Calories are a unit of energy. Your body uses energy to keep you alive, to digest food and to move. Eat more than you burn over time and you gain weight; eat less and you lose it. Knowing roughly how much you burn is therefore the starting point for any eating plan, whether you want to lose fat, maintain or build muscle. Our calorie calculator does the sums for you; this guide explains what is behind them.
This guide is general information, not medical or dietary advice. Talk to a GP or registered dietitian before major changes, especially if you are pregnant, have a medical condition or a history of disordered eating.
The building blocks of daily energy use
Total daily energy expenditure (TDEE) has four parts:
- Basal metabolic rate (BMR): energy for breathing, circulation and organ function at rest, usually 60–70% of the total.
- Thermic effect of food: energy used to digest food, roughly 10%.
- Non-exercise activity (NEAT): walking, standing, fidgeting, housework. This varies hugely between people.
- Exercise: planned workouts.
Calculators estimate BMR with a formula and then multiply it by an activity factor that covers parts 2 to 4.
Step 1: calculate BMR with Mifflin-St Jeor
The Mifflin-St Jeor equation (1990) is widely regarded as the most accurate simple formula for adults:
Men: BMR = 10 × weight (kg) + 6.25 × height (cm) − 5 × age (years) + 5
Women: BMR = 10 × weight (kg) + 6.25 × height (cm) − 5 × age (years) − 161If you use pounds and inches: weight (kg) = lb × 0.4536, height (cm) = inches × 2.54.
Example: woman, 35, 70 kg, 165 cm
- 10 × 70 = 700
- 6.25 × 165 = 1,031.25
- 5 × 35 = 175
- BMR = 700 + 1,031.25 − 175 − 161 = 1,395 kcal/day
Example: man, 30, 80 kg, 180 cm
- 10 × 80 = 800
- 6.25 × 180 = 1,125
- 5 × 30 = 150
- BMR = 800 + 1,125 − 150 + 5 = 1,780 kcal/day
Step 2: multiply by an activity factor
| Activity level | Description | Factor |
|---|---|---|
| Sedentary | Desk job, little exercise | 1.2 |
| Lightly active | Light exercise 1–3 days a week | 1.375 |
| Moderately active | Moderate exercise 3–5 days a week | 1.55 |
| Very active | Hard exercise 6–7 days a week | 1.725 |
| Extra active | Physical job plus hard training | 1.9 |
- Woman above, moderately active: 1,395 × 1.55 ≈ 2,163 kcal/day
- Man above, lightly active: 1,780 × 1.375 ≈ 2,448 kcal/day
Most people overestimate their activity. If you are unsure, pick the lower level and adjust after a few weeks.
Step 3: set a target for your goal
Losing weight
A pound of body fat is often equated with about 3,500 kcal (about 7,700 kcal per kg). That is a simplification, because the body adapts as you lose weight, but it gives a useful starting point:
- A deficit of 500–600 kcal a day typically leads to about 0.5 kg (1 lb) a week at first. The NHS weight-loss plan is built around this rate.
- For the woman in our example, that means eating about 1,600–1,650 kcal a day.
- Avoid going below your BMR for long periods without medical advice. Very low-calorie diets (under 800 kcal a day) should only be done under supervision.
Maintaining
Eat around your TDEE and track your weight weekly. If it drifts up or down by more than about 1 kg over a month, adjust by 100–200 kcal.
Gaining muscle
A small surplus of 200–300 kcal a day combined with strength training limits fat gain while supporting muscle growth.
Protein basics
Protein helps preserve muscle when eating less and keeps you fuller.
- The UK reference nutrient intake is 0.75 g per kg of body weight a day for adults; the US RDA is 0.8 g/kg.
- Many sports nutrition bodies suggest about 1.2–2.0 g/kg for people who train regularly or are in a calorie deficit.
- Example: a 70 kg person needs at least 52.5 g (UK RNI), or roughly 84–140 g if training hard.
- Spread protein across meals; good sources include fish, poultry, eggs, dairy, pulses, tofu and nuts.
Protein and carbohydrate provide about 4 kcal per gram, fat about 9 kcal per gram and alcohol about 7 kcal per gram.
Reference values and food labels
- UK and EU food labels use reference intakes based on 2,000 kcal a day for an average adult. The NHS gives about 2,000 kcal for women and 2,500 kcal for men as general guidance.
- US labels also use 2,000 calories as the Daily Value reference.
- “Calories” on labels are kilocalories (kcal). 1 kcal = 4.184 kJ.
Limits of calorie formulas
- Formulas are estimates; individual metabolism can differ by 10% or more.
- They do not account well for very muscular people or those with obesity; body-composition based formulas such as Katch-McArdle may suit athletes better.
- Fitness trackers often overestimate calories burned during exercise.
- Medical conditions (thyroid disorders), medications and pregnancy change needs.
The best approach is to use the estimate as a starting point, track your weight for two to four weeks, then adjust.
Checklist
- Calculate BMR with Mifflin-St Jeor using accurate weight and height.
- Choose an honest activity factor to get TDEE.
- For weight loss, subtract about 500 kcal; for muscle gain, add 200–300 kcal.
- Hit at least 0.75–0.8 g protein per kg, more if training.
- Reassess every few weeks as your weight changes.
To see where your weight sits on the scale, read our BMI guide or try the BMI calculator.