In short
Your daily calorie need is your basal metabolic rate multiplied by an activity factor. Under the Mifflin-St Jeor equation, BMR is 10×weight + 6.25×height − 5×age − 161 for women and 10×weight + 6.25×height − 5×age + 5 for men. To lose weight, staying roughly 15-20% below that total produces a loss of about 0.3-0.7 kg per week.
Start with basal metabolic rate
Basal metabolic rate (BMR) is the energy your body spends in a day doing nothing at all — breathing, circulating blood, holding its temperature. It accounts for roughly 60-70% of most people’s total daily expenditure.
The equation used most often in clinical practice is Mifflin-St Jeor. Weight is in kilograms, height in centimetres, age in years.
- Women: BMR = (10 × weight) + (6.25 × height) − (5 × age) − 161
- Men: BMR = (10 × weight) + (6.25 × height) − (5 × age) + 5
Then apply an activity factor
To turn BMR into total daily energy expenditure (TDEE) you multiply it by an activity factor. The most common mistake here is overrating yourself: going to the gym three times a week does not make most people "very active", because the rest of the day is spent at a desk.
| Lifestyle | Factor |
|---|---|
| Sedentary — desk job, no regular exercise | 1.2 |
| Lightly active — light exercise 1-3 days a week | 1.375 |
| Moderately active — exercise 3-5 days a week | 1.55 |
| Very active — exercise 6-7 days a week | 1.725 |
| Extremely active — physical job or two sessions a day | 1.9 |
A worked example
Take a woman of 32, 168 cm tall, weighing 72 kg, who works at a desk and walks twice a week.
BMR = (10 × 72) + (6.25 × 168) − (5 × 32) − 161 = 720 + 1,050 − 160 − 161 = 1,449 calories. With an activity factor of 1.375, total daily expenditure is roughly 1,449 × 1.375 = 1,992 calories.
To lose weight she would target 15-20% below that: about 1,600-1,700 calories. That is a daily deficit of 300-400 calories, which corresponds to an average loss of around 0.3-0.4 kg per week.
How big should the deficit be?
A kilogram of fat tissue holds roughly 7,700 calories of energy. In theory a 500-calorie daily deficit means half a kilogram lost per week. In practice the number never lands exactly: BMR falls as you lose weight, water balance masks the weekly scale reading, and the body trims expenditure a little.
Very large deficits look attractive but raise the odds of losing muscle, feeling flat and quitting the plan. Losing 0.5-1% of body weight per week is a reasonable upper limit for most people.
- Daily deficit: 300-500 calories, ideally no more than 20% below your target.
- Protect your protein intake — it is the single most important variable for limiting muscle loss in a deficit.
- Judge progress by the weekly average on the scale, never by one day.
Where the formula gets it wrong
Mifflin-St Jeor is an estimate built on population averages. Individual variation can be around ±10%, so a calculated 2,000 calories might really be 1,800 or 2,200. The formula tends to underestimate expenditure in people carrying a lot of muscle and overestimate it in obesity.
Treat the number as a starting point. Hold the target for two weeks and measure the change; if it is slower than expected, drop the target by 100-150 calories, and if it is faster, raise it. Your own data always beats the equation.
Frequently asked questions
Should I eat back the calories I burn in exercise?
Not if you built exercise into your activity factor — it is already inside your daily target. Choosing the sedentary factor (1.2) and adding workout calories separately is also valid, but remember that fitness trackers tend to overstate how much a session burns.
Why am I not losing weight even though I hit my target?
Three reasons account for most cases: items that never get logged (oil, drinks, tastes while cooking), portions estimated smaller than they were, and water retention hiding the change on the scale. Measure for at least two weeks and look at averages; if the plateau holds, drop the daily target by 100-150 calories.
Would a very low calorie diet get me there faster?
It shows faster loss in the short term, but a substantial share of that loss is water and muscle. Adherence also collapses, and the plan gets abandoned. A sustainable deficit almost always wins over a longer horizon.
Sources
- 1.
Mifflin MD, St Jeor ST, Hill LA, Scott BJ, Daugherty SA, Koh YO A new predictive equation for resting energy expenditure in healthy individuals. The American Journal of Clinical Nutrition, 51(2):241-247, 1990.
- 2.
Frankenfield D, Roth-Yousey L, Compher C Comparison of predictive equations for resting metabolic rate in healthy nonobese and obese adults: a systematic review. Journal of the American Dietetic Association, 105(5):775-789, 2005.
- 3.
Hall KD, Sacks G, Chandramohan D, et al. Quantification of the effect of energy imbalance on bodyweight. The Lancet, 378(9793):826-837, 2011.