The short answer
- Your basal metabolic rate (BMR) is the energy you use at rest. Multiply it by an activity factor to get the calories you burn in a day.
- Eat that many calories to stay the same weight. Eat about 550 kcal a day less to lose about 0.5 kg (1 lb) a week.
- The NHS says to aim to lose 0.5 to 1 kg a week, and that diets of 800 kcal a day or less need medical supervision.
- Every figure is an estimate. Check your weight over three or four weeks and adjust.
What BMR means
Basal metabolic rate is the energy your body needs to keep running if you did nothing all day: your heart beating, your lungs breathing, your brain working and your body staying warm. For most adults it is the biggest part of the energy they use, more than half of the total.
BMR depends mainly on how much lean tissue you have. Muscle and organs use far more energy than fat, which is why larger and more muscular people burn more, and why BMR tends to drop slowly with age as muscle is lost.
From BMR to daily calories
Total daily energy expenditure (TDEE) is everything you burn in a day. It adds the energy you use moving about, exercising and digesting food to your BMR. Calculators estimate it by multiplying BMR by a standard activity factor:
| Activity level | Typical week | Multiplier |
|---|---|---|
| Sedentary | Desk job, little or no exercise | 1.2 |
| Lightly active | Light exercise 1 to 3 days | 1.375 |
| Moderately active | Moderate exercise 3 to 5 days | 1.55 |
| Very active | Hard exercise 6 to 7 days | 1.725 |
| Extra active | Physical job plus hard training | 1.9 |
Your TDEE is the number of calories that keeps your weight steady, sometimes called your maintenance calories.
A worked example
- Mifflin-St Jeor BMR10 × 70 + 6.25 × 165 − 5 × 35 − 1611,395 kcal
- Moderately activeBMR × 1.552,163 kcal
- To lose 0.5 kg a week0.5 × 7,700 ÷ 7 = 550 less a day1,613 kcal
- Goal weight 65 kg5 kg at 0.5 kg a week10 weeks
For a man aged 40, 180 cm tall and weighing 85 kg, the same steps give a BMR of 1,780 kcal, about 2,759 kcal a day to maintain at the same activity level, and 2,209 kcal to lose 0.5 kg a week.
The three formulas compared
Mifflin-St Jeor (1990) is the default. In studies it has come closest to measured resting energy for most adults. Harris-Benedict is older, first published in 1919 and revised in 1984, and usually gives a slightly higher figure. Katch-McArdle ignores age and sex and uses lean body mass instead, so it needs your body fat percentage. Our body fat calculator can estimate that from a tape measure.
The chart shows BMR for the woman in the worked example. The formulas differ by about 100 kcal, a reminder that each is an estimate.
Choosing your activity level
The activity level makes a bigger difference than the choice of formula. For the same woman, maintenance calories range from 1,674 kcal sedentary to 2,651 kcal extra active.
Count your job and your exercise together. A desk worker who goes to the gym twice a week is lightly active. People tend to overestimate how active they are, so if in doubt pick the lower level.
The NHS 2,000 and 2,500 figures
The NHS says an average man needs about 2,500 kcal a day and an average woman about 2,000 kcal to keep a steady weight. These are reference values, used on food labels and in public advice, not targets for any one person. A small, inactive older woman may need much less than 2,000 kcal, and a tall, active young man much more than 2,500 kcal.
Calories to lose weight
You lose weight when you take in less energy than you use. A common rule of thumb is that about 7,700 kcal equals 1 kg of body weight (about 3,500 kcal a pound). Spread over a week, that gives these daily changes:
| Rate a week | Change a day | Daily target | Weeks to lose 5 kg |
|---|---|---|---|
| 0.25 kg (½ lb) | −275 kcal | 1,888 kcal | 20 |
| 0.5 kg (1 lb) | −550 kcal | 1,613 kcal | 10 |
| 0.75 kg (1½ lb) | −825 kcal | 1,338 kcal | 6.7 |
| 1 kg (2 lb) | −1,100 kcal | 1,200 kcal (limit) | 5.7 |
The NHS’s own advice is to cut about 600 kcal a day, and its 12-week weight loss plan uses daily targets of 1,400 kcal for women and 1,900 kcal for men.
How fast is safe
The NHS suggests aiming to lose 0.5 to 1 kg (1 to 2 lb) a week. Faster loss is more likely to include muscle, can leave you tired and short of nutrients, and is harder to keep off. The calculator warns you if you choose a rate faster than 1 kg a week.
The real rule of thumb is an approximation
Early losses are often faster because of water, and later losses slower because a smaller body uses less energy. Use the weekly figure as a guide, not a promise.
The lowest target we show
Harvard Health advises that calorie intake should not fall below about 1,200 kcal a day for women or 1,500 kcal for men except under a health professional’s supervision. Below those levels it is hard to get the protein, vitamins and minerals you need. The calculator never shows a lower target and tells you when it has stopped at the limit.
- BMR1,058 kcal
- Maintenance× 1.21,269 kcal
- Maintenance − 550719 kcal
- Raised to the lower limit1,200 kcal
The NHS describes very low calorie diets, of 800 kcal a day or less, as suitable only for some people and only under medical supervision. For someone like this, more activity is a better lever than eating less.
Calories to gain weight
To gain weight, eat a little more than your maintenance figure. Gaining about 0.25 kg a week needs roughly 275 kcal a day extra, so the worked-example woman would eat about 2,438 kcal. A slow gain with strength training means more of the extra weight is muscle. If your BMI is under 18.5, check it with our BMI calculator and talk to your GP, as being underweight can have a medical cause.
Age, sex and size
In the Mifflin-St Jeor formula, each extra year of age takes 5 kcal off your BMR, so a decade lowers it by 50 kcal. Men get 166 kcal more than women of the same size (the +5 and −161 in the formula), reflecting more muscle on average.
| Age | BMR |
|---|---|
| 25 | 1,445 kcal |
| 45 | 1,345 kcal |
| 65 | 1,245 kcal |
Weight and height matter most. Every kilogram adds 10 kcal and every centimetre 6.25 kcal to BMR.
Protein, carbs and fat
Food energy comes from three macronutrients. Protein and carbohydrate give 4 kcal a gram and fat 9 kcal a gram. The calculator splits your target into grams using one of three common patterns:
| Split | Protein | Carbohydrate | Fat |
|---|---|---|---|
| Balanced (20/50/30) | 100 g | 250 g | 67 g |
| Higher protein (30/40/30) | 150 g | 200 g | 67 g |
| Lower carb (30/25/45) | 150 g | 125 g | 100 g |
The NHS Eatwell Guide suggests basing meals on starchy foods, eating plenty of fruit and vegetables, and choosing unsaturated fats in small amounts. Total calories matter most for weight, but more protein can help you feel full and keep muscle while losing weight.
Reading food labels
UK food labels show energy in kilojoules (kJ) and kilocalories (kcal). One kcal is 4.184 kJ, so 2,000 kcal is about 8,400 kJ. The "reference intake" percentages on the front of packs are based on 2,000 kcal a day: an average adult’s daily needs, not a personal target. If your target is lower, each item is a bigger share of your day than the label suggests.
Counting calories in practice
- Weigh food for a week or two to learn what a portion looks like.
- Count drinks, cooking oil, sauces and snacks; they are the easiest to miss.
- Use the NHS Weight Loss Plan app or a food diary for a few weeks.
- Weigh yourself once a week, at the same time of day.
Our stone to kg converter helps if your scales and your app use different units.
When progress stalls
Weight loss rarely runs in a straight line. If your weight has not changed for three or four weeks, recalculate with your new weight, check your activity level, and look again at portion sizes. Then lower your target by 100 to 200 kcal, staying above the lower limit. A smaller body needs fewer calories: for the worked-example woman, losing 5 kg lowers maintenance by about 78 kcal a day at the same activity.
Exercise and calories
The NHS recommends adults do at least 150 minutes of moderate activity a week, or 75 minutes of vigorous activity, and strengthening exercises on at least two days. Activity helps keep weight off and protects muscle while you lose weight. Fitness trackers often overestimate calories burned, so avoid eating back every calorie they report.
Who should not use this
- Pregnant or breastfeeding
- Ask your midwife or GP
- Under 18
- Growth charts apply
- Eating disorder history
- Speak to your GP
- Diabetes or other conditions
- Ask your care team
- Healthy adults
- Maintain, lose or gain
- Older adults
- Keep protein up
- Regular exercisers
- Pick an honest activity level
- Muscular people
- Try Katch-McArdle
Families on a low income who are pregnant or have young children may be able to get help buying healthy food: check the Healthy Start calculator.
Using kg, pounds or stones
Enter your height in centimetres or feet and inches, and your weight in kilograms, pounds or stones and pounds. The formulas work in metric, so the calculator converts using exact definitions: 1 lb is 0.45359237 kg and 1 inch is 2.54 cm. To convert a height yourself, use our feet to cm converter.
Getting help
In the UK, the free NHS Weight Loss Plan app offers a 12-week plan, and your GP can refer you to a local weight management service. In the US, ask your doctor about programs your health insurance covers. A registered dietitian can give personal advice on what and how much to eat.