Why thickness, not weight, sets sous vide time

Heat moving into a solid piece of food by conduction does not travel at a constant speed the way a car covers highway miles. The math behind one dimensional conduction says the time to heat the center of a slab scales with the square of its thickness, so doubling the thickness does not double the time, it roughly quadruples it, and tripling the thickness multiplies the time by roughly nine. This is why a recipe that says add a fixed number of extra minutes for every extra half inch of thickness is quietly wrong at anything beyond a small range: the real relationship curves upward much faster than a straight line does. A come-up time table built on this model, like the sous vide time calculator, reflects that curve rather than a flat rate per inch.

Weight is a measure of volume and density together, and neither of those tells you the distance from the surface to the center of the thickest point, which is the actual variable a conduction model cares about. A wide, thin flank steak and a narrow, thick filet mignon can weigh the same amount on a scale while heating at completely different rates, because the flank steak is thin enough that heat reaches its center quickly from two nearby faces, while the filet mignon's center sits much farther from any surface. Two chicken breasts of the same weight, one pounded flat and one left at its natural thickness, illustrate the same point: the pounded one comes up to temperature far faster, despite having identical mass. If you take away one thing from this page, it should be that a recipe listing a pound of steak without a thickness has not actually told you the number that matters.

Yes. A flat piece of food, a steak or a chop, is modeled as a slab, heated from two opposite faces. A rounder piece, a tenderloin or a sausage, is modeled as a cylinder, heated from every direction around its circumference at once, which means heat reaches the center faster than a slab of the same thickness because there is simply more surface delivering heat inward per unit of distance to the middle. This model applies a 0.7 factor to a cylinder's time relative to an equivalent slab, and an even lower 0.55 factor to a sphere, such as an egg in its shell, which is heated from every direction at once and has the shortest relative come-up time of the three shapes. A 1 inch thick filet mignon, modeled as a cylinder, comes up meaningfully faster than a 1 inch thick strip steak, modeled as a slab, purely because of that shape difference.

Because the two possible errors are not equally costly. If the calculated come-up time overshoots reality by twenty minutes, the food sits at bath temperature slightly longer than strictly necessary, which in most sous vide cooking costs a small amount of texture at worst. If the calculated time undershoots reality, you may pull food out, sear it and serve it while the actual center is still below the temperature you believed it had reached, which is a genuine safety question rather than a texture one. Given that asymmetry, a model calibrated to round long, rather than to hit the exact average case, is the more responsible choice for a number people use to decide when food is done.

Measure the thickest point of the food, not an average and not the edges, since the thickest point is what the model treats as the limiting distance heat has to travel. Use the sous vide time calculator or a specific cut page like the pork chop sous vide time page to get the calculated figure for that thickness rather than guessing from a rule of thumb tied to weight. If you want direct confirmation rather than relying on the calculation alone, a wireless probe thermometer such as the Typhur Sync 2 probe left in the food through the cook shows the actual core temperature climbing in real time, which is the most direct way to know the come-up period has finished.