How is calories in pound of fat determined?

We’ve all read that you need to use 3500 calories to lose a pound of fat. My questions is how the hell is that determined?

My initial thought was that since there are approximately 9 calories (on a side note, does anyone know exactly how many calories are in a gram of fat. I’ve typically read that 9 is an approximation) in a gram of fat to multiply the number of grams in a pound (453.59237) by 9 but that gives me 4082.33133. So that doesn’t work out.

Anyone know the answer?

Is this strictly a physics question?

It could be but I’m never been to great at physics. I’ve just read that there are 3500 Calories in a pound over and over again but always was curious on how that number was derived.

How about this as an explanation:

Are they talking about 1 troy pound as opposed to 1 Avoirdupois pound? This would mean that 1 gram of fat=9.02kCal and 1 Pound (Troy) would equal 373.24g so that would give 3366.64Kcal, which is alot closer to the 3500Kcal you are suggesting.

(BTW got the Kcal value for fat by searching on the USDA National Nutrient Database for Standard Reference for 100g of lard)