How this instrument works
Basal metabolic rate is the energy your body spends at complete rest — keeping the heart beating, the lungs filling, the brain running, and every cell maintained. The Mifflin-St Jeor equation estimates it from four things a scale and a tape measure can tell you. Each kilogram of body weight contributes 10 kcal per day, each centimetre of height adds 6.25, each year of age removes 5, and a closing constant — plus 5 for men, minus 161 for women — shifts the whole line to fit each sex.
The equation comes from a 1990 study by Mifflin, St Jeor and colleagues, who measured resting energy expenditure in 498 adults by indirect calorimetry and fitted the simplest line that predicted it well. It displaced the older Harris-Benedict formula of 1919, whose subjects ate, moved, and were built rather differently from anyone alive now. The American Dietetic Association's evidence review later rated Mifflin-St Jeor the most reliable of the common predictive equations, typically landing within 10% of measured values.
The output is a floor, not a budget. Nobody spends a day at true basal conditions — digestion, fidgeting, walking, and exercise all sit on top. To estimate total daily burn, multiply the resting figure by an activity factor between 1.2 (desk-bound) and 1.9 (hard daily training). What this instrument gives you is the fixed part of the ledger: the calories your body claims before you choose to do anything with it.
- Set the Sex toggle first — it selects the equation's closing constant: +5 for male, −161 for female.
- Enter Weight; the unit menu accepts kilograms, pounds, or stone and converts before computing.
- Enter Height in centimetres, metres, inches, or feet — internally the equation works in centimetres.
- Set Age in years, then read BMR (kcal/day); the working block shows each term of the sum.
Worked example — 70 kg, 172 cm, age 30
A 30-year-old man, 70 kg and 172 cm. Weight term: 10 × 70 = 700. Height term: 6.25 × 172 = 1075. Age term: 5 × 30 = 150, subtracted. Add the male constant of 5 and the ledger closes: 700 + 1075 − 150 + 5 = 1630 kcal per day — the cost of simply being him, before a single step is taken.
Flip the Sex toggle and change nothing else: the same 70 kg, 172 cm body at 30 now closes with −161 instead of +5, giving 700 + 1075 − 150 − 161 = 1464 kcal per day. The 166-kcal gap is what the equation attributes, on average, to differences in lean mass between the sexes at equal size.
Questions
Is BMR the number of calories I should eat?
No — it is the resting floor, not a target. Eating exactly your BMR would ignore everything you do while awake. Multiply the figure by an activity factor (1.2 sedentary, 1.375 lightly active, 1.55 moderate, 1.725 very active, 1.9 athlete) to estimate total daily energy expenditure, then adjust from there for your weight goal.
How accurate is the Mifflin-St Jeor equation?
Within about 10% of a metabolic-cart measurement for most healthy adults — the best record among the common predictive formulas, per the American Dietetic Association's evidence review. It drifts for outliers: very muscular people run hotter than predicted, while very obese or elderly individuals can run cooler, because the equation sees weight but not body composition.
Why do men and women get different constants?
Because at the same weight, height, and age, men carry more lean mass on average, and lean tissue burns far more energy at rest than fat does. Rather than demand a body-fat measurement, the 1990 study fitted one offset per sex: +5 for men, −161 for women. It is a statistical average — an unusually muscular woman or a lean, wiry man will sit off the line.
What is the difference between BMR and RMR?
BMR is measured under strict laboratory conditions — fasted twelve hours, fully rested, lying still in a temperature-controlled room. RMR, resting metabolic rate, relaxes those rules and runs a few percent higher. The Mifflin-St Jeor study measured resting energy expenditure, so strictly speaking this instrument predicts RMR; in everyday planning the two terms are used interchangeably.
Does losing weight lower my BMR?
Yes, mechanically: each kilogram lost removes about 10 kcal per day from the weight term. Real bodies often drop a little further than the arithmetic predicts during sustained dieting — adaptive thermogenesis — which is one reason weight loss slows over time. Re-run the calculation as your weight changes and rebuild your intake target on the new figure.
References
Read this first: This instrument computes a screening figure from population formulas — it is not a diagnosis, and it cannot see the whole picture a clinician can. Use it to inform a conversation, not to replace one.