How many calories do I need?
There are two steps. First the calculator estimates your basal metabolic rate (BMR), the energy your body uses at complete rest to breathe, circulate blood and keep warm. It then multiplies this by an activity factor to estimate your total daily energy expenditure (TDEE).
To keep your weight steady you eat about your TDEE. To lose weight you eat less than this, and to gain weight you eat more. As a rough guide, the NHS says an average man needs around 2,500 kcal a day and an average woman around 2,000 kcal.
The Mifflin-St Jeor equation
Men: BMR = 10 × weight (kg) + 6.25 × height (cm) − 5 × age + 5
Women: BMR = 10 × weight (kg) + 6.25 × height (cm) − 5 × age − 161
Published in 1990, this equation has been found to predict resting energy use in healthy adults more accurately than the older Harris-Benedict equation, and it is widely used by dietitians.
| Activity level | Multiplier |
|---|---|
| Sedentary | 1.2 |
| Lightly active (1 to 3 days a week) | 1.375 |
| Moderately active (3 to 5 days a week) | 1.55 |
| Very active (6 to 7 days a week) | 1.725 |
| Extra active | 1.9 |
Calories for weight loss or gain
One kilogram of body fat stores roughly 7,700 kcal. A deficit of 500 kcal a day is about 0.45 kg (1 lb) a week. The NHS suggests aiming to lose 0.5 to 1 kg (1 to 2 lb) a week.
Macros: the calculator sets protein by body weight (1.8 g per kg when losing or gaining, 1.4 g per kg when maintaining), fat at 25% of calories, and the rest as carbohydrate. Protein and carbohydrate provide 4 kcal per gram and fat 9 kcal per gram.
Worked example 1: maintaining weight
Man, 30, 80 kg, 180 cm, moderately active
BMR = 10 × 80 + 6.25 × 180 − 5 × 30 + 5 = 1,780 kcal
TDEE = 1,780 × 1.55 = 2,759 kcal
Macros: 112 g protein, 77 g fat, 405 g carbohydrate
Worked example 2: losing weight
Woman, 35, 75 kg, 165 cm, moderately active, goal −500 kcal
BMR = 750 + 1,031.25 − 175 − 161 = 1,445 kcal
TDEE = 1,445.25 × 1.55 = 2,240 kcal; target = 2,240 − 500 = 1,740 kcal
Expected loss about 0.45 kg a week; 135 g protein, 48 g fat, 191 g carbohydrate
Worked example 3: the minimum target
Woman, 25, 60 kg, 165 cm, sedentary, goal −500 kcal
TDEE = 1,345 × 1.2 ≈ 1,614 kcal; a 500 kcal deficit would give 1,114 kcal
That is below the recommended minimum, so the target is raised to 1,200 kcal
Moving more is a safer way to increase the deficit than eating less
Limitations
- Equations give an estimate for an average person; your real needs may differ by around 10%. Track your weight for a few weeks and adjust.
- People with a lot of muscle may burn more than the equation predicts.
- The calculator is not suitable during pregnancy or breastfeeding, when energy needs rise.
- If you have diabetes, a thyroid or kidney condition or an eating disorder, set targets with your GP or a registered dietitian.
- To check whether your weight is in a healthy range for your height, use the BMI Calculator.