🥗 Calorie Deficit

How it works

Weight loss comes down to one unglamorous principle: eat fewer calories than you burn. This calculator first estimates how many calories your body uses per day — your Total Daily Energy Expenditure, or TDEE — based on your size, age, sex, and activity level. Then it subtracts a daily deficit to give you a calorie target that should produce steady, sustainable fat loss.

A deficit of about 500 calories per day leads to roughly 0.5 kg (1 lb) per week, since a kilogram of body fat stores around 7,700 calories. Bigger deficits lose weight faster but are harder to sustain and more likely to cost you muscle; smaller deficits are slower but easier to live with.

The formula

First, basal metabolic rate via the Mifflin-St Jeor equation. Men: BMR = 10 × weight(kg) + 6.25 × height(cm) − 5 × age + 5. Women: the same, but − 161 instead of + 5. Then TDEE = BMR × activity factor (1.2 sedentary, 1.375 lightly active, 1.55 moderately active, 1.725 very active, 1.9 extremely active). Daily target = TDEE − deficit.

Worked example

A 40-year-old man, 90 kg, 180 cm, moderately active: BMR = 10 × 90 + 6.25 × 180 − 5 × 40 + 5 = 900 + 1,125 − 200 + 5 = 1,830 calories. TDEE = 1,830 × 1.55 ≈ 2,837. With a 500-calorie deficit, the target is about 2,337 calories per day — expected loss around 0.5 kg per week.

Frequently asked questions

Why did my weight loss stall even though the math says I should be losing?
Water weight masks fat loss week to week, and your body adapts to long deficits by burning slightly less. Track the trend over 3–4 weeks, not day to day, and recalculate your target as you get lighter.

How low can I safely go?
As a general guideline, most adults should not drop below about 1,500 calories (men) or 1,200 (women) per day without medical supervision — below that it gets hard to meet nutrient needs.

Is it better to exercise more or eat less?
Eating less moves the needle faster — it is far easier to skip 500 calories than to burn them off. But exercise preserves muscle and improves health, so the best results come from combining a moderate deficit with regular activity.

How to read your calorie target

This calculator subtracts a deficit from your maintenance calories and projects the result. The standard conversion behind the projection is that one kilogram of body fat stores roughly 7,700 calories, so a steady 500 calorie daily deficit sums to 3,500 calories a week, or about 0.45 kg of fat. Treat that as an average over weeks, not a daily promise. In the first week or two the scale usually drops faster than fat loss alone explains, because eating less carbohydrate and salt sheds stored water; later, water retention can hide real fat loss for a week or more at a time. A sensible rate for most people is 0.5 to 1 percent of body weight per week. Faster loss raises the share of muscle in what disappears and makes the plan harder to hold. Your maintenance level is also a moving target: a lighter body costs less to run, so a deficit that worked at 95 kg will be smaller at 85 kg even if you change nothing. Very low calorie diets belong under medical supervision, and if you have a medical condition or a history of disordered eating, a clinician should be part of the plan from the start.

A worked example

Consider a man of 38 who weighs 95 kg and maintains on about 2,650 calories a day. He chooses a moderate 500 calorie deficit, so his target is 2,150 calories. The maths projects 3,500 calories of deficit per week, divided by 7,700, giving about 0.45 kg of fat loss a week, or roughly 5.5 kg over 12 weeks. His chosen pace is about 0.5 percent of his body weight weekly, at the gentle end of the recommended band, which suits someone who wants to keep training hard and keep the diet livable. If the first week shows a 2 kg drop, most of that is water, and if week four shows no change at all, that is usually water arriving as fat leaves, not the deficit failing.

Common mistakes

• Building a deficit so large it collapses at the weekend. Five disciplined days at 500 under saves 2,500 calories; two relaxed days at 1,250 over maintenance erase the entire week.
• Eating back exercise calories at face value. Machines and watches often overstate exercise burn substantially, and eating the full estimate back can quietly cancel the deficit.
• Drinking the deficit. Lattes, juice, alcohol and cooking oils are the most common places a careful food diary leaks several hundred calories.
• Never recalculating after real progress. Every 5 to 7 kg lost, re-run your maintenance estimate, or you will eventually stall and blame your willpower instead of arithmetic.
• Judging the plan on three days of scale data. Weigh daily if you like, but judge only the weekly average, and give any target at least three weeks.

Frequently asked questions

How large should my calorie deficit be?

A deficit of 300 to 500 calories a day suits most people, producing roughly 0.3 to 0.5 kg of fat loss a week. Larger deficits work on paper but cost hunger, energy, training quality and muscle, and they fail far more often. Slow enough to sustain beats fast enough to quit.

Why has my weight not moved after two weeks?

Fat loss and scale weight are different things. Water retention from stress, salt, sore muscles and hormonal cycles can mask a kilogram or more of genuine fat loss for days or weeks. If your weekly average weight is flat after three honest weeks, trim another 100 to 200 calories.

Can I save calories for the weekend?

Yes, within reason. What drives fat loss is your average intake across the week, so eating slightly less on weekdays to allow a larger Saturday can work. The trap is scale: two very large days can quietly erase five careful ones, as the worked example above shows.

Is it safe to eat fewer than 1,200 calories?

Very low calorie diets carry real risks, including gallstones, nutrient deficiencies and heavy muscle loss, and they are normally used only with medical supervision. Commonly cited minimums are around 1,200 calories for women and 1,500 for men, and even those are floors, not targets. Ask a clinician before going lower.