How your calorie needs are worked out
First the calculator estimates your resting energy, the calories your body uses at rest, with the Mifflin-St Jeor equation from 1990. For men it's 10 × weight in kg + 6.25 × height in cm − 5 × age + 5. For women it's the same but − 161 at the end.
That figure is then multiplied by an activity factor: 1.2 for little or no exercise, 1.375 for light exercise, 1.55 for moderate exercise, 1.725 for hard exercise and 1.9 for very hard exercise or a physical job. The result is the calories you need to maintain your weight.
The lose and gain targets are 500 kcal a day below and above that. A difference of about 500 kcal a day changes weight by roughly 0.5 kg a week.
Worked examples
A 35-year-old woman who is 165 cm tall, weighs 68 kg and does light exercise has a resting energy use of 10 × 68 + 6.25 × 165 − 5 × 35 − 161 = 1,375 kcal a day. Multiplied by 1.375, she needs about 1,891 kcal a day to maintain her weight, 1,391 to lose about 0.5 kg a week or 2,391 to gain about 0.5 kg a week.
A 40-year-old man who is 180 cm tall, weighs 85 kg and does moderate exercise has a resting energy use of 1,780 kcal a day. He needs about 2,759 kcal a day to maintain his weight.
Treat the result as a starting point
These are estimates. Two people with the same details can burn quite different amounts, and most people find it hard to judge their activity level. Track your weight for a few weeks and adjust if it isn't changing as expected.
Weight change slows as you lose or gain weight, because your energy needs change too. Re-enter your details from time to time.
The calculator is for adults and isn't suitable during pregnancy or breastfeeding. It's not medical advice: if you have a health condition or want to lose a lot of weight, talk to your GP before changing your diet.
Inputs for the calorie calculator
Sex
The Mifflin-St Jeor equation has a different constant for men and women.
Age
Your age in whole years, from 18 to 100. Resting energy use falls by 5 kcal a day for each extra year.
Height
Your height in centimetres.
Weight
Your current weight in kilograms.
Activity level
Pick the option that best matches a typical week, including exercise and how active your job is. If you're unsure, choose the lower option, because people often overestimate how active they are.
CALCULATOR QUESTIONS
Calorie calculator questions
How many calories should I eat to lose weight?
Eating about 500 kcal a day less than your maintenance figure should lose roughly 0.5 kg a week. The calculator shows this as the 'lose' figure.
What is BMR?
BMR (basal metabolic rate) is the energy your body uses at rest to keep you alive, such as breathing and keeping warm. Your daily needs are higher because you also move around.
Why am I not losing weight on the suggested calories?
The figures are estimates and your real needs may be lower. It's also easy to underestimate how much you eat. Track your intake and weight for a few weeks and adjust gradually.
Is it safe to eat much less than the 'lose' figure?
Very low-calorie diets aren't suitable for everyone and should only be followed with medical supervision. Talk to your GP before cutting your calories sharply.