Calorie Deficit Calculator
Work out how many calories to eat to lose weight at a pace you can hold.
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Learn the details
Eating below your TDEE creates the deficit: 500 cal/day yields roughly 1 lb/week of loss, 1000 cal/day about 2 lbs/week. Moderate deficits (500-750 cal/day) best preserve muscle and adherence.
A deficit is simple to describe and much harder to hold, so here is the honest version: where the numbers came from, how far they can be trusted, and what I would actually watch once you have a target.
What a deficit really is
A calorie deficit just means you are taking in less energy than your body spends, so it makes up the difference from stored fuel. The familiar 3,500 calories per pound figure traces back to Max Wishnofsky in 1958, who estimated the energy sitting in a pound of body fat tissue. It is a decent starting rule and a poor long-term one, because your maintenance level is not fixed. As you get lighter you burn a little less moving through the day, and most people also move slightly less without noticing, so the same food intake that started as a 500 calorie deficit quietly becomes a smaller one.
A worked example
Say your maintenance lands at 2,400 calories a day. Eating 1,900 leaves a 500 calorie gap, which is 3,500 across a week, or roughly a pound. Run that for ten weeks and the early weeks will usually outpace the math while the later ones fall behind, because at a lighter bodyweight maintenance might now sit nearer 2,300. Same plate, smaller gap. That is not failure, it is the arithmetic catching up, and it is why I recalculate every few weeks instead of riding one number for months.
How big to go
A reasonable target for most people is losing about 0.5 to 1 percent of bodyweight per week, so roughly 0.9 to 1.8 pounds at 180 pounds. Larger deficits do move the scale faster, but the cost shows up in places the scale cannot see: more of the loss comes from lean tissue, training quality drops, hunger climbs, and adherence gets brittle. A deficit you can hold for twelve weeks beats an aggressive one you abandon in three.
Why the scale lies week to week
Daily weight is mostly water. Sodium, carbohydrate and the glycogen it drags along, digestion, sleep, training soreness, and the menstrual cycle can all swing the reading by several pounds while your actual fat mass barely moves. Weigh under similar conditions and compare weekly averages rather than single days, and read it alongside waist measurements, how your lifts feel, and photos every few weeks. Any one of those on its own is noisy. Together they tell a fairly clear story.
What to do with your number
Treat the target as a starting estimate and adjust from real trends. Keep protein high and keep lifting, since those two do most of the work of holding onto muscle while you lose fat. If the target looks off, sanity check the maintenance figure behind it with the TDEE calculator, and if you want a realistic sense of how long this takes, run the goal timeline. For the same maths worked all the way through on a specific target, see how many calories it takes to lose 20 pounds. If you have a health condition or take medication that affects appetite or weight, set the plan with your doctor rather than a calculator.
Sources: Mifflin et al. 1990
I would start you at 500 cal/day, pushing protein up and keeping training as the deficit grows. Plan a diet break every 8-12 weeks and reassess every 2-3 weeks against actual weight trends.