How Big Should a Calorie Deficit Be?
Max Wishnofsky published the rule in 1958: a pound of body fat stores about 3500 calories, so a deficit of 500 calories a day should cost you a pound a week. It is printed on gym walls and baked into most diet apps.
It is also wrong, and predictably so. The figure assumes the tissue you lose is pure fat and that your energy expenditure holds still while you shrink. Neither happens. Kevin Hall at the National Institutes of Health modelled this properly and found the rule overpredicts long term loss substantially: sustained deficits deliver roughly half the linear prediction over a year, because expenditure falls as you do.
The rule still works as a starting point for the first four to six weeks. Just stop expecting it to hold in month six.
Sensible deficit sizes
Set the deficit as a percentage of your maintenance rather than a flat number, because 500 calories means something very different to a 55 kg person than to a 110 kg one. Get your maintenance figure from the calorie and TDEE calculator before doing any of this arithmetic.
- 10 to 15% below maintenance. Slow, easy to hold, minimal effect on training. Roughly 0.25 to 0.5% of body weight per week. This is the right setting if you have under 10 kg to lose.
- 20 to 25% below maintenance. Noticeable but workable for most people with a decent amount to lose. Around 0.5 to 0.75% of body weight per week.
- Beyond 25%. Sustainable only for short blocks and only under supervision if you are already lean. Muscle loss and adherence failure both climb sharply here.
For the 2759 calorie TDEE from the worked example in the guide to BMR and TDEE, a 20% cut lands at about 2210 calories a day. The calculator’s own mild loss preset subtracts a flat 500, giving 2259, which is close to the same place. Both are reasonable. Neither is the 1780 BMR figure that people mistakenly aim at.
Why aggressive deficits backfire
Three things happen when the cut is too steep. You lose more lean tissue, because your body will strip muscle for fuel when the shortfall outpaces what fat can supply. You move less without noticing, which erodes the deficit you thought you had created. And adherence collapses, which is the one that actually ends most diets.
The muscle loss point is worth dwelling on, because lean mass is what keeps your maintenance calories high. Every kilogram of muscle you give up lowers the intake you can eat at forever after. A slower cut that preserves muscle leaves you in a better position at the same body weight, which shows up clearly if you are tracking with the Navy tape method for body fat rather than the scale alone.
Protein and resistance training are not optional extras
Two things reliably protect lean mass in a deficit. Eat 1.6 to 2.2 g of protein per kilogram of body weight, which for an 80 kg person is 128 to 176 g a day. And keep lifting, with roughly the same loads you used at maintenance. Cutting training volume because you are eating less is exactly backwards; the training stimulus is the signal that tells your body the muscle is worth keeping.
Reading the results without panicking
Weigh yourself at the same time each morning after using the bathroom and before eating, then look only at the seven day average. Individual days are meaningless. A single reading can move 2 kg on water alone, which is covered in detail in why scale weight jumps overnight.
Judge the plan on a fortnight of data. If the average has not moved after two weeks of consistent eating, cut another 150 to 200 calories or add deliberate walking. If it is dropping faster than 1% of body weight per week, eat more; you are losing tissue you will want back.
When to stop cutting
Deficits are a phase, not a lifestyle. Run one for eight to sixteen weeks, then spend several weeks eating at your new maintenance before deciding whether to go again. The break is not a failure of discipline. It restores training quality, gives hormones and appetite signalling a chance to normalise, and makes the next block work better than grinding through would have.