How your daily calories are calculated
What is a daily calorie target?
It is an estimate of how much energy your body uses in a day, adjusted for what you want your weight to do. It is built in three steps: work out what your body burns at rest, scale that up for how active your daily life is, then add or subtract an amount for your goal.
Every step below is an estimate with a known error margin. Treat the output as a starting point you adjust from real results, not a fact about your body.
Step 1 — Resting energy (BMR)
This is what you would burn lying still all day. The most widely validated equation for adults without a body-composition measurement is Mifflin-St Jeor [1]:
s = +5 for men, −161 for women
A 30-year-old man at 82 kg and 180 cm gets a BMR of about 1,800 kcal.
One limitation worth knowing: Mifflin under-predicts by roughly 12% at a BMI of 40 or above. No weight-and-height-only equation does better without measuring body composition, so the honest answer is that targets run slightly low for the heaviest people rather than that a different formula would fix it.
Step 2 — Activity (TDEE)
Multiply resting energy by a factor for how much you move in ordinary life:
| Daily life | Multiplier |
|---|---|
| Desk job, little walking | 1.4 |
| On your feet part of the day | 1.5 |
| Active job or a lot of walking | 1.6 |
| Physically demanding job | 1.75 |
| Heavy manual labour all day | 1.9 |
These multipliers describe your daily life, not your workouts. Most calculators pair a multiplier with a label like "exercises 3–5 times a week", which counts training twice if your workouts are also logged separately. Here they are deliberately non-exercise, and logged training is credited on top.
The values are also higher than the familiar 1.2–1.9 ladder. Doubly-labelled-water measurements put real free-living activity around 1.6–1.75 on average, and the sedentary band tops out near 1.39 [3][4] — a true 1.2 is closer to bed rest than to a desk job.
Our 30-year-old at a desk job: 1,800 × 1.4 ≈ 2,520 kcal. That is maintenance.
Step 3 — Your goal
To lose weight you eat under maintenance. The default pace is 0.5 kg per week for loss and 0.25 kg per week for gain, capped at 1.0 and 0.5 respectively. A kilogram of body fat is treated as roughly 7,700 kcal, so 0.5 kg/week works out to about 550 kcal per day below maintenance.
Two limits then apply, and the higher one wins:
- Never more than 25% below maintenance. For our example that floor is about 1,890 kcal.
- Never below the point where meeting your vitamin and mineral needs from food gets hard — about 1,500 kcal for men and 1,200 for women [5].
If maintenance is already at or under that floor, there is no safe room to cut at all, and the honest advice is more movement rather than less food.
Why does the calculator's timeline rarely come true?
Because the 7,700 kcal-per-kilogram figure is a static model. It assumes your energy needs stay fixed while you lose weight, and they do not — resting energy falls as you get lighter. Measured loss runs roughly 27% behind what the fixed rule predicts [6]. Use it to size a deficit; do not use it to promise a date.
Is this different for children and teenagers?
Yes, fundamentally. Mifflin-St Jeor is fitted on adults and is not valid for a growing body. Under 18, the standard is the IOM Estimated Energy Requirement, which is measured total expenditure plus an explicit allowance for the energy laid down as growth [2].
A growing child should not be given a weight-loss deficit. Restricting a child's calories is associated with both disordered eating and worse long-term weight outcomes, and the paediatric guideline puts clinician-led behaviour treatment at the centre of care [7]. Any weight concern before 18 belongs with a paediatrician who can read growth curves.
What should you actually do with the number?
Treat it as a hypothesis. Eat to it for two to three weeks, weigh yourself consistently, and compare the trend to what was predicted. If your weight is not moving the way the estimate said, the estimate was wrong for you — adjust it. That feedback loop matters more than the precision of the starting equation.
Keep reading
How fast can you safely lose weight?
0.5 kg a week, and why the predicted finish date slips.
How to split your calories into carbs and fat
Protein first, fat floor, carbs absorb the rest.
Sources
- Mifflin MD, St Jeor ST, Hill LA, Scott BJ, Daugherty SA, Koh YO. A new predictive equation for resting energy expenditure in healthy individuals. Am J Clin Nutr 1990;51:241–247.
- Institute of Medicine (now NASEM). Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids, 2005 — EER equations, physical-activity coefficients, and the Acceptable Macronutrient Distribution Ranges.
- Pontzer H, et al. Daily energy expenditure through the human life course. Science 2021;373:808–812.
- FAO/WHO/UNU. Human Energy Requirements, Report of a Joint Expert Consultation, 2004 — physical-activity level (PAL) bands.
- National Heart, Lung, and Blood Institute. Clinical Guidelines on the Identification, Evaluation, and Treatment of Overweight and Obesity in Adults, 1998 — low-calorie-diet ranges.
- Thomas DM, et al. Time to correctly predict the amount of weight loss with dieting. J Acad Nutr Diet 2013 — why the fixed 7,700 kcal/kg rule overestimates real loss.
- American Academy of Pediatrics. Clinical Practice Guideline for the Evaluation and Treatment of Children and Adolescents With Obesity. Pediatrics 2023;151(2):e2022060640.