The one-percent-a-week rule is a reasonable heuristic with a hidden assumption: that the person following it has enough fat mass to supply the deficit.
Why the rate has to scale
100 kg at 30% body fat → 1 kg/week from ~30 kg of fat mass
80 kg at 10% body fat → 0.8 kg/week from ~8 kg of fat mass
Weekly draw as a share of available fat: 3.3% vs 10%
Same rule, triple the relative demand. The lean lifter's body meets the shortfall partly from somewhere else, and that somewhere else is what they were training to keep.
So: 0.5 to 1% per week, with leaner people at the bottom and people carrying more fat able to sit at or above the top.
Working the deficit out backwards
Pick the rate, then derive the deficit. Do not pick the deficit.
At roughly 7,700 kcal per kilogram of body mass, 0.5 kg a week needs about 550 kcal a day. 0.25 kg needs about 275.
Then the awkward step: that number is a difference from your maintenance, and most people’s maintenance figure is a formula estimate carrying a couple of hundred calories of error. A 550 deficit off a maintenance figure that is 200 high is really a 350 deficit, which is why cuts routinely run at half the expected rate and get blamed on metabolism.
What actually protects muscle
Resistance training, which is the largest factor by a distance. A deficit without training loses lean mass; a deficit with training mostly does not.
Adequate protein, in the 1.6 to 2.2 g/kg range, with the higher end more defensible when the deficit is aggressive or you are already lean.
A rate you can sustain. An aggressive cut abandoned in week three loses less fat than a moderate one completed.
Neither is a substitute for knowing your intake. A deficit you cannot measure is a deficit you cannot troubleshoot.
Questions we get asked
How fast can I lose fat without losing muscle?
Roughly 0.5 to 1% of body weight per week, and where you sit in that range depends on how much fat you are carrying. Someone with substantial fat mass can run the top of it or above with little lean loss; a lifter at 10% body fat should be at the bottom or below, because the body draws increasingly on lean tissue as the fat available to mobilise falls.
How big should my deficit be?
Work backwards from the rate rather than picking a number. At roughly 7,700 kcal per kilogram, a 0.5 kg weekly loss needs about a 550 kcal daily deficit. Then check that against your maintenance figure — and if you do not know your maintenance within a couple of hundred calories, establish that first, because the deficit is a difference between two numbers and one of them is a guess.
Will I lose muscle in a deficit?
Some lean mass loss is normal and much of it is not muscle — glycogen and its associated water account for a meaningful share of early loss. Resistance training and adequate protein are what protect actual muscle, and both matter more than the exact size of the deficit within a sensible range.
Caleb Ostrowski
Editor · Stronger by Math
Coaches lifters and writes the arithmetic down. Nine years of client logs, most of them unglamorous. Buys every app on this site at retail and cancels most of them. No affiliate links anywhere on this desk.
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