Resting energy before activity
Mifflin–St Jeor first estimates resting energy from mass, height, age, and a sex constant: 10 times weight in kilograms plus 6.25 times height in centimetres minus 5 times age, then plus 5 for the displayed male setting or minus 161 for the displayed female setting. That figure is multiplied by the selected activity factor. It is an estimate, not a metabolic measurement.
The default calculation
For the default 32-year-old woman at 68 kg and 165 cm, resting energy is 10 × 68 + 6.25 × 165 − 5 × 32 − 161 = 1,390.25 kcal/day. At 1.55 activity, 1,390.25 × 1.55 is 2,154.9, shown as 2,155 kcal/day. The arithmetic is checkable; whether 1.55 represents ordinary life is the uncertain part.
Activity is an assumption
Walking at work, standing, commuting, childcare, recovery days, and unplanned movement all affect daily expenditure. A gym session alone cannot select a multiplier. A wearable and this equation can disagree without either calculation being wrong because they infer activity from different incomplete information.
Food intake is not energy use
Two menus with equal calories can differ in protein, fibre, alcohol, sodium, micronutrients, timing, and satiety. The output cannot choose meal composition or prove maintenance intake. A food log and a body-weight trend over several weeks give a better reality check than one day’s scale movement.
When a population equation is too narrow
Growth, pregnancy, breastfeeding, fever, recovery, thyroid disease, medication changes, and disordered eating can make a generic equation a poor basis for a personal plan. Use this number as a tentative range only. Nutrition or medical concerns affecting symptoms or treatment need individual professional advice.
Check the arithmetic in context
At 1.35 activity the default resting estimate becomes about 1,877 kcal/day; at 1.75 it becomes about 2,433. The 556 kcal spread is much larger than a rounding difference, so the activity assumption matters more than displaying a whole-number result.
Keep the date, measured weight, height unit, age, equation setting, and activity assumption with the output. If observed intake and trend consistently differ from the scenario, revise the assumption or seek guidance; do not claim a formula has measured an individual metabolism.
It is informational only and is not medical or professional advice.