BMR Calculator
Basal Metabolic Rate — the calories your body burns at complete rest. Foundation for any calorie plan.
Total Daily Energy Needs (BMR × Activity)
What BMR Actually Means
Basal Metabolic Rate (BMR) is the calories your body would burn over 24 hours if you stayed completely at rest, lying still, after a full night's sleep, in a thermoneutral environment. It powers your heart, brain, kidneys, lungs, body temperature — all the involuntary metabolic activity. For most adults, BMR accounts for 60-70% of total daily energy expenditure (TDEE). The remaining 30-40% comes from activity, digestion (thermic effect of food), and non-exercise movement (NEAT). See Wikipedia on BMR for the physiology details.
The Mifflin-St Jeor Equation
The most accurate widely-used equation for the general population, derived in 1990 by Mifflin et al. and validated in numerous peer-reviewed studies:
Men: BMR = 10 × kg + 6.25 × cm − 5 × age + 5
Women: BMR = 10 × kg + 6.25 × cm − 5 × age − 161
The Mifflin-St Jeor equation replaced the older Harris-Benedict equation (1919), which overestimates BMR by 5-10% in modern populations because body composition has shifted over the past century. The Wikipedia Harris-Benedict article covers the historical equations. For very lean or very muscular individuals who know their body-fat percentage, the Katch-McArdle equation (which uses lean body mass) is more accurate.
Two 40-year-old men, same height, same weight, different composition
| Attribute | Sedentary Office Worker | Trained Athlete |
|---|---|---|
| Height | 180 cm | 180 cm |
| Weight | 85 kg | 85 kg |
| Body fat % | 28% | 12% |
| Lean body mass | 61.2 kg | 74.8 kg |
| Mifflin-St Jeor BMR | 1,780 cal | 1,780 cal |
| Katch-McArdle BMR | 1,716 cal | 1,997 cal |
The standard Mifflin-St Jeor gives identical numbers — it doesn't see body composition. Katch-McArdle correctly captures the ~280-cal/day gap that comes from extra muscle. For athletes, fitness models, and anyone with body composition far from average, use Katch-McArdle if you know your body fat %.
From BMR to Calorie Goals
BMR alone isn't your calorie target — you move, work, exercise, digest food. Multiply BMR by an activity factor to get TDEE (Total Daily Energy Expenditure):
| Activity Level | Multiplier | Description |
|---|---|---|
| Sedentary | × 1.2 | Desk job, no exercise |
| Lightly active | × 1.375 | Light exercise 1-3 days/week |
| Moderately active | × 1.55 | Moderate exercise 3-5 days/week |
| Very active | × 1.725 | Hard exercise 6-7 days/week |
| Athlete / physical job | × 1.9 | Twice/day training or manual labor |
For weight management: 500 calories below TDEE ≈ 1 lb/week loss; 500 above ≈ 1 lb/week gain; at TDEE = maintenance. For more on this, see our calorie calculator.
Why BMR Drops With Age (and What to Do)
Lean muscle mass declines by roughly 1% per year after age 30 in inactive adults — a process called sarcopenia. Muscle is metabolically active; fat is mostly inert. Less muscle = lower BMR. Combined with hormonal shifts (testosterone in men, estrogen in women), BMR typically drops 1-2% per decade after 30 without intervention.
The single best counter: resistance training. Adults who lift 2-3 times per week from age 30 onward maintain 80-90% of peak muscle mass into their 60s. The CDC's physical activity guidelines recommend muscle-strengthening activity on 2+ days per week for exactly this reason.
Frequently Asked Questions
How accurate is the Mifflin-St Jeor equation?
±10% for typical adults — meaning a calculated BMR of 1,500 could actually be anywhere from 1,350 to 1,650 for any specific person. People at the extremes (very lean, very obese, very athletic, very old) get less accurate estimates. The gold standard is indirect calorimetry in a metabolic lab, but it's rarely necessary outside research.
Why does BMR drop with age?
Muscle mass decline (sarcopenia), hormonal shifts, and reduced cellular metabolic rate all contribute. Resistance training and adequate protein (1.2-1.6 g/kg body weight after age 50) are the strongest counter-measures.
Can I increase my BMR?
Yes — the most effective lever is building muscle. Every pound of muscle burns roughly 6-10 calories per day at rest (the exact number is debated). Building 10 lb of muscle over a year could raise BMR by 60-100 cal/day. Other things like green tea or "metabolism-boosting" supplements have negligible effect.
Does eating frequently boost BMR?
No — the popular "6 small meals stoke the metabolism" claim isn't supported by research. Total calorie intake matters; meal frequency mostly doesn't. Studies show no significant TDEE difference between 3 and 6 meals/day at the same total calories.
Why does my smartwatch show a different number?
Smartwatches estimate using heart-rate data and proprietary algorithms that include activity. They typically report TDEE, not BMR, and they're often off by 15-25% in either direction. Use them for relative comparisons over time, not absolute numbers.