Fahrenheit to Celsius Formula Explained

4 min read•June 1, 2025•Reviewed: June 1, 2026

✓Reviewed by Apex Conversion Editorial Team · Last reviewed June 1, 2026

The formula for converting Fahrenheit to Celsius is °C = (°F − 32) × 5/9. The subtraction of 32 aligns the zero points of the two scales; the multiplication by 5/9 (= ÷ 1.8) adjusts for the different sizes of each degree. Both steps are always required.

A quick mental estimate — subtract 32 and divide by 2 — gives results within about 10% of the exact value. For example: 68°F → (68 − 32) ÷ 2 = 18°C (exact: 20°C). More accurate: subtract 32 and multiply by 0.55.

The Formula

°C = (°F − 32) × 5/9
°C = (°F − 32) ÷ 1.8  (equivalent)

Worked examples:
   32°F: (32 − 32) × 5/9 =   0°C  (water freezes)
   68°F: (68 − 32) × 5/9 =  20°C  (comfortable room)
   98.6°F: (98.6 − 32) × 5/9 = 37°C  (body temp)
  212°F: (212 − 32) × 5/9 = 100°C  (water boils)
  350°F: (350 − 32) × 5/9 = 176.7°C  (common oven temp)
  -40°F: (-40 − 32) × 5/9 = -40°C  (scales meet)

Quick estimate (subtract 32, divide by 2):
  68°F: (68−32) ÷ 2 = 18°C  (exact: 20°C)
  100°F: (100−32) ÷ 2 = 34°C  (exact: 37.8°C)

Why Subtract 32 First?

The two scales have different zero points: 0°F ≠ 0°C. Water freezes at 32°F but 0°C. Before comparing the scale sizes, you must bring both to a common reference point. Subtracting 32 from the Fahrenheit value shifts it to start at the same reference as Celsius (the freezing point of water).

Once aligned, the scale-size difference is 5/9 (Fahrenheit degrees are smaller: 180°F covers the same range as 100°C, so 1°F = 100/180°C = 5/9°C). Multiplying by 5/9 converts the Fahrenheit-degree count to a Celsius-degree count.

Quick Tips

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Key anchors: 32°F = 0°C, 68°F = 20°C, 98.6°F = 37°C, 212°F = 100°C.
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Quick estimate: (°F − 32) ÷ 2 — within 10% for common temperatures.
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Better estimate: (°F − 32) × 0.55 — closer to exact (0.5556 is the true factor).
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Common oven temps: 350°F = 177°C, 375°F = 191°C, 400°F = 204°C, 425°F = 218°C, 450°F = 232°C.
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Body temperature fever threshold: 100.4°F = 38°C.

Frequently Asked Questions

What is 72°F in Celsius?

(72 − 32) × 5/9 = 40 × 5/9 = 22.2°C — a warm, comfortable room temperature.

What is 350°F in Celsius?

(350 − 32) × 5/9 = 318 × 5/9 = 176.7°C — the most common baking temperature in the US.

What is 100°F in Celsius?

(100 − 32) × 5/9 = 68 × 5/9 = 37.8°C — a high fever or a very hot summer day.

Can I use 'subtract 30 and halve' instead?

Subtract 30 then halve is a slightly different shortcut: 72°F → 42 ÷ 2 = 21°C (exact: 22.2°C). Subtract 32 then divide by 2 is slightly more accurate and only slightly harder.

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