← Kitchen Clock · Lesson 2 of 3
Why a big roast takes less than twice as long
Heat gets into a roast through its surface, and the surface grows more slowly than the weight. Double the weight of a roast and its surface grows by only about 59%, so the cooking time grows roughly with the weight to the power of two thirds: about 1.59 times as long, not twice.
That is why cookbooks give a rule like “90 minutes plus 11 minutes a pound” rather than “so many minutes a pound”: a straight line with a head start fits the curve well over the sizes people actually cook. Children on SaffronLab compare the two rules and see where they agree.
A roast twice as heavy as the one you usually cook needs roughly how much more oven time?
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