Nutrition & Body Composition
BMR Calculator (Mifflin-St Jeor & Harris-Benedict)
Estimate your Basal Metabolic Rate — the energy your body uses at complete rest — using the Mifflin-St Jeor or revised Harris-Benedict equation. Results update automatically as you enter your details.
Calculate Your BMR
Estimated TDEE by Activity Level
Resting energy expenditure typically accounts for a substantial share of daily energy needs, though the exact proportion varies with activity level. Here is what your estimated total daily expenditure would look like at three common activity levels:
This calculator provides estimates for adults ages 18–90 and is not medical advice. Predictive equations can differ from measured resting energy expenditure.
What Is Basal Metabolic Rate (BMR)?
Basal Metabolic Rate (BMR) is an estimate of how much energy your body uses at complete rest to support essential functions — breathing, circulation, cell production, and maintaining normal body temperature.
BMR is only one part of your total daily energy needs. Movement, exercise, digestion, and other activities require additional energy on top of BMR. That is why BMR should never be treated as your maintenance calorie target.
This calculator estimates BMR using two widely used predictive equations: the Mifflin-St Jeor equation (1990) and the revised Harris-Benedict equation (Roza & Shizgal, 1984).
How RepMaths Calculates Your BMR
This calculator is a predictive tool, not a measurement. Here is exactly how the number is produced.
- Inputs. You provide age, sex, body weight, and height. Weight and height are read in either imperial (lb, ft-in) or metric (kg, cm) units, depending on your selection.
- Unit conversion. Internally, values are stored in
canonical metric units (kg and cm). When you switch unit systems,
the calculator converts using
1 lb = 0.45359237 kgand1 in = 2.54 cm, then keeps that canonical value in memory so that repeated toggles do not accumulate rounding error. If you edit a converted field manually, the canonical value updates from your new input. - Equation. The Mifflin-St Jeor or revised Harris-Benedict equation is applied using the exact canonical values, not the rounded display values.
- Rounding. The final result is rounded to the
nearest whole kcal/day. Kilojoule output is derived from the rounded
kcal value using
1 kcal = 4.184 kJ. - Limitations. The equations are population-level predictions. They do not directly measure your individual metabolic rate, and they do not adjust for body composition (lean vs fat mass), hormonal status, or medical conditions.
How to Calculate BMR — Formula and Worked Example
BMR equations use body weight, height, age, and sex to predict resting energy expenditure. The Mifflin-St Jeor equation is widely considered the more accurate predictive formula for modern adults, and the Academy of Nutrition and Dietetics has endorsed it as the preferred equation for clinical use.
BMR = 10 × weight(kg) + 6.25 × height(cm) − 5 × age(y) + 5 Mifflin-St Jeor — Women
BMR = 10 × weight(kg) + 6.25 × height(cm) − 5 × age(y) − 161 BMR = 13.397 × weight(kg) + 4.799 × height(cm) − 5.677 × age(y) + 88.362 Revised Harris-Benedict — Women
BMR = 9.247 × weight(kg) + 3.098 × height(cm) − 4.330 × age(y) + 447.593 Worked Example (Mifflin-St Jeor)
Consider Mike, a 30-year-old male who weighs 175 lb (79.4 kg) and is 5′10″ (177.8 cm) tall.
- Convert to metric:
175 lb ÷ 2.2046 = 79.4 kg;5′10″ = 70 in = 177.8 cm - Apply the Mifflin-St Jeor equation for men:
BMR = (10 × 79.4) + (6.25 × 177.8) − (5 × 30) + 5 - Solve step by step:
BMR = 794 + 1,111.25 − 150 + 5
BMR ≈ 1,760 kcal/day
The same inputs with the revised Harris-Benedict equation produce approximately 1,830 kcal/day. The two equations can produce different estimates depending on individual inputs, which is why the Mifflin-St Jeor estimate is generally preferred for planning purposes.
The live calculator works with the exact values you enter, so your result may differ slightly from the rounded example above.
Typical BMR Ranges for Adults
The following reference values show typical BMR ranges for healthy non-athlete adults at common body weights. Individual BMR depends on body size, body composition, and other factors, so use these as reference points — not strict targets.
| Group | Example weight | Typical BMR range |
|---|---|---|
| Adult male, 20–40 years | 175 lb (79.4 kg) | 1,700–1,900 kcal/day |
| Adult male, 40+ years | 175 lb (79.4 kg) | 1,550–1,750 kcal/day |
| Adult female, 20–40 years | 140 lb (63.5 kg) | 1,350–1,500 kcal/day |
| Adult female, 40+ years | 140 lb (63.5 kg) | 1,250–1,400 kcal/day |
Athletes and very muscular individuals often have BMR values above these ranges. Values below the ranges are not automatically a medical issue, but persistent concerns are worth discussing with a qualified healthcare professional.
BMR vs TDEE vs RMR — What Is the Difference?
BMR, RMR, and TDEE all describe energy expenditure, but at different levels of activity and measurement strictness.
| Metric | What it measures | Typical share of TDEE |
|---|---|---|
| BMR (Basal Metabolic Rate) | Energy at complete rest under strict conditions | ~60–70% |
| RMR (Resting Metabolic Rate) | Energy at rest under relaxed conditions | ~60–75% |
| TDEE (Total Daily Energy Expenditure) | Total daily energy including activity and digestion | 100% |
Predictive equations like Mifflin-St Jeor technically estimate RMR rather than strictly measured BMR, but the difference is small (usually under 10%) and the terms are often used interchangeably in consumer calculators.
To estimate your total daily energy needs, use our TDEE Calculator.
Factors That Affect Your BMR
- Body size and weight: Larger bodies generally require more resting energy.
- Height: Height is a variable in every predictive BMR equation.
- Age: Mifflin-St Jeor subtracts 5 kcal/day for each year of age, reflecting typical declines in lean mass and metabolic activity over time.
- Sex: Predictive equations use different constants for men and women because of average differences in body composition.
- Body composition: Lean tissue is more metabolically active than fat tissue, so muscle mass affects resting energy needs.
- Individual variation: Genetics, hormones, and health conditions can shift measured BMR away from any predictive equation's output.
Because of this variation, two people with identical height, weight, age, and sex can still have different real-world resting energy needs.
Using BMR as a Starting Point for Weight Loss
BMR by itself should not be used as a weight-loss calorie target. Eating below your BMR for extended periods may not meet your energy or nutrient needs and is generally not recommended without professional guidance.
A more practical approach is to estimate your TDEE first, then apply a moderate calorie deficit. Use the Calorie Deficit Calculator to plan a daily target and projected timeline, and the Macro Calculator to split calories between protein, carbohydrate, and fat.
Calculator results are estimates. Real-world weight trends over several weeks are the most useful feedback for adjusting your intake.
Sources & Methodology
- Mifflin et al. (1990) — A new predictive equation for resting energy expenditure in healthy individuals, Am J Clin Nutr
- Roza & Shizgal (1984) — The Harris-Benedict equation reevaluated, Am J Clin Nutr
- Frankenfield et al. (1998) — Comparison of predictive equations for resting metabolic rate, J Am Diet Assoc
- NIH StatPearls — Basal Metabolic Rate (physiology reference)
Frequently Asked Questions
What is the difference between BMR and TDEE?
BMR estimates the energy your body uses at complete rest. TDEE (Total Daily Energy Expenditure) estimates your total daily energy needs after adding movement, exercise, and digestion. BMR is typically a large share of TDEE — often cited as 60–75% for most adults — but the exact share varies with activity level. Use our TDEE Calculator for a complete daily calorie estimate.
What is the difference between BMR and RMR?
BMR (Basal Metabolic Rate) and RMR (Resting Metabolic Rate) both describe resting energy expenditure, but BMR requires stricter measurement conditions. In practice, RMR tends to be slightly higher (about 5–10%) and is what most predictive equations like Mifflin-St Jeor actually estimate.
Which BMR formula is more accurate — Mifflin-St Jeor or Harris-Benedict?
The Mifflin-St Jeor equation (1990) is generally considered the more accurate predictive formula for modern adults. The Academy of Nutrition and Dietetics has endorsed it as the preferred equation for clinical use. In the original Mifflin-St Jeor comparison research, the older Harris-Benedict equation tended to overestimate resting energy needs by roughly 5%.
What is a good BMR?
There is no single BMR value that is good or bad for everyone. BMR varies with body size, height, weight, age, sex, and body composition. Typical adult BMRs range from about 1,200–1,500 kcal/day for women and 1,500–1,900 kcal/day for men, but individual variation is wide. A calculator estimate should be interpreted in context, not as a health score.
Is a BMR of 1400 good or bad?
A BMR of 1400 kcal/day is a common resting energy value for an average-sized adult female. It simply means the body burns about 1400 calories per day just staying alive at rest. It is not inherently healthy or unhealthy — BMR alone is not a health rating.
How accurate are online BMR calculators?
Validation studies often show predictive BMR equations accurate within about 10% for average-weight, healthy adults. Accuracy tends to be lower for very muscular individuals, people with obesity, and some clinical populations. If you need a precise measurement, resting energy expenditure can be measured with indirect calorimetry in a clinical or research setting.
Should I eat my BMR or my TDEE to lose weight?
Neither by default. Eating at BMR is usually too low for most adults because BMR excludes daily activity. A more practical approach is to estimate your TDEE, choose a moderate deficit, and then track real-world progress over several weeks. Use the Calorie Deficit Calculator for a target that fits your goal.
Can I use BMR to calculate calories for weight loss?
BMR alone does not represent your total daily calorie needs. A more useful approach is to estimate your TDEE and use that as a starting point. Then use the Calorie Deficit Calculator to plan a daily target and timeline.
How often should I recalculate my BMR?
Recalculate BMR whenever your body weight changes by about 10 lb (4.5 kg), your age changes, or your body composition shifts significantly. Predictive equations depend on weight, height, age, and sex, so meaningful changes in any of those affect the estimate.
Can I increase my BMR naturally?
BMR is largely determined by body size and composition. The most meaningful lever is increasing lean muscle mass through resistance training, though the effect on total daily calorie needs is usually smaller than commonly claimed. Adequate protein intake, sleep, and overall health also support a normal BMR.
Does building muscle increase BMR?
Yes, muscle tissue is metabolically active. However, the effect of gaining muscle on total daily energy needs is often overstated online. Estimates for the resting energy cost of muscle tissue vary widely across studies — commonly cited values are around 6–10 calories per pound per day, so a realistic 5–10 lb (2.3–4.5 kg) muscle gain might add roughly 30–100 kcal/day to BMR.
Why can my BMR change when I lose weight?
Your estimated resting energy needs decrease as body weight decreases, because a smaller body has less tissue mass to maintain at rest. Beyond that, some people experience a slightly larger drop known as adaptive thermogenesis, where energy expenditure falls a bit more than predicted by weight loss alone.
Can BMR be measured instead of calculated?
Yes. Resting energy expenditure can be measured using indirect calorimetry under controlled conditions in clinical or research settings. Online BMR calculators provide predictive estimates from variables such as age, sex, height, and weight.