How your calorie needs are worked out
Your daily calorie requirement is built in two steps. First, your Basal Metabolic Rate — the energy your body uses at complete rest to run your heart, brain, kidneys and everything else. For most people this is 60% to 70% of total daily energy use.
Second, BMR is multiplied by an activity factor covering everything else: exercise, walking, fidgeting and digesting food. The result is your Total Daily Energy Expenditure, the intake at which your weight stays stable.
The Mifflin-St Jeor equation
This calculator uses Mifflin-St Jeor, published in 1990 and recommended by the Academy of Nutrition and Dietetics as the most accurate predictive equation for healthy adults. It estimates BMR within 10% for about 82% of people.
Women: BMR = (10 × kg) + (6.25 × cm) − (5 × age) − 161
- kg
- Body weight in kilograms
- cm
- Height in centimetres
- age
- Age in years
The constant differs by sex because men carry more lean mass on average at the same height and weight, and lean tissue is metabolically more active than fat.
Creating a deficit that works
Roughly 7,700 calories equal one kilogram of body fat, or about 3,500 per pound. So a deficit of 500 calories a day predicts about 0.5 kg of loss a week.
In practice loss is slower than the arithmetic suggests, and slows further over time. As you lose weight your BMR falls because there is less of you to maintain, and your body reduces spontaneous movement. This is why plateaus happen at fixed intake and why recalculating every 4 to 6 kg matters.
| Daily deficit | Weekly loss | Suitable for |
|---|---|---|
| 250 kcal | 0.25 kg / 0.5 lb | Small amounts to lose, preserving muscle |
| 500 kcal | 0.5 kg / 1 lb | The standard recommendation for most people |
| 750 kcal | 0.75 kg / 1.5 lb | Higher starting body fat, short term |
| 1,000 kcal | 1 kg / 2 lb | Medical supervision advised |
What this cannot account for
Predictive equations estimate a population average for your measurements. Individual metabolic rates vary by 200 to 300 calories a day at the same height, weight, age and sex, driven by body composition, thyroid function, genetics and past dieting history.
- Body composition. Two people at 75 kg — one at 12% body fat, one at 30% — have meaningfully different BMRs. Mifflin-St Jeor cannot see this. If you know your body fat percentage, the Katch-McArdle equation is more accurate; find yours with the body fat calculator.
- Activity self-assessment. The largest source of error is not the equation but the multiplier people choose. Most overestimate.
- Adaptive thermogenesis. Extended dieting reduces energy expenditure beyond what weight loss alone predicts.
Treat the number as a starting point. Track your intake and weight for two to three weeks, then adjust based on what actually happens. Your own data beats any equation.
Frequently asked questions
How many calories should I eat to lose weight?
Take your TDEE — shown above — and subtract 500 for about 0.5 kg a week. The calculator does this for you once you select a goal.
Do not go below your BMR. If the target would fall under it, choose a gentler rate or increase activity to widen the gap instead.
Is the Mifflin-St Jeor equation accurate?
It is the most accurate of the widely used predictive equations, estimating BMR within 10% for roughly 82% of healthy adults. It outperforms the older Harris-Benedict equation, particularly for people who are overweight.
For anyone with an unusual amount of muscle or a known body fat percentage, Katch-McArdle is better because it works from lean mass directly.
Why am I not losing weight at my calculated deficit?
Most often, intake is higher than believed — studies consistently find people under-report by 20% to 40%, mainly through cooking oils, drinks and untracked bites. Weigh food for a fortnight to establish the truth.
Other causes: an overestimated activity level, water retention masking fat loss on the scale, or metabolic adaptation after a long diet. Judge by a weekly average, not a single weigh-in.
Should I eat back the calories I burn exercising?
No, if you selected an activity level above sedentary — exercise is already included in the multiplier, so eating it back double-counts.
If you selected sedentary and log workouts separately, add roughly half of what your tracker reports. Fitness trackers commonly overestimate calorie burn by 25% to 90%.
How often should I recalculate?
Every 4 to 6 kg of change, or roughly every six weeks. Your BMR falls as you lose weight, so a target set at your starting weight becomes too generous.