How the BMR formula works, step by step
Follow a complete Mifflin–St Jeor example and see exactly what the calculator can—and cannot—tell you.
Start with the question the tool answers
A resting-energy calculator answers a narrow question: what does a prediction equation estimate from the measurements you enter? It does not observe your metabolism. It does not know what you ate yesterday, whether you slept well, or how much you moved. Understanding that distinction makes the output easier to use without treating it as an instruction.
Many websites use the familiar label BMR. This site uses the Mifflin–St Jeor equation, whose published outcome is resting energy expenditure, or REE. A true basal measurement requires controlled conditions. We retain the searchable BMR label but identify the estimated quantity beside the result.
Work through an illustrative example
Take a fictional adult aged 30, weighing 70 kg and measuring 175 cm, using the male equation coefficient. First multiply 70 by 10 to get 700. Next multiply 175 by 6.25 to get 1,093.75. Multiply the age, 30, by 5 to get 150. Combine those terms and add 5: 700 + 1,093.75 − 150 + 5 = 1,648.75 kcal/day.
The displayed result rounds to 1,649. With the female coefficient, replace +5 with −161; the same illustrative measurements give 1,482.75. These are mathematical examples, not reference targets for other people. The original equation offers two sex-specific coefficients; it does not model every variation in body composition or hormonal context.
Keep the inputs and units together
Before comparing results, write down the age, height, weight, units, and chosen coefficient. Enter 175 in a centimetres field, not 1.75. Enter 70 in a kilograms field, not 154.3. A plausible-looking result is not proof that the units were correct. If you switch to another website, check all five items again.
The original study included healthy participants aged 19–78. This tool deliberately restricts energy estimates to that age range. Its height and weight limits are input safeguards, not a statement that every combination inside those limits has been clinically validated.
Read the result without turning it into a diet
A resting estimate is not a daily food allowance, a universal minimum intake, or a test for a “fast” or “slow” metabolism. Daily activity and other energy costs belong to a different question. Use the TDEE tool to inspect an activity scenario, remembering that its multiplier adds another assumption.
If your purpose is treatment, pregnancy or breastfeeding nutrition, recovery from illness, or support with an eating disorder, use advice from a qualified professional who can consider the full situation. The useful outcome of this exercise is understanding the calculation—not deciding that a precise-looking number must be followed.