Advanced options
Mifflin-St Jeor estimate. Individual metabolism varies; this is general information, not medical or dietetic advice.
What The BMR Calculator Works Out
- Your BMR: the energy your body burns doing nothing at all, keeping your heart beating, lungs breathing, brain running and temperature steady. It is typically 60% to 70% of everything you burn in a day. The default formula is Mifflin-St Jeor (10 x weight in kg + 6.25 x height in cm - 5 x age, plus 5 for men or minus 161 for women), the one dietitians generally use; a 30-year-old 80 kg, 180 cm man gets about 1,780 kcal.
- Rough maintenance range: BMR multiplied by 1.2 (sedentary) up to 1.725 (very active), the span of daily calories most adults need to hold their weight. BMR rises with weight and height and falls with age, and men average higher than women largely through muscle mass.
- Headline formula (advanced): switch to the revised Harris-Benedict equation (1984), which usually reads 50 to 150 kcal higher, or to Katch-McArdle, which works from lean mass and so needs a body fat percentage. Mifflin-St Jeor has been found most accurate for modern populations.
- Body fat % (advanced): unlocks the Katch-McArdle estimate (370 + 21.6 x lean mass in kg). Typical UK adult ranges are roughly 15% to 25% for men and 25% to 35% for women.
- Your activity level (advanced): picks the single multiplier that matches you, giving a daily and weekly maintenance figure instead of a range. Most office workers who train two or three times a week sit at "lightly active".
- Imperial entry (advanced): type weight in stones and pounds or height in feet and inches and the calculator converts them (1 stone = 6.35 kg, 1 inch = 2.54 cm), overriding the metric boxes. The "inputs used" card shows exactly what went into the sums.
- Activity table (advanced): maintenance calories at all five levels, plus a 500 kcal daily deficit (about 0.5 kg of fat loss a week) and a 250 kcal surplus for gradual gain. The deficit column never drops below BMR, because eating below BMR is rarely sensible without professional supervision.
- Caveats: muscle burns more at rest than fat, so strength training nudges BMR up over time, while crash dieting pulls the other way. These are estimates only; for personal dietary guidance speak to a GP or registered dietitian.
Source and further reading: NHS: managing a healthy weight.