Cooking science

Thickness and cooking time

Cooking time does not scale with weight and it does not scale linearly with thickness. It scales roughly with the square of thickness, which is why a joint twice as thick takes about four times as long — and why almost everybody underestimates a thick cut.

Why it is not linear

Heat has to travel from the surface to the centre. Doubling the thickness doubles the distance — but it also halves the temperature gradient driving the heat inwards, because the same temperature difference is now spread over twice the distance.

Two effects multiplying gives something close to a squared relationship. Double the thickness, roughly quadruple the time. Triple it, and the time goes up by something like a factor of nine.

This is why "twenty minutes per kilo" is such a poor rule. Weight tells you almost nothing; the shortest distance from surface to centre tells you nearly everything.

What this predicts

All of these follow from the same relationship and all of them surprise people.

Two small joints beat one big one

Halving the thickness cuts the time by around three quarters, and the meat is more evenly cooked.

A spatchcocked chicken is much faster

Flattening reduces the maximum thickness dramatically. Often half the roasting time.

A thin steak has no margin

The centre reaches the surface temperature almost immediately, so there is no window between raw and overdone.

A rolled joint is slower than it looks

Rolling increases diameter. A rolled shoulder takes far longer than the same meat laid flat.

A wide shallow tray is not slower

More food at the same depth takes roughly the same time.

A deep cake needs a lower oven

The extra distance means a longer cook, and a longer cook at high heat burns the outside.

Using it to estimate

• Measure the shortest distance to the centre, not the length or the weight. For a joint, that is usually half the smallest cross-section.
• Compare with something you have cooked before of a known thickness, and scale by the square of the ratio.
• Halve the thickness when you are short of time — butterfly, spatchcock, pound flat or cut into two thinner pieces.
• Expect a thick cut to need a lower temperature, or a two-stage cook.
• Buy steaks thick and chops thick. The margin between perfect and overdone widens with thickness.
• A probe removes the guesswork for anything expensive.

Reducing thickness when you are short of time

Every one of these works by cutting the distance heat has to travel.

1. Spatchcock a bird — cut out the backbone and press it flat. Often halves the roasting time.
2. Butterfly a thick piece of meat, opening it out like a book.
3. Pound an escalope between two sheets of paper until even.
4. Cut a large joint into two smaller ones and roast them side by side.
5. Halve or quarter large vegetables rather than roasting them whole.
6. Slice a loaf-shaped bake into a wider, shallower tin.
7. In each case reduce the time far more than seems reasonable, and check early.

Common questions