error: usually 1–2°. Fine for weather, not for cooking or medical.
  • ▸ Reference points worth memorising: 0°C = 32°F (freezing), 100°C = 212°F (boiling), 37°C = 98.6°F (body).

Examples

  • Weather: 25°C
    Exact: 25 × 9/5 + 32 = 77°F. Quick: 25 × 2 + 30 = 80°F. 3° off — fine for "is it warm".
  • Oven: 350°F
    Exact: (350 − 32) × 5/9 = 177°C. Most UK ovens have a 180°C setting — close enough.
  • Fever: 101°F
    Exact: (101 − 32) × 5/9 = 38.3°C. Quick: (101 − 30) ÷ 2 = 35.5°C — big error at high F. Use exact for medical.

When to use which tool

Related

Frequently asked questions

› Why is the quick approximation inaccurate at high temperatures? Troubleshooting

The formula C = (F − 32) × 5/9 uses 5/9 ≈ 0.556, but the quick version uses 0.5. The error scales with the temperature difference from 32°F.

› Is there a cross-over point where F = C? Trust & accuracy

Yes: −40°. Minus 40 Fahrenheit equals minus 40 Celsius exactly. Useful trivia, not much else.

› How accurate are online calculators and converters? Trust & accuracy

Online calculators are only as accurate as the numbers, units, assumptions, and rounding choices you enter. Recheck the input values first, then compare the formula against your real situation. For legal, tax, medical, financial, or professional decisions, treat the result as a planning estimate, not advice.

› What inputs should I double-check first? Troubleshooting

Double-check units, dates, percentages, decimal placement, and whether the input is before-tax, after-tax, gross, net, original, or final. Most calculator mistakes come from feeding the right formula the wrong base. If the result feels off, rebuild it from a simple worked example.

› Why do two calculators sometimes give different answers? Comparison

Two calculators may round at different steps, use different defaults, or interpret the same label differently. Percent, time, finance, and unit tools are especially sensitive to basis and rounding rules. Compare the formula, not just the final number, before deciding which result to trust.