Most adults need roughly 1,600–3,000 kcal a day to keep their weight stable. Your number depends on sex, age, height, weight and, above all, how active you are. You estimate it in two steps: work out your basal metabolic rate with the Mifflin-St Jeor equation, then multiply it by your physical activity level.
What makes up your daily calorie needs?
Your daily needs are the sum of three parts: basal metabolism, the energy spent digesting food, and the energy spent moving. Basal metabolism is the largest: the energy your body uses at rest to breathe, circulate blood, keep warm and run its organs.
| Component | What it is | Share of the total, roughly |
|---|---|---|
| Basal metabolic rate (BMR) | Energy used at complete rest | 50–70% |
| Thermic effect of food | Energy used to digest and absorb food | 5–15% (Westerterp, 2004) |
| Physical activity | Exercise, but also walking, standing and housework | 15–40% |
The split varies a lot between people. For someone at a desk who does no exercise, activity is a small slice; for a construction worker or an athlete it can be more than a third.
How do you calculate your basal metabolic rate?
Basal metabolism is best estimated with the Mifflin-St Jeor equation, published in 1990. A 2005 systematic review of the equations used in clinical practice found that Mifflin-St Jeor predicted resting metabolic rate within 10% of the measured value in more people, with and without obesity, than any other equation.
With weight in kilograms, height in centimetres and age in years:
- Men: 10 × weight + 6.25 × height − 5 × age + 5
- Women: 10 × weight + 6.25 × height − 5 × age − 161
Example: a 30-year-old woman, 165 cm, 60 kg. 10 × 60 + 6.25 × 165 − 5 × 30 − 161 = 600 + 1,031 − 150 − 161 ≈ 1,320 kcal a day. That is what her body needs with no activity at all.
If you think in stones and feet, convert first: 1 stone is 6.35 kg and 1 inch is 2.54 cm.
Which activity level fits you?
The physical activity level (PAL) is the ratio between the energy you burn in a day and your basal metabolism. EFSA uses PAL values of 1.4, 1.6, 1.8 and 2.0 for adults, from sedentary to very active.
| PAL | What a typical day looks like |
|---|---|
| 1.4 | Desk job, travel by car, no exercise |
| 1.6 | Desk job, but daily walking or 2–3 workouts a week |
| 1.8 | Job on your feet, or exercise almost every day |
| 2.0 | Heavy manual work or hard daily training |
Most people overrate their activity. If in doubt, pick the lower value and correct it after the first few weeks.
How many calories do different people need?
The table shows estimated needs (BMR × PAL) for a few typical profiles, rounded to 10 kcal. They are starting points for weight maintenance, not personal targets.
| Profile | BMR | PAL 1.4 | PAL 1.6 | PAL 1.8 | PAL 2.0 |
|---|---|---|---|---|---|
| Woman, 30, 165 cm, 60 kg | 1,320 kcal | 1,850 | 2,110 | 2,380 | 2,640 |
| Woman, 45, 165 cm, 75 kg | 1,395 kcal | 1,950 | 2,230 | 2,510 | 2,790 |
| Woman, 60, 160 cm, 65 kg | 1,189 kcal | 1,660 | 1,900 | 2,140 | 2,380 |
| Man, 30, 178 cm, 75 kg | 1,718 kcal | 2,400 | 2,750 | 3,090 | 3,440 |
| Man, 45, 180 cm, 90 kg | 1,805 kcal | 2,530 | 2,890 | 3,250 | 3,610 |
| Man, 60, 175 cm, 80 kg | 1,599 kcal | 2,240 | 2,560 | 2,880 | 3,200 |
Two things stand out. Age lowers the estimate by only 5 kcal a year in the equation, or 50 kcal a decade. Activity changes far more: the gap between PAL 1.4 and 1.8 is 500–700 kcal a day.
Why is a calculator only an estimate?
Because equations describe the average of a group, not your body. Even the best one is more than 10% off for some people, and a PAL picked by feel can add another 10–15%. For someone who needs 2,200 kcal, that is a margin of a few hundred calories.
The best test is your own body. Eat roughly the calculated amount for 2–3 weeks, log your food, and weigh yourself under the same conditions (in the morning, after the toilet) several times a week. Then:
- If your average weight holds, you have found your maintenance intake.
- If it falls steadily, your real needs are higher than what you ate.
- If it rises steadily, your real needs are lower.
Weight swings by 0.5–1 kg from day to day because of water, salt and gut contents, so compare weekly averages, not two single days.
How do your needs change as your weight changes?
Your needs move with your weight, so recalculate after every 4–5 kg lost or gained. A lighter body uses less energy both at rest and when moving. Models published by Kevin Hall and colleagues at the NIH show that weight responds slowly to a change in intake: about half of the effect appears in the first year and the rest over the following years.
In practice, if you started at 2,000 kcal and lost 8 kg, the same 2,000 kcal may become your maintenance intake a few months later. That is not a “broken metabolism”, just the physics of a smaller body.
When do you need more or fewer calories?
A few situations change your needs compared with the standard calculation:
- Pregnancy: EFSA estimates about 70 extra kcal a day in the first trimester, 260 kcal in the second and 500 kcal in the third.
- Breastfeeding: about 500 extra kcal a day in the first 6 months for a woman of normal weight.
- Competitive sport: needs can exceed PAL 2.0; work them out with a specialist.
- Illness and medication: thyroid conditions, some medicines and recovery from surgery change your needs.
In all these cases, agree your starting figure with your doctor or a registered dietitian. For a quick calculation on your own numbers, use the calorie calculator in our calculators section; in Rod, the nutrition app we are building, your target recalculates itself when you update your weight.
How do you use the number in practice?
Use it as a starting point for your goal, not as a fixed rule:
- To maintain: eat around your needs and check your weight monthly.
- To lose weight: subtract 300–500 kcal, not more at first.
- To build muscle: add 200–300 kcal and do strength training.
Whatever the goal, food quality still matters. The WHO recommends keeping free sugars below 10% of daily energy and salt below 5 g a day, however many calories you eat.
