Calorie deficit
calculator

Enter your maintenance calories and a daily deficit. You get the intake target, the weekly and monthly loss, the deficit as a share of maintenance, and the point where the 3,500-kcal rule stops holding.

How the math works

A calorie deficit is maintenance calories minus what you eat. Your maintenance figure is your TDEE — the calories that hold your weight steady. Subtract a daily deficit from it and the difference is your intake target.

The loss rate comes from the energy content of body-weight change: about 3,500 kcal per pound (Wishnofsky 1958). A deficit of 500 kcal/day is 3,500 kcal a week — roughly a pound.

The percentage line is the one most calculators skip. A 500 kcal deficit is a mild cut for someone at 2,770 kcal maintenance and a steep one for someone at 2,108 kcal — same number, different diet. Reading the deficit as a share of maintenance tells you which one you're on.

Worked example

A 30-year-old, 5'10" (178 cm), 180 lb (82 kg) male at moderate activity, with a maintenance TDEE of 2,770 kcal, running a standard 500 kcal/day deficit.

• Intake target: 2,770 − 500 = 2,270 kcal/day
• Deficit of maintenance: 500 / 2,770 = 18.1%
• Weekly loss: 500 × 7 / 3,500 = 1.0 lb (0.45 kg)
• Four-week loss: 4.0 lb (1.8 kg)
• To lose 20 lb (9.1 kg): a linear 20 weeks — but past week 8 expect the loss to slow (see below)

When this calculator is wrong

The 3,500-kcal-per-pound rule is a useful approximation that breaks down past about 8 weeks. It works well for short cuts — for the first 4–8 weeks a 500 kcal/day deficit lands within 10–15% of measured loss. Beyond that window it systematically over-predicts, because the body adapts: NEAT drops, BMR falls, and water and glycogen shift. Hall et al. (2011) put the realistic long-term ratio closer to 7,000 kcal per pound — roughly half the rate. So a "20-week" linear timeline is really the optimistic floor; a long cut takes longer. The exception is that short cut: inside the first month or two, the linear number is close enough to plan with.

• Your maintenance figure is an estimate too. TDEE from a formula carries its own 10–15% error, and this calculator inherits it. If the deficit isn't producing the predicted loss after two weeks, the maintenance number is the first thing to re-check.
• A bigger deficit isn't a faster diet past a point. At 750 kcal/day and above, lean-mass loss and adherence problems rise (stats.md flags this tier as short-term only). The scale moves faster; the body-composition result can be worse.
• Protein isn't in the intake number, and it should go up. In a deficit the protein target rises to 2.3–3.1 g/kg of lean body mass (Helms et al. 2014) to hold onto muscle — higher than at maintenance. Most deficit calculators say nothing about it.

What to do with the result

Treat the intake target as a two-week experiment, not a verdict. Track intake and weight for two weeks and take the average — the first week is noisy from water, glycogen, and gut contents. If the average weight is falling at roughly the predicted rate, the number is right. If it isn't moving, your real maintenance is lower than the estimate; drop another 100–200 kcal and re-check.

Set protein first, then fill the rest with carbs and fat. On a cut that means about 2.3–3.1 g/kg of lean body mass (Helms et al. 2014); the protein calculator works the grams out, and the macro calculator splits the remaining calories. When the loss stalls for two to three weeks despite hitting the target, that's the adaptation showing — take a short break at maintenance or trim another small amount rather than cutting deeper and deeper.

Common questions

How big should my calorie deficit be?

For most people a 250–500 kcal/day deficit — roughly 0.5–1 lb (0.23–0.45 kg) per week — is the sustainable range. Read it as a share of maintenance: aiming for something in the region of 15–25% below maintenance keeps the loss steady without the lean-mass and adherence costs that show up at larger deficits. A 750 kcal/day deficit is aggressive and best kept to short blocks.

How long will it take to lose the weight?

At a 500 kcal/day deficit the linear math is 1 lb (0.45 kg) per week, so 20 lb (9.1 kg) reads as 20 weeks. That's the optimistic version. Past 8 weeks the body adapts and loss slows — Hall et al. (2011) put the long-run cost closer to 7,000 kcal per pound — so a long cut runs past its linear estimate. Use the timeline as a floor, not a promise.

Why isn't a 1,000 kcal deficit twice as good as 500?

On paper it doubles the weekly loss. In practice, larger deficits raise the share of weight lost as muscle rather than fat, get harder to stick to, and trigger the metabolic adaptation sooner. The scale can move faster while the body-composition outcome gets worse. Faster isn't automatically better here.

Do I eat back exercise calories?

Not if your maintenance figure already includes exercise via a TDEE activity multiplier — the training is baked in, and eating it back again double-counts. Only add exercise calories on top if you set maintenance from a sedentary baseline and log workouts separately, which is the less reliable approach.

Should I eat below my BMR?

The intake target here can dip below your resting metabolic rate at large deficits, and this calculator will flag it when it does. That's not automatically dangerous for a short block, but it's a sign the deficit is aggressive — a smaller deficit over a longer timeline usually preserves more muscle and is easier to hold. Persistent very low intakes are a reason to involve a dietitian.

Why does the same deficit feel different for two people?

Because the absolute number hides the ratio. A 500 kcal deficit is 18% of a 2,770 kcal maintenance but about 24% of a 2,108 kcal one. The smaller person is on a meaningfully steeper cut at the identical calorie figure — which is exactly why the percentage is worth reading, and why this page shows it.