How to use this calculator
- Age, height, weight and sex — the four inputs the equation needs.
- Activity level — be honest here. Most people overestimate. Desk job with a few gym sessions is "lightly active", not "very active".
The formula and how the maths works
This uses the Mifflin-St Jeor equation, generally considered the most accurate of the widely used formulas for the general population.
Men: BMR = (10 × weight kg) + (6.25 × height cm) − (5 × age) + 5
Women: BMR = (10 × weight kg) + (6.25 × height cm) − (5 × age) − 161
BMR is then multiplied by an activity factor to give TDEE (Total Daily Energy Expenditure):
| Activity level | Description | Factor |
|---|---|---|
| Sedentary | Desk job, little deliberate exercise | 1.2 |
| Lightly active | Light exercise 1–3 days a week | 1.375 |
| Moderately active | Moderate exercise 3–5 days | 1.55 |
| Very active | Hard exercise 6–7 days | 1.725 |
| Extra active | Physical job plus daily training | 1.9 |
Worked example
A 32-year-old woman, 162 cm, 65 kg, lightly active:
| Step | Working |
|---|---|
| 10 × weight | 650 |
| 6.25 × height | 1,012.5 |
| −5 × age | −160 |
| −161 (female) | −161 |
| BMR | ≈ 1,342 kcal |
| TDEE (× 1.375) | ≈ 1,845 kcal |
So roughly 1,845 kcal a day maintains her current weight at that activity level.
What the number means in practice
TDEE is your maintenance level. Eating consistently around it keeps weight roughly stable.
- To lose weight gradually — a modest deficit of roughly 300–500 kcal below TDEE is the usual recommendation, giving around 0.25–0.5 kg a week.
- To gain weight or build muscle — a similar-sized surplus, combined with resistance training.
If you have a medical condition, are pregnant or breastfeeding, take medication that affects appetite or metabolism, or have any history of disordered eating, please work with a doctor or registered dietitian rather than a calculator. And if thinking about food and numbers has started to feel distressing or all-consuming, that's worth talking to someone about — it's a common experience and support helps.
Why the scale doesn't move in a straight line
Day-to-day weight swings of 1–2 kg are normal and come from water, sodium, glycogen and digestion — not fat. Weigh yourself at the same time under the same conditions, and judge trends over two to three weeks rather than day to day.
Limits of this estimate
- The equation doesn't know your body composition. Two people of the same weight with different muscle mass have genuinely different BMRs.
- Activity factors are broad categories, and the gap between 1.375 and 1.55 is around 200 kcal.
- Thyroid conditions, some medications and certain medical conditions shift metabolic rate meaningfully.
- Calorie labels on food carry their own error margin, often 10–20%.
The practical approach: use the estimate for two to three weeks, track your weight trend, and adjust based on what actually happens rather than what the formula predicted.
Frequently Asked Questions
What is BMR?
Basal Metabolic Rate — the energy your body uses at complete rest to run basic functions like breathing, circulation and cell repair. It's typically 60–70% of total daily energy use.
What is the difference between BMR and TDEE?
BMR is energy used at rest. TDEE adds everything else — movement, exercise and digestion — and is the figure that reflects what you actually need in a day.
Which formula is most accurate?
Mifflin-St Jeor is generally considered the most reliable for the general population and is what this calculator uses. Harris-Benedict is older and tends to overestimate slightly.
How accurate is this estimate?
Individual metabolic rate typically varies 10–15% around the predicted value. Use it as a starting point and adjust based on how your weight actually responds over a few weeks.
How large a deficit should I use to lose weight?
A moderate deficit of roughly 300–500 kcal below maintenance is the usual recommendation. Larger deficits tend to cost muscle and energy and are harder to sustain.
Does metabolism slow with age?
Somewhat, largely because muscle mass tends to decline. Resistance training helps preserve muscle and therefore metabolic rate.
Should I eat back the calories I burn exercising?
If you selected an activity level that already includes your exercise, it's counted. Adding it again on top double-counts. Pick the activity factor that reflects your overall week.
Why is my weight not changing despite a deficit?
Common reasons include water retention, underestimating portions, and normal day-to-day fluctuation. Judge the trend over two to three weeks. If it genuinely doesn't shift over a longer period, it's worth speaking to a doctor or dietitian.
Related calculators and guides
Health disclaimer. This calculator provides general estimates for healthy adults and is not medical advice. It cannot account for your individual health conditions, medications or circumstances. Please speak to a doctor or a registered dietitian before making significant changes to your diet, exercise or fluid intake — particularly if you have any medical condition, are pregnant or breastfeeding, or have any history of disordered eating.