BMR, TDEE and the difference between them
BMR is what you would burn lying still all day: heart, lungs, brain, temperature regulation, cell repair. It is typically 60–70% of total energy use. TDEE multiplies BMR by an activity factor to include movement, exercise and the energy cost of digesting food.
Mifflin–St Jeor replaced the older Harris–Benedict equation in the 1990s and is now the standard recommendation of the Academy of Nutrition and Dietetics, because it is measurably more accurate for modern populations. It is still a population average: individual metabolisms vary by roughly ±10%, and more in people with unusual body composition.
Choosing an honest activity level
The single biggest source of error in every online calorie calculator is people over-rating their activity. Some guidance:
- Sedentary (1.2) — desk job, mostly seated, under 5,000 steps a day. This is most office workers even if they go to the gym twice a week.
- Lightly active (1.375) — 1–3 moderate sessions a week, or a job on your feet.
- Moderately active (1.55) — 3–5 real training sessions a week, plus general movement.
- Very active (1.725) — 6–7 hard sessions a week, or a physical trade.
- Athlete (1.9) — twice-daily training or heavy manual labour. Very few people belong here.
If in doubt, pick the level below the one you were about to choose.
Making the number work in practice
A calorie target is only useful if you can hit it without living inside a food-tracking app. Four things do most of the work.
- Weigh food for two weeks, then stop. Most people underestimate intake by 20–50% — a finding replicated repeatedly under controlled conditions, and it applies to nutrition professionals too. Two weeks of weighing recalibrates your eye permanently. You do not need to do it forever.
- Protein and fibre first. They are the two most satiating things on the plate. A deficit built on protein, vegetables and whole grains is far easier to sustain than the same deficit built on smaller portions of the same food.
- Weigh yourself daily, judge weekly. Daily weight swings by 1–2 kg on water, salt and digestion alone. The seven-day average is the signal; any single morning is noise.
- Liquid calories are invisible. Juice, lattes, alcohol and smoothies carry real energy with almost no satiety return.
Adjusting when the scale stalls
If your seven-day average has not moved in three weeks, do not slash calories. Drop by 100–150 kcal, or add 2,000 steps a day, and give it another two weeks. Small corrections applied patiently beat large ones abandoned quickly.
And a note on the days you go over: one high day does not undo a week. A 1,000 kcal overshoot on Saturday inside a 3,500 kcal weekly deficit still leaves you in deficit. The people who fail are rarely the ones who overeat occasionally — they are the ones who treat overeating as a reason to stop.
Why 3,500 calories per pound is only roughly true
The classic rule says a 500 kcal daily deficit produces a pound of fat loss per week, because a pound of body fat stores about 3,500 kcal. It works reasonably for the first several weeks, then drifts.
As you lose weight you become smaller, so BMR falls; you also move slightly less without noticing, and the body becomes somewhat more efficient. This is called adaptive thermogenesis, and it is why plateaus are normal rather than a personal failing. The fix is to recalculate at your new weight every 4–6 kg, not to keep cutting indefinitely.
This calculator is general information, not medical advice. If you are pregnant or breastfeeding, under 18, managing a chronic condition, or have a history of disordered eating, please work with a clinician or registered dietitian rather than an online tool.