“Double it and add 30” is close enough to fool you — until you need the real Celsius to Fahrenheit number

There's a trick everyone half-remembers for Celsius to Fahrenheit: double it and add 30.

This is the second post in a short series about conversion factors people half-remember. The first one covered why 1 mile is not 1.6 km — same pattern: a rounded rule that is right at one point and quietly wrong everywhere else.

It's not wrong. It's not right either. It's a straight line that happens to cross the real one at exactly 10°C, and then drifts away from it in both directions.

The exact formula

°F = °C × 9/5 + 32 °C = (°F − 32) × 5/9 

That's it. The Fahrenheit scale puts freezing at 32 and boiling at 212 — 180 degrees apart — while Celsius uses 100. The ratio 180/100 is 9/5, and the 32 is the offset. Everything else is arithmetic.

How far the shortcut drifts

The shortcut is 2C + 30. Subtract the exact value and you get a clean expression for the error:

error = (2C + 30) − (1.8C + 32) = 0.2C − 2 

So the error is zero at 10°C, and grows by 2°F for every 10°C you move away from it:

For weather, that's fine. "It's 30 outside" doesn't need to be 86 rather than 90. For anything you'll act on precisely, it stops being fine quickly.

Where it actually bites: body temperature

Normal body temperature is about 37°C = 98.6°F. A fever of 38°C is 100.4°F.

Run the reverse shortcut — subtract 30, divide by 2 — on 100.4 and you get 35.2°C. That's not a fever. That's hypothermia territory, from a reading that means the opposite. This is the one place where a "close enough" conversion is worse than no conversion at all.

The exact reverse formula gives 38.0°C, as it should.

The mental model worth keeping

If you want a shortcut that behaves predictably, use this one instead: double it, subtract 10%, add 32.

°F ≈ C × 2 × 0.9 + 32 = 1.8C + 32 

That's algebraically identical to the exact formula, so the error is zero everywhere. It's one more step than "double and add 30", and it removes the entire error table above.

Two other anchors worth memorising:

• −40 is the same in both scales. It's the only point where Celsius and Fahrenheit agree.
• Every 5°C is 9°F. So 0 → 32, 5 → 41, 10 → 50, 15 → 59, 20 → 68. If you know one, you can walk up and down the scale in steps of 9.

When I need the number rather than the trick, I use a converter with a pre-computed table: celsius to fahrenheit and the reverse direction at fahrenheit to celsius. Both run in the browser, no upload, no account.

The shortcut isn't a lie. It's a rounded line that crosses the truth once, at 10°C — which is exactly why it feels trustworthy.