The formula
hours ≈ weight × hours per kilo for the method
Where it comes from
Defrosting is a heat conduction problem, so what governs it is not only the weight but the thickness: the cold has to come out from the centre and the distance it travels grows with the square. A two-kilo joint and eight two-hundred-and-fifty-gram steaks weigh the same and take very different times. The hours-per-kilo rules work well on normal pieces; on a really thick one the time runs longer, and this says so rather than inventing a coefficient for it.
How to work it out by hand
- Weigh the piece while it is still frozen
- Multiply by the hours per kilo for the method: 6 in the fridge, half an hour in cold water
- Add a margin if the piece is thick: the cold has to come from the centre
- Work backwards from the time you plan to cook
What is worth knowing
Cold water is twelve times faster than the fridge because water conducts heat far better than air, but it has to be changed every half hour or it stops being cold, and the piece has to be in a well-sealed bag. One detail few people know: something defrosted in the fridge can be refrozen without cooking, because it never left refrigeration temperature; something defrosted in water or a microwave cannot. And a microwave thaws the outside before the inside, so whatever comes out of it has to be cooked straight away.