What this calculator does
Basal metabolic rate is the energy your body uses simply staying alive: circulation, breathing, brain function, cell repair and maintaining temperature. It is measured at complete rest, and typically accounts for around 60 to 70 per cent of total daily energy use for most people.
This calculator uses the Mifflin-St Jeor equation, which has been the standard since 1990 and generally outperforms the older Harris-Benedict formula in validation studies. Both are shown so you can see how much the choice of equation matters: usually a few per cent.
The formula
Both equations are regressions fitted to measured metabolic data, with weight, height and age as predictors and separate constants by sex. Weight carries the largest coefficient because lean tissue is the main driver of resting energy use; age enters negatively because metabolic rate declines gradually through adulthood.
| Term | Meaning |
|---|---|
| BMR | Energy used at complete rest over 24 hours, in kilocalories. |
| Mifflin-St Jeor | The modern standard equation, published in 1990. |
| Harris-Benedict | The original 1919 equation, revised in 1984. Tends to read slightly higher. |
The inputs explained
| Field | What to enter |
|---|---|
| Weight (kg) | Body weight in kilograms. |
| Height (cm) | Height in centimetres. |
| Age (years) | Age in years. Resting metabolism declines gradually with age, largely through loss of lean tissue. |
| Sex | Biological sex. The equations use different constants because of average differences in body composition. |
When to use it
Establishing a baseline before anything else
BMR is the floor of your energy needs. Total daily requirements build on top of it through activity, which is what the TDEE calculator adds.
Understanding why energy needs fall with age
Hold everything else constant and change the age. The decline is gradual but real, and much of it reflects lost muscle rather than an inevitable slowing.
Seeing the effect of body size
A larger body costs more to maintain. This is why energy requirements change as weight changes, and why an intake that once created a deficit may eventually stop doing so.
Comparing the two equations
Harris-Benedict typically reads a few per cent higher than Mifflin-St Jeor. Neither is exact for an individual: both are population averages with meaningful variation around them.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
BMR at different body weights
A 30-year-old man of 175 cm at a range of body weights.
| Weight | BMR | Harris-Benedict estimate | Burned per hour at rest |
|---|---|---|---|
| 55 kg | 1,499 kcal/day | 1,495 kcal/day | 62 kcal |
| 65 kg | 1,599 kcal/day | 1,629 kcal/day | 67 kcal |
| 75 kg | 1,699 kcal/day | 1,763 kcal/day | 71 kcal |
| 85 kg | 1,799 kcal/day | 1,897 kcal/day | 75 kcal |
| 95 kg | 1,899 kcal/day | 2,031 kcal/day | 79 kcal |
| 110 kg | 2,049 kcal/day | 2,232 kcal/day | 85 kcal |
How BMR changes with age
The same body at different ages.
| Age | BMR | Harris-Benedict estimate | Kilojoules |
|---|---|---|---|
| 20 years | 1,749 kcal/day | 1,819 kcal/day | 7,317 kJ/day |
| 30 years | 1,699 kcal/day | 1,763 kcal/day | 7,108 kJ/day |
| 40 years | 1,649 kcal/day | 1,706 kcal/day | 6,898 kJ/day |
| 50 years | 1,599 kcal/day | 1,649 kcal/day | 6,689 kJ/day |
| 60 years | 1,549 kcal/day | 1,592 kcal/day | 6,480 kJ/day |
| 75 years | 1,474 kcal/day | 1,507 kcal/day | 6,166 kJ/day |
Questions
What is the difference between BMR and RMR?
Basal metabolic rate is measured under strict conditions: fasted, fully rested, at a controlled temperature. Resting metabolic rate is measured under less stringent conditions and typically reads a few per cent higher. In everyday use the terms are often interchanged.
Which equation should I use?
Mifflin-St Jeor is generally the more accurate for the broad population and is what most practitioners now use. Harris-Benedict is shown for comparison because a great deal of older material still quotes it.
How accurate is a BMR estimate?
These are population regressions, so an individual can sit perhaps 10 per cent either side of the prediction. Body composition is the largest source of variation: the equations use total weight, not lean mass.
Should I eat at my BMR?
No. BMR is what your body uses at complete rest, before any movement at all. Eating at that level would leave a substantial deficit for anyone who gets out of bed. The TDEE calculator gives the more useful figure.
Can I change my BMR?
Somewhat. Building lean muscle raises it modestly, since muscle is more metabolically active than fat. The effect is real but smaller than often claimed: a few tens of kilocalories per kilogram of muscle gained.
To add activity and get your full daily requirement, use the TDEE calculator. For energy used in exercise, see calories burned.