Calorie Estimator: How It Works
A calorie target is an estimate of the energy your body uses in a day, adjusted for what you want to happen to your weight. It is a starting hypothesis to test against reality, not a prescription — and this page is as much about how to adjust it as how to calculate it.
How the estimate is built
- BMR — resting energy use, from the Mifflin-St Jeor equation.
- TDEE — BMR multiplied by an activity factor (1.2 sedentary through 1.9 extremely active).
- Goal adjustment — a deficit to lose, a surplus to gain, or TDEE to maintain.
Men: BMR = (10 × kg) + (6.25 × cm) − (5 × age) + 5
Women: BMR = (10 × kg) + (6.25 × cm) − (5 × age) − 161
Sizing a deficit sensibly
Roughly 7,700 kcal is commonly associated with a kilogram of body mass, so a daily deficit of 500 kcal corresponds to about 0.5 kg a week in theory.
| Daily adjustment | Approx. weekly change | Notes |
|---|---|---|
| −250 kcal | ≈ 0.25 kg loss | Slow, easy to sustain, minimal muscle loss |
| −500 kcal | ≈ 0.5 kg loss | The usual recommended pace |
| −750 to −1000 kcal | ≈ 0.75–1 kg loss | Only appropriate at higher starting weights, and harder to sustain |
| +250 to +500 kcal | ≈ 0.25–0.5 kg gain | For gaining, paired with resistance training |
Larger deficits do not reliably produce proportionally better results. They increase muscle loss, hunger and the likelihood of abandoning the attempt, and below roughly 1,200 kcal a day for women or 1,500 for men it becomes difficult to meet micronutrient needs. Very low intakes should only be undertaken with medical supervision.
Why the scale disagrees with the arithmetic
Weight moves day to day for reasons that have nothing to do with fat.
- Water. Each gram of stored glycogen holds about 3 g of water. A change in carbohydrate intake can move the scale 1–2 kg in either direction within days.
- Sodium and hydration. A salty meal shifts fluid balance overnight.
- Digestive contents. Ordinary variation of several hundred grams.
- Menstrual cycle. Fluid retention commonly adds 1–2 kg in the luteal phase.
The consequence is practical: judge progress on a weekly average, not a single morning. A whole week of no movement is within normal noise; three weeks of no movement is data worth acting on.
Why estimates drift
The equations estimate expenditure within about 10%, which on a 2,200 kcal maintenance is ±220 kcal — potentially the entire deficit. Activity multipliers are self-reported and consistently overestimated, and both food labels and portion estimates carry error in the same direction. Expenditure also falls as you lose weight, since a smaller body costs less to maintain, so a target that worked at the start stops working after 5–10% of body weight is lost.
The reliable method is empirical: eat at the estimate for two to three weeks, track weight daily and average it weekly, then adjust by 100–200 kcal based on what actually happened rather than on what the formula predicted.
What the number does not capture
Calories describe energy, not nutrition. Two diets at the same calorie level can differ enormously in protein — which affects how much of any loss is muscle rather than fat — in fibre, micronutrients, and in how full they leave you. Adequate protein and resistance training together are what determine whether weight lost is fat or lean tissue. A calorie target with neither of those in place will still produce weight loss, and a worse body composition at the end of it.
This page is general information, not medical or dietary advice. If you have a health condition, are pregnant or breastfeeding, are under 18, or have a history of disordered eating, speak to a doctor or registered dietitian before changing your intake. If your relationship with food or your body feels distressing, a healthcare professional is the right place to start.