How many calories are in a kilogram — or a pound — of fat?
Roughly 7,700 kcal per kilogram, or 3,500 kcal per pound. That is the figure behind almost every calorie-deficit plan you will read, and it is the only constant Caldown needs to turn “I want to lose 10 kg (22 lb)” into a number that counts down.
It is worth understanding where it comes from, because it is an estimate — a useful one, but an estimate.
Where 7,700 comes from
Pure fat holds about 9,400 kcal per kilogram (4,260 per pound). Body fat is not pure fat, though. Adipose tissue is roughly 87% lipid, with the remainder water, protein and cellular structure, none of which carries that energy density.
Multiply it out:
9,400 kcal/kg × 0.87 ≈ 8,200 kcal/kgThe commonly quoted 7,700 figure (3,500 kcal per pound, in the units it was first published in) is a slightly more conservative version of that arithmetic, dating to a 1958 paper by Max Wishnofsky. Different textbooks land anywhere between 7,000 and 8,000. The number is a convention, not a physical constant.
Why it is only an approximation
The 7,700 rule assumes every calorie of your deficit comes out of fat. Over a real diet, several things complicate that:
- Water and glycogen move first. Each gram of stored glycogen holds roughly three grams of water with it. The dramatic drop in week one is mostly this, and it is why the scale seems to stall in week three when it stops.
- Some of the loss is lean tissue. How much depends on your protein intake, whether you lift, and how aggressive the deficit is. Lean tissue carries far less energy per kilogram than fat, so losing it “costs” fewer calories.
- Your burn falls as you shrink. A lighter body costs less to run, and metabolic adaptation reduces it a little further. A deficit calculated on day one is a smaller deficit by month three.
The practical consequence: the rule tends to be reasonably good over the first several kilograms and gets progressively more optimistic the longer and larger the cut.
Using it anyway
None of that makes the number useless — it makes it a starting estimate rather than a guarantee. Its real value is converting a vague goal into something countable:
| Metric | Imperial | Total deficit required |
|---|---|---|
| 1 kg | 2.2 lb | 7,700 kcal |
| 5 kg | 11 lb | 38,500 kcal |
| 10 kg | 22 lb | 77,000 kcal |
| 20 kg | 44 lb | 154,000 kcal |
Seventy-seven thousand is a large, slightly intimidating number, and that is rather the point. It reframes the task honestly: not “eat less this week” but a fixed quantity of energy to work through, where every walk and every night’s sleep takes a piece out of it.
Why 3,500 and 7,700 do not quite agree
If you divide 7,700 by 2.205 you get 3,492, not 3,500. The two conventions were rounded separately — one in pounds, one in kilograms — and never reconciled.
The gap is about 0.2%, which is nothing against a figure that is itself an approximation of an approximation. Use whichever matches the units you think in and do not convert between them.
What this looks like day to day
A 500 kcal daily deficit against 77,000 kcal is about 154 days. A 750 kcal deficit is about 103. Neither is fast, and anything promising faster is either counting water or asking for a deficit that is hard to hold.
How long it takes to lose 10 kg (22 lb) works through those numbers properly, including why real life runs slower than the arithmetic. For the whole picture, see the calorie deficit guide.
This is general information, not medical advice. If you have a health condition or are planning a substantial change to how you eat, talk to a doctor or a registered dietitian.