What is a calorie deficit?
A calorie deficit means eating fewer calories than your body burns in a day. Your burn is your total daily energy expenditure — the TDEE calculator breaks down where it comes from. When intake sits below TDEE, the missing energy has to come from somewhere, and most of it comes from stored fat. Every diet that works, works through this mechanism. The food rules just change how the deficit gets created.
How this calculator works
It estimates your resting burn with the Mifflin-St Jeor equation (American Journal of Clinical Nutrition, 1990):
- Men: BMR = 10 × weight (kg) + 6.25 × height (cm) − 5 × age + 5
- Women: BMR = 10 × weight (kg) + 6.25 × height (cm) − 5 × age − 161
That number times an activity factor (1.2–1.9) is your maintenance; the BMR calculator covers the resting piece alone, and entering a body fat percentage switches the math to Katch-McArdle. Your chosen tier is then subtracted, and the target never drops below 1,200 kcal — under that line it is hard to meet protein and micronutrient needs without medical supervision.
The 3,500-calorie rule, honestly
A pound of body fat stores about 3,500 kcal (7,700 per kg), so a 500-kcal daily deficit maps to roughly 1 lb (0.45 kg) per week on paper. Reality is slower: as you lose weight, a lighter body burns fewer calories, and metabolism adapts a little beyond that. The NIH Body Weight Planner, the reference model for this, shows real-world losses running behind the straight-line math. That is why the projection above adds 10–20% to the naive timeline. A plan that survives month three beats one that flatters you in week one.
How big should your calorie deficit be?
A useful ceiling is 0.5–1% of body weight per week. The leaner you are, the closer to the bottom of that range you should sit — the body fat calculator gives a quick estimate if you are unsure where you stand.
Mild: 250 kcal a day
About 0.25 kg (0.5 lb) per week. Best for people close to their goal, anyone prone to rebound eating, and lifters trying to hold muscle. Hunger is barely noticeable. Patience is the price.
Moderate: 500 kcal a day
About 0.45 kg (1 lb) per week, the default recommendation for a reason: large enough to see monthly progress, small enough to leave normal meals intact. Unsure what zero-deficit eating looks like for you? The maintenance calorie calculator shows the baseline.
Aggressive: 750–1,000 kcal a day
Around 0.7–0.9 kg (1.5–2 lb) per week. Justified at higher starting weights or under professional care; costly otherwise, because larger deficits lose more muscle, hurt training quality, and fail more often on adherence. The preset here uses 750 kcal, every target respects the 1,200 floor, and men should generally stay above about 1,500 without supervision.
How long will it take to reach your goal weight?
Enter a goal weight and the tool divides the distance by your weekly rate, then puts a date on it. Two honesty notes. The early weeks flatter you: starting a diet drops stored carbohydrate and the water bound to it, so the scale falls fast before settling into the true fat-loss rate. And the finish stretches: expect the projection to run 10–20% long as your body adapts. Judge progress by weekly averages, never by single weigh-ins, which swing 1–2 kg on water alone.
Why you might not lose weight in a deficit
Three causes cover almost every stall. The deficit is smaller than you think — logging studies find people eat 20–50% more than they record, mostly oils, drinks, and bites while cooking. Water is masking the loss — new training, extra salt, poor sleep, and the menstrual cycle each hold enough water to hide weeks of fat loss. Or your maintenance has dropped, and the intake that used to be a deficit now holds you steady. Track carefully for two to three weeks, then adjust by 100–200 kcal if the trend still disagrees.
How to eat in a calorie deficit
Protein first: 1.6–2.2 g per kg of body weight protects muscle and blunts hunger — the protein calculator gives your number. Fill volume with fiber: vegetables, fruit, potatoes, and legumes buy fullness cheaply. Keep lifting, because the deficit decides how much weight you lose and training decides how much of it is fat. To turn the calorie target into gram targets, the macro calculator does the split. And log what you eat: a deficit you do not measure is a guess.
