What this calculator works out
Basal metabolic rate is the energy your body uses at complete rest — keeping your heart beating, your lungs working and your temperature stable. This calculator estimates it using the Mifflin-St Jeor equation from your weight, height, age and sex.
BMR is not the amount you should eat. It is the floor before any movement at all is added, and for most people it accounts for roughly two-thirds of total daily energy use.
Enter your details
Before you rely on the figure
- Use weight in kilograms and height in centimetres.
- Select the sex term the equation should use. It is a constant offset in the formula rather than a statement about you.
- This is a resting figure. To reach a daily total, multiply it by an activity factor — the Daily Calorie Calculator does this.
- Individual metabolic rates vary by around 10% either side of any equation's prediction, and more in some medical conditions.
Muscle, organs and brain tissue use energy at rest; fat tissue uses very little. Two people of the same weight can differ meaningfully in BMR if one carries more lean mass. Equations based on weight alone cannot see this, which is the main source of their error.
How this calculator works
The Mifflin-St Jeor equation, widely regarded as the most accurate of the simple predictive formulas:
BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age) + s
where s is +5 for men and −161 for womenThe 166-calorie gap between the two constants reflects average differences in body composition rather than anything measured from you directly.
The equation was published in 1990 and validated against indirect calorimetry. It typically predicts within about 10% for most adults, which on a 1,600 calorie BMR is a range of roughly 160 calories.
Worked example: 75 kg, 175 cm, age 40
Using the default figures — 75 kg, 175 cm, 40 years old, male:
- 10 × 75 = 750
- 6.25 × 175 = 1,093.75
- 5 × 40 = 200, subtracted
- Plus the male constant of 5
- BMR: 1,649 kcal/day
That is the energy used lying still for 24 hours. A moderately active person with this BMR uses roughly 2,600 calories a day in total, so activity accounts for around 950 of them. Note the 10% error range: the honest version of this result is "somewhere around 1,500 to 1,800", and any plan built on the precise figure is built on more precision than exists.
Common mistakes
- Eating to your BMR. It excludes all activity, including simply being awake and upright.
- Entering height in metres. The field expects centimetres.
- Treating it as measured. It is predicted from population data with roughly 10% error.
- Assuming it is fixed. BMR falls with age and with weight loss, which is part of why maintaining weight loss is harder than achieving it.
- Using it during pregnancy or breastfeeding. Energy needs are substantially different and the equation does not account for them.
Frequently asked questions
Which equation is best?
Mifflin-St Jeor is generally considered the most accurate of the simple predictive equations for the general population, and it is the one most commonly used clinically. Harris-Benedict is older and tends to overestimate slightly. Katch-McArdle uses lean body mass instead of total weight and is better where body composition is known, which for most people it is not.
Does a slow metabolism explain weight gain?
Rarely to the extent people expect. Measured metabolic rates cluster fairly tightly once body size and composition are accounted for, and genuine metabolic disorders such as an underactive thyroid are diagnosable and treatable. Differences in unconscious daily movement and in how accurately intake is estimated usually account for more variation than metabolic rate does.
Why does BMR fall when I lose weight?
A smaller body needs less energy to maintain, and some lean tissue is usually lost alongside fat. The reduction is real and is one reason maintaining weight loss requires ongoing effort. Resistance training helps preserve lean mass, which limits the fall.
Should I eat below my BMR?
This is not something to decide from a calculator. Very low intakes carry real risks and are used clinically only with supervision. If you are considering significant changes to how much you eat, a GP or a registered dietitian is the right place to take that question.
Is what I enter stored?
No. Your figures are processed in your browser and never transmitted or retained. If thinking about weight, food or body shape is causing you distress, these tools are unlikely to help and may make things harder. Beat, the UK eating disorder charity, runs a free helpline and online support, and a GP can refer you to specialist services.
Related tools
References
- NHS — guidance on energy needs, healthy eating and weight management
- GOV.UK — Scientific Advisory Committee on Nutrition reports on energy requirements
Sources are checked at publication and can change — how I choose and check references.