How many calories should I eat to lose weight?

Step 1: your resting burn

Your basal metabolic rate (BMR) is what your body burns just keeping you alive: heart, brain, breathing and temperature. The most widely used formula today is Mifflin-St Jeor from 1990, built from measurements of 498 healthy adults¹:

• Women: 10 × weight in kg + 6.25 × height in cm, minus 5 × age, minus 161¹.
• Men: 10 × weight in kg + 6.25 × height in cm, minus 5 × age, plus 5¹.

So a 40 year old woman who is 170 cm and 80 kg has a resting burn of about 1,500 kcal. A major review found Mifflin-St Jeor came within 10% of measured values for more people than any other formula². It is still an estimate, though, and any one person can be some way off².

Step 2: your everyday activity

On top of your resting burn comes everything you do during the day. The calculator multiplies by a factor from 1.2 if you mostly sit, up to 1.725 if your days are very active. Our example woman, with a desk job and a few walks (factor 1.375), burns about 2,065 kcal a day.

Step 3: take off the deficit

A kilo of body fat holds roughly 7,700 kcal³. To lose half a kilo a week, you need a deficit of about 3,850 kcal a week, or around 550 kcal a day. The NHS suggests something very similar: eat about 600 calories a day less⁵.

For our example woman, that puts her daily target at around 1,500 kcal. It isn't a crash diet. It's a level you can live with for months.

Why progress slows over time

As you lose weight you burn a little less, simply because there is less of you to run. Research shows the body responds slowly to a change in how much you eat, taking around a year to get halfway to its new balance⁴. That's why simple rules of thumb drift over the long run⁴.

So a slower pace doesn't mean you're doing something wrong. It just means it's time to work out your daily target again after you've lost a few kilos.

Why slow and steady

The NHS advises aiming for 0.5 to 1 kg a week⁶. A pooled analysis of controlled trials found that gradual weight loss gave the same total loss as rapid weight loss, but around 1 kg more fat lost and a better preserved resting burn⁷.

To be honest, pace alone doesn't decide whether the weight stays off. In an Australian trial, both the fast and the slow groups regained most of it over the following three years or so⁸. What matters most is choosing a pace you can keep, and habits you can live with afterwards.

Don't go too low

The NHS describes very low calorie diets, under roughly 800 to 1,200 calories, as unsuitable for many people and something to do only with professional support⁶. That's why the calculator never goes below 1,200 kcal for women or 1,500 kcal for men. If you're breastfeeding, different rules apply, covered in our guide to losing weight while breastfeeding.

• Have protein at every meal to help hold on to muscle. Read more about protein and weight loss.
• Choose food that fills you up for fewer calories: vegetables, beans and lentils, lean meat, fish and wholegrain bread.
• Watch your drinks. They rarely fill you up, but they still count.