Why time does not double with weight
Heat penetrates from the surface, so cooking time scales roughly with the square of thickness, not with volume. In practice, roasting time follows weight to the power of about 0.6 — doubling a roast adds around 50% more time, not 100%.
The two exponents
- Roasting: weight^0.6 — a 2 kg bird at 90 minutes becomes about 137 minutes at 3 kg
- Baking: weight^0.4 for same-depth tins (spread over more area)
- Thicker batter in the same tin: add 15–25% and lower the temperature slightly
- Never rely on time alone — use a probe thermometer
Temperature beats time
For meat, internal temperature is the only reliable signal: poultry 74 °C, ground meat 71 °C, pork 63 °C with rest, beef as you like it. Time is a planning estimate, not a safety guarantee.
Checking without ruining it
Open the oven at 60–75% of the calculated time — earlier for a shallow bake, later for a deep roast. Every opening loses 10–15 °C, so be quick and close the door firmly.
Frequently Asked Questions
If I double the recipe, do I double the time?
No — roughly ×1.5 for a roast, and often the same time if you split between two tins.
How much longer for a deeper cake?
Add 15–25% and drop the oven by 10 °C so the centre sets before the edges burn.
Is time or temperature more reliable?
Temperature, always. Use time to plan and a probe to decide.
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