What basal metabolic rate (BMR) is
Your basal metabolic rate is the energy your body spends just to stay alive while you are awake and at rest: breathing, circulating blood, keeping body temperature stable and maintaining cells. For most people it is the largest single part of daily energy use, often more than half of the total, which makes it the natural starting point when planning how much to eat. Strictly, BMR is measured after an overnight fast, lying quietly, by analyzing the gases you breathe. Resting metabolic rate (RMR) is measured under slightly looser conditions, but in everyday use the two terms are treated as the same thing. Since that kind of lab test is not practical for most people, prediction equations estimate it from weight, height, age and sex.
The equation this calculator uses: Mifflin-St Jeor
Published in 1990 by Mifflin, St Jeor and colleagues in the American Journal of Clinical Nutrition, the equation was derived from measurements on healthy adults, and it is widely used in nutrition practice because it tends to perform well for both normal-weight and overweight adults.
- Men: BMR = 10 Γ weight (kg) + 6.25 Γ height (cm) β 5 Γ age + 5
- Women: BMR = 10 Γ weight (kg) + 6.25 Γ height (cm) β 5 Γ age β 161
Each extra kilogram adds 10 kcal, each centimeter of height adds 6.25 kcal, and each year of age subtracts 5 kcal. The +5 versus β161 constants reflect the fact that, on average, men carry more lean mass at the same size. They do not mean every man burns exactly 166 kcal more than every woman, and a muscular woman may well burn more than the formula says.
Other equations for comparison
- Harris-Benedict (revised): the 1919 original was revised in 1984 by Roza and Shizgal. Men: 88.362 + 13.397 Γ weight + 4.799 Γ height β 5.677 Γ age. Women: 447.593 + 9.247 Γ weight + 3.098 Γ height β 4.330 Γ age. It appears in a great deal of older material and tends to give slightly higher numbers than Mifflin-St Jeor.
- Katch-McArdle: BMR = 370 + 21.6 Γ lean body mass (kg). It ignores sex, height and age and instead relies on knowing your body fat percentage, so it can suit muscular or very lean people better. Enter a body fat percentage above and the calculator adds this row.
No equation reproduces an individual's true metabolism. Differences of around 10% between a prediction and a measurement are commonly reported, so treat any figure as a starting point and adjust after watching your actual weight trend for two to four weeks.
From BMR to TDEE: activity multipliers
TDEE (total daily energy expenditure) adds the energy used in daily movement, exercise and digesting food. This calculator multiplies BMR by an activity factor from the five levels below. These factors are conventions rather than measured constants, so they can miss your real life by a wide margin.
The table applies them to two example people: A, a 30-year-old man, 175 cm and 70 kg (BMR 1,649 kcal), and B, a 45-year-old woman, 160 cm and 55 kg (BMR 1,164 kcal). All values were calculated when this page was built.
| Activity level | Multiplier | A: TDEE | B: TDEE |
|---|---|---|---|
| Little or no exercise (mostly seated, few steps) | 1.2 | 1,979 kcal | 1,397 kcal |
| Light exercise (1-3 sessions a week) | 1.375 | 2,267 kcal | 1,601 kcal |
| Moderate exercise (3-5 sessions a week) | 1.55 | 2,556 kcal | 1,804 kcal |
| Hard exercise (6-7 sessions a week) | 1.725 | 2,844 kcal | 2,008 kcal |
| Very hard exercise (physical job or two sessions a day) | 1.9 | 3,133 kcal | 2,212 kcal |
The most common mistake is overrating activity. Someone who trains four times a week but sits at a desk the rest of the day is usually closer to "light" than "moderate". Judge from your daily step count, the physical demands of your job and the hours of training together, and when in doubt choose the lower level.
BMR at 170 cm by age and weight
This reference table shows how the Mifflin-St Jeor result moves with age and weight at a fixed height of 170 cm (kcal per day).
| Age | M 50 kg | M 60 kg | M 70 kg | M 80 kg | F 50 kg | F 60 kg | F 70 kg | F 80 kg |
|---|---|---|---|---|---|---|---|---|
| 20 | 1,468 | 1,568 | 1,668 | 1,768 | 1,302 | 1,402 | 1,502 | 1,602 |
| 30 | 1,418 | 1,518 | 1,618 | 1,718 | 1,252 | 1,352 | 1,452 | 1,552 |
| 40 | 1,368 | 1,468 | 1,568 | 1,668 | 1,202 | 1,302 | 1,402 | 1,502 |
| 50 | 1,318 | 1,418 | 1,518 | 1,618 | 1,152 | 1,252 | 1,352 | 1,452 |
| 60 | 1,268 | 1,368 | 1,468 | 1,568 | 1,102 | 1,202 | 1,302 | 1,402 |
Every ten years of age lowers the estimate by 50 kcal, and every 10 kg of weight raises it by 100 kcal. The age effect is a straight line in the equation, but in real life much of the decline comes from losing muscle, which regular resistance training can slow considerably.
Worked examples
Example 1: man, 30, 175 cm, 70 kg, moderate exercise
10 Γ 70 + 6.25 Γ 175 β 5 Γ 30 + 5 = 700 + 1,093.75 β 150 + 5 = 1,648.75, about 1,649 kcal (BMR). Multiplying by 1.55 gives 2,555.6, about 2,556 kcal (TDEE). The revised Harris-Benedict equation gives 1,695.7 kcal for the same person, about 47 kcal higher. With a body fat of 20% the lean mass is 56 kg, and Katch-McArdle gives 370 + 21.6 Γ 56 = 1,579.6 kcal, a little lower. A spread of about 100 kcal between equations is normal.
Example 2: woman, 45, 160 cm, 55 kg, light exercise
10 Γ 55 + 6.25 Γ 160 β 5 Γ 45 β 161 = 550 + 1,000 β 225 β 161 = 1,164 kcal (BMR). Times 1.375 gives 1,600.5, about 1,601 kcal (TDEE). A 500 kcal daily cut would land at roughly 1,101 kcal, which is below her BMR of 1,164. In a case like this a smaller deficit of 300 kcal or less, or more daily movement, is the more realistic route.
Example 3: man, 25, 180 cm, 80 kg, light exercise
10 Γ 80 + 6.25 Γ 180 β 5 Γ 25 + 5 = 800 + 1,125 β 125 + 5 = 1,805 kcal (BMR). With the 1.375 factor the TDEE is 2,482 kcal. A 300 kcal reduction gives 2,182 kcal a day, comfortably above BMR, and at the common rule of thumb of about 7,700 kcal per kilogram of body fat it corresponds to roughly 0.27 kg a week.
Using the numbers for cutting or bulking
- Maintain: eat about your TDEE.
- Lose weight: a deficit of 300 to 500 kcal below TDEE is a commonly used range. With the widely quoted estimate of about 7,700 kcal per kilogram of body fat, 500 kcal a day is roughly 0.45 kg a week. That rule of thumb drifts from reality over longer periods because the body adapts, so use it as a guide, not a promise. The goal table in the calculator uses the same conversion.
- Gain weight: a surplus of 300 to 500 kcal above TDEE, combined with resistance training and enough protein, helps the gain be mostly lean mass.
- Check the floor: if your target intake falls below your BMR, the risk of nutrient shortfalls and muscle loss rises, so reduce the deficit. The calculator shows a warning when that happens.
Limits, and when not to use these numbers
- Pregnancy or breastfeeding: energy needs are different and are not covered by the standard equations.
- Under 18: growth adds energy needs, and adult equations do not fit. Use pediatric guidance.
- Thyroid conditions, diabetes, eating disorders or medications that affect metabolism: true expenditure can differ greatly from the formula.
- Very lean, very heavy or highly muscular bodies: prediction errors tend to be larger.
- Very low calorie plans: do not follow intakes such as under 1,200 kcal a day without professional supervision.
Turning the result into a plan
- Eat at your TDEE for two weeks and record your weight at the same time of day. If it holds steady, your maintenance estimate is right; if it drifts, adjust by 100 to 300 kcal and test again.
- If weight loss stalls, consider adding walking or training before cutting food further.
- Once calories are set, use the Daily Protein Intake Calculator to set a protein target, then divide the remaining calories between carbohydrate and fat.
- If you suspect you are much more or less muscular than average, enter a body fat percentage to compare the Katch-McArdle estimate.
Please note
This calculator provides reference information from statistical equations and does not replace medical or dietitian advice. If you have a chronic condition or plan a large change in weight, speak with a physician or registered dietitian. Your inputs are processed in your browser only. Activity multipliers are conventional values, and official dietary reference intakes in your country may differ, so check national guidance for formal recommendations.
Frequently Asked Questions
Is this result an exact medical figure?
No. It is a statistical estimate, and individual results can differ by about 10% or more. Real BMR can only be measured with specialized testing such as respiratory gas analysis. Use the number as a starting point and refine it using your actual weight trend.
Why is the formula different for men and women?
On average men carry more lean mass than women of the same size, which raises BMR, so Mifflin-St Jeor uses different constants. A woman with a lot of muscle may burn more than the formula suggests.
Will I lose weight faster by eating below my BMR?
Short-term the scale may fall faster, but muscle loss, nutrient shortfalls, fatigue and rebound eating become more likely. The usual advice is to keep intake from dropping below BMR, even while cutting.
How should I choose an activity level?
When unsure, pick the lower one. Think about steps per day and your job, not only gym time. After two to three weeks at the same intake, a stable weight means the level is right; if you gain or lose, move it up or down a step.
Does metabolism really slow with age?
The equation subtracts 5 kcal per year, but that is an average trend. A large part of the decline comes from losing muscle, which strength training and adequate protein can help limit.
What changes when I enter body fat percentage?
The calculator computes lean mass (weight minus fat) and adds a Katch-McArdle row. That figure tends to be higher than Mifflin-St Jeor for muscular people and lower for people with more body fat. The Body Fat Calculator can give you an estimate.