How to calculate your calorie deficit
It takes two steps. First, your resting energy needs (your basal metabolic rate, or BMR): the energy your body uses at rest. The Mifflin-St Jeor equation estimates it from your weight, height, age and sex.
Then this figure is multiplied by an activity factor, which depends on your day and your exercise. The factor starts at 1.45 if you are mostly sitting, 1.65 if you are up and about and 1.85 for a physical job, then rises with the number of exercise sessions in your week. The result is your maintenance calories, and your deficit is taken off that.
An example: a 35-year-old woman, 165 cm (5 ft 5 in) and 70 kg (11 st, or 154 lb), mostly sitting, with no exercise. At rest, 10 × 70 + 6.25 × 165 − 5 × 35 − 161 ≈ 1,395 kcal; times 1.45, about 2,020 kcal a day. The calculator gives her 1,922 to 2,124 kcal a day to keep her weight, and 1,372 to 1,574 kcal a day to lose 0.5 kg a week: about 550 kcal less.
The calculator above opens on the same woman exercising once or twice a week: 1,962 to 2,168 kcal to keep her weight, 1,412 to 1,618 kcal to lose 0.5 kg a week.
| Formula | |
|---|---|
| Woman | 10 × weight + 6.25 × height − 5 × age − 161 |
| Man | 10 × weight + 6.25 × height − 5 × age + 5 |
| Other | 10 × weight + 6.25 × height − 5 × age − 78 (the average of the two) |
What is a calorie deficit?
A calorie deficit means eating a little less than your maintenance calories, so your body draws on its reserves. A kilogram of body fat holds about 7,700 kcal (about 3,500 kcal per pound). Losing 0.25 kg a week (about half a pound) takes a deficit of about 275 kcal a day; losing 0.5 kg a week (about a pound), about 550 kcal.
- Never more than 880 kcal a day below maintenance.
- Never below 1,200 kcal a day for a woman, 1,350 kcal if you choose “other”, or 1,500 kcal for a man.
- No deficit at all during pregnancy or breastfeeding: the range stays at maintenance.
Building muscle works the other way round: a small surplus above maintenance, at a slow pace of 0.5 kg a week at most, and never more than 1% of your body weight a week.
What to eat in a calorie deficit
A calorie deficit diet isn’t a special list of foods: it’s ordinary meals in slightly smaller amounts. What changes is the plate:
- A full portion of protein at each meal: meat, fish, eggs, dairy, pulses or tofu.
- Generous vegetables.
- A smaller, measured portion of the starchy food: rice, pasta, potatoes or bread.
- Weighed amounts rather than guesses by eye.
The last point decides the rest: a deficit of a few hundred kcal is easily lost in a heaped serving.
A range, not a number
Your needs vary from day to day, and the equation is an estimate. That’s why the result is a range, and why the real judge is your weight over several weeks.