Most of the time in a kitchen, you don't actually need decimal-level precision on an oven temperature conversion — ovens themselves run 10-15 degrees off their dial anyway. So here's the fast mental version I actually use, plus the exact formula for the rare case it matters.
The Fast (Approximate) Method
Subtract 30 from the Fahrenheit number, then divide by 2. It's not exact, but it's close enough for oven temperatures and can be done in your head without a calculator.
Example: 350°F − 30 = 320, ÷ 2 = 160°C. The real answer is 176.7°C — off by about 17 degrees, which sounds like a lot, but ovens are rarely accurate to within 17 degrees of their dial anyway, so in practice this shortcut gets you in the right neighborhood fast.
The Exact Formula
°C = (°F − 32) × 5/9. For 350°F: (350 − 32) = 318, × 5/9 = 176.7°C. This is the number ovens and recipes actually use when they list "precise" conversions, typically rounded to the nearest 10.
Why the Fast Method Works Roughly
The real formula subtracts 32 and multiplies by about 0.56 (5/9). The fast method subtracts 30 (close to 32) and divides by 2 (close to multiplying by 0.56, though not identical — dividing by 2 is multiplying by 0.5). The two roundings partially cancel out across the oven temperature range most recipes use (300-450°F), which is why it holds up reasonably well specifically in that range, even though it's not mathematically exact.
The Standard Lookup Table (For When You Want It Exact)
| °F | °C (exact, rounded) |
|---|---|
| 300°F | 149°C |
| 325°F | 163°C |
| 350°F | 177°C |
| 375°F | 191°C |
| 400°F | 204°C |
| 425°F | 218°C |
Most recipes round these to the nearest 10 (180°C instead of 177°C, for instance) since oven dials rarely offer that level of precision anyway.
When the Fast Method Isn't Good Enough
For candy-making, sugar work, or anything with a genuinely narrow safe temperature window, use the exact formula or a proper thermometer rather than the mental shortcut — the 15-20 degree margin of error in the fast method matters a lot more there than it does for a general oven bake.
A Worked Example
Recipe calls for 425°F, oven only displays Celsius. Fast method: 425 − 30 = 395, ÷ 2 ≈ 198°C. Exact answer: 218°C. That's a bigger gap than at 350°F, since the fast method's accuracy declines a bit at the higher end of the range — worth double-checking with the exact formula or the tool for higher temperatures specifically.
FAQ
Is the fast method accurate enough for everyday baking?
For most home baking, yes — the gap is smaller than the natural variation
between different home ovens anyway.
Why not just always use the exact formula?
No reason not to if you have a calculator handy — the fast method exists
purely for quick mental math when you don't.
Does gas mark fit into either of these formulas?
No — gas mark isn't derived from a formula at all; it's a fixed lookup
table. See our full
oven temperature guide for the complete gas mark reference.