How big should your calorie deficit be?

Every weight-loss approach, whatever it is called, works by creating a calorie deficit: taking in less energy than you use. The question is how big that gap should be. Too small and progress is invisible; too big and it is miserable, costs muscle and rarely lasts. The calorie deficit calculator turns a target into daily numbers, and this guide explains how to pick one.

What a deficit is

Your daily energy use, or TDEE (total daily energy expenditure), is your basal metabolic rate, the energy to keep you alive at rest, plus the energy for digestion, daily movement and exercise. The calorie calculator estimates it from your age, sex, height, weight and activity. A deficit is the difference between that figure and what you eat. Eat 2,000 kcal when your TDEE is 2,500 and your deficit is 500 kcal a day; the body makes up the shortfall from its stores, mostly fat if you eat enough protein and stay active. How daily calorie needs are calculated — BMR, activity multipliers and what a deficit means explains the formulas.

Choosing a size

A deficit of about 300 to 500 kcal a day is a common, sustainable starting point for most adults, typically producing loss of around 0.25 to 0.5 kg a week at first. The NHS weight-loss plan uses a slightly larger gap, around 600 kcal a day below maintenance. A useful way to scale it is as a percentage: a deficit of 10 to 25 per cent of TDEE. Larger bodies can handle larger deficits; someone with a TDEE of 3,200 kcal losing 700 a day is in a very different position from someone with a TDEE of 1,800 doing the same. Start moderate, watch the trend for three or four weeks, and adjust. How fast should you lose weight? What a safe, lasting rate looks like covers the target rate.

The 3,500-calorie rule and its limits

The classic rule says a pound of fat contains about 3,500 kcal (about 7,700 kcal per kilogram), so a 500 kcal daily deficit loses a pound a week. It is a fair estimate for the first few weeks, which is why calculators use it. Over months it overestimates, because as you lose weight your energy needs fall: a lighter body burns less, and the body adapts a little to eating less. Researchers at the US National Institutes of Health built dynamic models showing that a fixed deficit produces steadily slowing loss that eventually levels off. The practical lesson is to recalculate your TDEE every 5 kg or so, and expect the same habits to deliver less over time.

Minimums and safety

Big deficits make it hard to get enough protein, vitamins and minerals and increase the risk of muscle loss, fatigue and gallstones. A widely used floor for unsupervised eating is about 1,200 kcal a day for women and 1,500 kcal for men; the calculators here warn when a target falls below it. Very-low-calorie diets of around 800 kcal a day exist for specific medical situations and should be followed under supervision. Children, teenagers, pregnant or breastfeeding women, and people with diabetes, eating disorders or other conditions should get individual advice rather than use a generic deficit.

Keeping muscle and sticking with it

The quality of the deficit matters as well as the size. Eating enough protein (often 1.2 to 1.6 g per kilogram of body weight when losing weight) and doing resistance exercise two or three times a week help preserve muscle, so more of what you lose is fat. Filling meals with vegetables, fruit and fibre makes a deficit easier to live with. Many people find it easier to eat slightly less every day than to swing between strict and relaxed days, though weekly averages are what count. Track your weight trend with the weight tracker and change one thing at a time when progress stalls. The macro calculator splits your calories into protein, carbohydrate and fat if you want more structure.