Basal Metabolic Rate (BMR) Calculator

Mifflin-St Jeor (1990) is generally considered more accurate for modern US adults and is the formula endorsed by the Academy of Nutrition and Dietetics for non-obese individuals. A 2005 systematic review found Mifflin-St Jeor predicted measured RMR within 10 percent in about 82 percent of non-obese adults, versus around 69 percent for the original Harris-Benedict. The 1984 revised Harris-Benedict closed much of that gap but Mifflin-St Jeor still edges it out on average. Both formulas land within roughly 50 to 100 calories of each other for most adults, so for practical diet planning the difference is small. Use Mifflin-St Jeor if you have to pick one; use Harris-Benedict if a clinician or program specifically asks for it.

No, both Harris-Benedict and Mifflin-St Jeor underestimate BMR for muscular individuals because they only use total body weight and do not distinguish lean mass from fat mass. A 200 lb male athlete at 10 percent body fat has 180 lb of lean mass; a 200 lb sedentary male at 30 percent body fat has 140 lb of lean mass. Their BMRs can differ by 200 calories or more even though the equation gives the same answer. If you are clearly more muscular than average, use the Katch-McArdle formula, which uses lean body mass directly; you will need a body fat percentage from DEXA, BodPod, or a high-quality bioimpedance scan. As a quick correction, athletes often run 5 to 10 percent above the Mifflin-St Jeor estimate.

Using the Harris-Benedict revised formula for a 35-year-old 180 lb (81.6 kg) 6'0" (182.9 cm) man: BMR is approximately 1,861 calories per day. Using Mifflin-St Jeor, it is approximately 1,789 calories per day. The gap of about 70 calories is normal between the two equations. Age affects this significantly: at age 25 the same man would have a BMR around 1,917 (Harris-Benedict) or 1,839 (Mifflin-St Jeor); at age 55 it drops to roughly 1,748 or 1,689. To get total daily calorie needs, multiply by an activity factor between 1.2 (sedentary) and 1.9 (extra active).

One pound of body fat stores about 3,500 calories of energy, so a 500 calorie daily deficit (500 x 7 = 3,500) theoretically yields 1 lb of fat loss per week. Start by calculating your TDEE (BMR multiplied by your activity factor), then subtract 500 calories. For a typical 30-year-old US woman, 150 lb, 5'5", lightly active: BMR is about 1,420, TDEE is about 1,953, target intake for 1 lb per week loss is about 1,450 calories per day. In practice, expect 0.5 to 1.5 lb per week actual loss; the 3,500 calorie rule is a useful approximation but adaptive thermogenesis, water shifts, and tracking error all create noise. Do not drop below your BMR without medical supervision, and aim for no more than 1 percent of body weight lost per week to preserve muscle.

BMR (basal metabolic rate) is measured after a 12-hour fast, full overnight sleep, and 30 minutes of supine rest in a thermoneutral lab, capturing the absolute minimum energy needed to stay alive. RMR (resting metabolic rate) is measured under less strict conditions: no overnight fast or pre-test rest required, just sit quietly for a few minutes before testing. RMR runs about 10 percent higher than true BMR because digestion (TEF) and minor activity are still happening. Outside the lab, most calculators and apps use RMR and BMR interchangeably, which is fine for diet planning since the difference is smaller than the equation's prediction error. If a clinical report distinguishes the two, take BMR as the cleaner baseline.

Often, yes. The standard multipliers (1.2 to 1.9) were developed in the late 20th century and many people, especially those tracking moderate gym attendance, overstate their activity. A common pattern: someone with a desk job who works out 4 days a week picks the 1.55 moderately active multiplier, but their non-exercise activity (NEAT) is actually low because they sit for 9 hours. A more conservative starting estimate is to use one tier lower than feels right (e.g., 1.375 instead of 1.55) and adjust based on real-world weight change over 2 to 3 weeks. Fitness trackers can help but their calorie burn estimates have their own 10 to 30 percent error margins, especially during strength training.

Generally not, and not without medical supervision. Eating below BMR for extended periods (weeks to months) is associated with adaptive thermogenesis (your body lowers RMR 5 to 15 percent below predicted to defend against perceived starvation), muscle loss, fatigue, hormonal disruption (lower thyroid output, lower testosterone or estrogen, menstrual irregularity), and increased risk of regaining weight after the diet ends. The Academy of Nutrition and Dietetics recommends a moderate deficit of 500 to 750 calories below TDEE for most adults wanting to lose weight, which usually keeps intake above BMR. Very low calorie diets (VLCDs) below 800 calories total are clinically appropriate in specific circumstances like pre-bariatric surgery prep, but only under physician oversight.

BMR drops roughly 1 to 2 percent per decade after age 20, mostly because of gradual loss of lean muscle mass (sarcopenia) and to a lesser extent reduced organ activity. A 60-year-old will typically have a BMR 100 to 200 calories lower than they did at 25 at the same body weight. The good news: this is largely preventable. Resistance training 2 to 3 times per week preserves and rebuilds muscle, blunting most of the drop. Adequate protein intake (0.7 to 1 g per lb of body weight for active adults) supports muscle maintenance. Cardio supports cardiovascular health but does not preserve BMR the way strength training does. Older adults who lift weights regularly often have BMRs comparable to sedentary 30-year-olds at the same body weight.

Recalculate any time your weight changes by more than 5 to 10 lb, since weight is the largest variable in the equation. Also recalculate on each birthday milestone (every 5 years) to account for age effects. If you are actively dieting or bulking, recalculate every 4 to 6 weeks as your weight changes. The formula itself does not need updating: Harris-Benedict revised (1984) and Mifflin-St Jeor (1990) have not been superseded by any new standard. What does change is your individual response, so treat the calculated number as a starting estimate and adjust based on 2 to 3 weeks of consistent tracking and real weight trend.