How to calculate a calorie deficit for weight loss

What a calorie deficit is

Your body needs energy every minute: to keep your heart beating and organs working, to digest food, and to move. That energy is measured in kilocalories, usually just called calories (food labels in many countries also show kilojoules; 1 kcal = 4.184 kJ). When you eat about as many calories as you use, your weight stays roughly stable. When you eat more, the surplus is stored, mostly as fat. When you eat less, your body makes up the gap from its stores, and you lose weight. That gap is the calorie deficit.

The NHS puts it simply: to lose weight, eat and drink fewer calories than your body uses and be more active. It gives averages of about 2,000 calories a day for women and 2,500 for men, but stresses that the right number depends on your age, weight, height and activity (NHS, Understanding calories). That’s why a personal calculation is more useful than a generic target.

This guide is general information for healthy adults, not medical advice. If you’re pregnant, under 18, have diabetes, an eating disorder or another health condition, talk to a doctor or registered dietitian before changing how much you eat.

Step 1: estimate your BMR

Basal metabolic rate (BMR) is the energy your body uses at complete rest. It usually makes up the largest part of daily energy use. You can’t measure it at home, but equations estimate it well for most people. The Mifflin-St Jeor equation, published in 1990, is the one most dietitians use today and the one our calculator uses:

• Men: BMR = 10 × weight (kg) + 6.25 × height (cm) − 5 × age + 5
• Women: BMR = 10 × weight (kg) + 6.25 × height (cm) − 5 × age − 161

If you know your weight in pounds, divide by 2.2046 for kilograms (or use the kg to lbs converter); for height, multiply inches by 2.54. Our BMR formula guide compares Mifflin-St Jeor with the older Harris-Benedict equation and the Katch-McArdle formula, which uses lean body mass.

Step 2: find your maintenance calories

Maintenance calories, also called total daily energy expenditure (TDEE), add the energy you use moving about. The usual estimate multiplies BMR by an activity factor:

Most people overestimate their activity. If you sit most of the day and exercise three times a week, “light” is often closer to reality than “moderate”. The TDEE calculator shows all five levels side by side, and our guide on how to calculate TDEE explains how to pick yours and check it against your real weight trend.

Step 3: choose your deficit

The calorie deficit formula is simply:

Daily calorie target = maintenance calories − deficit

How big should the deficit be? A common starting point is about 500 calories a day. Using the widely quoted estimate that a kilogram of body fat stores about 7,700 kcal (roughly 3,500 kcal per pound), a 500-calorie daily deficit adds up to 3,500 calories a week, or roughly 0.45 kg (1 lb) of fat. The NHS recommends losing weight steadily, aiming for 0.5 to 1 kg (1 to 2 lb) a week (NHS, Obesity treatment). A deficit of about 10–20% of your maintenance calories is a practical way to stay in that range without feeling starved.

• Small (about 250–300 kcal): slow, easy to sustain, good if you’re close to your goal.
• Moderate (about 500 kcal): the usual default.
• Large (750–1,000 kcal): faster at first, harder to keep up, and only sensible with plenty of weight to lose.

Worked example

A 30-year-old woman, 165 cm tall, weighing 65 kg, who exercises 3–5 days a week:

1. BMR: 10 × 65 + 6.25 × 165 − 5 × 30 − 161 = 650 + 1,031.25 − 150 − 161 = 1,370 kcal.
2. Maintenance: 1,370.25 × 1.55 = about 2,124 kcal a day.
3. Deficit of 500 kcal: 2,124 − 500 = about 1,624 kcal a day.
4. Expected loss at first: 500 × 7 = 3,500 kcal a week ÷ 7,700 kcal per kg ≈ 0.45 kg a week.

A 35-year-old man, 180 cm, 80 kg, with a desk job and little exercise: BMR = 10 × 80 + 6.25 × 180 − 5 × 35 + 5 = 1,755 kcal; maintenance = 1,755 × 1.2 ≈ 2,106 kcal; with a 500-calorie deficit, about 1,606 kcal a day. If he started exercising three times a week (factor 1.375, maintenance about 2,413 kcal), the same 1,606-calorie intake would become a deficit of about 800 calories. Moving more and eating a little less is usually easier than cutting food alone.

Put your own numbers in the calorie calculator: it shows your BMR, maintenance and targets for losing or gaining at different rates, and it won’t suggest intakes below a sensible floor.

The 3,500-calorie rule and why loss slows

The “3,500 calories per pound” rule (7,700 per kilogram) is a fair guide to the first few weeks. Over months it overestimates weight loss, for two reasons. First, as you lose weight your body needs less energy: a lighter body has a lower BMR and burns fewer calories moving. A deficit that was 500 calories at the start shrinks as you go. Second, the body adapts to eating less, and early loss includes water and stored carbohydrate, not just fat.

Researchers at the US National Institutes of Health built a model of these changes, and the National Institute of Diabetes and Digestive and Kidney Diseases offers it as the free Body Weight Planner. It shows how weight changes over time for a given intake, and how many calories you’d need to maintain the new weight. The practical lesson: recalculate your maintenance calories every 4–5 kg, and expect the scale to move more slowly in month three than in month one.

Tracking and adjusting

Every formula is an estimate; individuals differ by a few hundred calories a day. Treat your first target as a starting guess and adjust from real results:

1. Weigh yourself at the same time of day (for example, after waking) several times a week and look at the weekly average, not single days.
2. Track what you eat honestly for 2–3 weeks. Food labels and a kitchen scale help; drinks, oils and snacks are the usual blind spots.
3. If your weekly average isn’t moving after 3 weeks, reduce intake by about 100–200 calories or add activity. If you’re losing much faster than planned, eat a little more.

Weight naturally swings by 1–2 kg from day to day with water, salt and meal timing, which is why averages matter.

How big a deficit is safe?

Very low-calorie diets exist (the NHS describes them as under 800 or 1,200 calories a day), but the NHS says they’re not suitable or safe for everyone and are usually only recommended when weight is affecting health, with support from a professional. For most people, a moderate deficit is easier to keep up, preserves more muscle, and fits a balanced diet with enough protein, fibre, vitamins and minerals. The Dietary Guidelines for Americans (dietaryguidelines.gov) and the WHO’s healthy lifestyle advice (WHO Europe) both emphasise the quality of food, not only the quantity.

Our calculator never suggests targets below about 1,200 kcal for women or 1,500 kcal for men, which are common practical floors for unsupervised dieting. Strength training and enough protein help keep muscle while you lose fat.

Common mistakes

• Choosing too high an activity level. This inflates maintenance calories and hides the deficit.
• Counting exercise twice. If your activity factor already includes your workouts, don’t eat back the calories a fitness watch says you burned.
• Never recalculating. Maintenance falls as you lose weight.
• Judging by one weigh-in. Use weekly averages.
• Cutting too hard. Big deficits often end in rebound eating.
• Ignoring BMI context. Check whether weight loss is right for you at all with the BMI calculator and our BMI formula guide.