Calorie deficit calculator

A small, steady deficit beats a dramatic one. Pick how big a gap you want and we work out a daily target, never below your resting energy or a safe floor, with a realistic estimate of the pace.

Your details

A gentle daily target

1,380

kcal a day

Your everyday energy1,883 kcal
Deficit used500 kcal
Likely pace at first0.5 kg (1.0 lb) a week
Expected loss by 12 weeks (Hall model)4.2 kg (9.3 lb)
Expected loss by a year (Hall model)14.3 kg (31.4 lb), against 23.6 kg on the straight-line rule

Weight loss slows as you go, because a lighter body uses less. That is normal, not failure.

Your numbers

What the model expects

For the example woman of 40 at 11 st and 5 ft 5 in who is a bit active, a 500 kcal cut starts at about 0.45 kg a week. Kept at 500, the Hall model expects about 4.2 kg by 12 weeks and about 14 kg by a year, against 23.6 kg on the straight-line rule. Your own rows use your sex, age, height and activity, so they move when those do.

The working

Everyday energy minus a gap

We take your TDEE (Mifflin-St Jeor times your activity), take off the deficit you pick, and then check the result against two floors: your BMR and the NHS based floor of 1,200 kcal for women or 1,500 kcal for men. We use whichever is highest.

Example: a TDEE of 1,883 kcal minus 500 is 1,383 kcal. The BMR here is 1,369, so 1,383 stands, and we show 1,380 kcal. A 500 kcal gap is about 3,500 kcal a week, or roughly 0.45 kg (1 lb).

The pace

Why it slows

The old rule says 7,700 kcal is a kilo of body fat. It is fine for the first few weeks, but research by Kevin Hall and colleagues shows weight loss slows as your body gets lighter and adapts. So expect a slower second month than first, and treat any weeks-to-goal figure as a guess.

NHS advice

Steady is the aim

The NHS suggests 0.5 to 1 kg (1 to 2 lb) a week, often with a deficit of around 600 kcal. Bigger gaps tend to leave you hungry, tired and more likely to bounce back. The free NHS Weight Loss Plan app can help you plan meals around your number.

Read with care

What this cannot tell you

BMI under 18.5We will not work out a deficit if you are already below the healthy range.
Pregnancy and breastfeedingNot the time to diet. Your midwife or health visitor can help.
MedicinesIf you take diabetes medicines, eating less can change your doses.

Questions

Common questions

What is a safe calorie deficit?

For most adults, 300 to 600 kcal a day below maintenance. That usually means losing 0.25 to 0.75 kg a week, which the NHS considers steady and sustainable.

Why does the calculator not go lower?

Eating below your resting energy, or under about 1,200 kcal for women and 1,500 for men, makes it hard to get enough nutrients. Very low calorie diets need medical supervision.

How long will it take to reach my goal?

Add a goal weight and we give two answers: weeks at your starting pace, and weeks if you keep the same daily cut while your body adapts (the Hall 2011 model). The second is usually longer, which is normal.

Sources

Last checked 21 October 2026. Spotted something wrong? Tell us and we will fix it.

If food feels hard

If counting calories leaves you anxious, guilty or thinking about food all the time, please stop and talk to someone. Beat has a free helpline on 0808 801 0677, and your GP can help too.