BMR & TDEE Calculator

They were developed on different populations at different times. Harris-Benedict (1919, revised 1984) was calibrated on subjects whose body composition skewed leaner than modern averages, so it tends to overestimate for contemporary bodies. Mifflin-St Jeor (1990) used a more representative sample. Katch-McArdle (1977) bypasses the problem by using lean mass directly — it's the most accurate of the three *if* you have a good BF% measurement. A spread of ±100 kcal across formulas is normal and not alarming.

Sedentary is a desk job with little deliberate movement outside work. Light is 1-3 structured workouts per week. Moderate is 3-5 workouts or a physically active job (nursing, teaching, warehouse). Active is 6-7 workouts or heavy manual labor. Very active is two-a-day training, competitive endurance athletes, or manual labor plus training. Most self-described 'moderately active' office workers are actually 'light' once their weekend is honest.

We strongly recommend against it, and the warning you see when your goal drops below BMR is not decorative. Sustained sub-BMR intake reliably triggers adaptive thermogenesis (your body slowing its own metabolic rate by 10-25%), increased hunger hormones, and preferential loss of lean tissue over fat. The result is a lower plateau weight, worse body composition, and a harder time maintaining losses. A moderate 300-500 kcal deficit sustained for months beats a 1,000 kcal deficit that you crash out of in two weeks. Talk to a registered dietitian before anything more aggressive.

Weight is the dominant input, but body composition matters too. A 75 kg lean athlete has more metabolically active tissue than a 75 kg sedentary person with higher body fat, so the athlete's true BMR is typically 100-200 kcal higher. Mifflin and Harris-Benedict both ignore this — they just see 75 kg. That's why we also compute Katch-McArdle whenever BF% is available: it's the only formula of the three that directly rewards lean mass.

Yes, but less than conventional wisdom says. A landmark 2021 study (Pontzer et al., *Science*) found that adjusted metabolic rate is remarkably stable from age 20 to 60 and only declines meaningfully after 60. The age term in Mifflin-St Jeor reflects population averages (about 5 kcal per year of life), but most of what people perceive as 'slowing metabolism' in their 40s is actually gradual loss of lean mass and reduced NEAT — both addressable with strength training and walking.

Yes. As you lose weight, both BMR and TDEE decrease. Someone who loses 10 kg will burn roughly 100-150 kcal less at rest than they did at the start. If you keep the same deficit target, the rate of loss will gradually slow and eventually stall. Recalculate every 4-5 kg of change and adjust. Many diet failures are this math problem, not a willpower problem.

Reasonably for moderate-training athletes; less so at the extremes. Endurance athletes in heavy training (marathoners during peak weeks, cyclists on tour) routinely burn 4,000-7,000 kcal/day — numbers the 1.9 activity factor ceiling cannot reach. Very muscular strength athletes may find Katch-McArdle more accurate than Mifflin. For serious performance work, pair the calculator estimate with a food diary and a training log, and adjust based on what your body weight actually does across a 4-week block.