You enter the same age, weight, and height into three different BMR calculators — and get three different numbers. One says 1,685. Another says 1,810. A third says 1,592. That's a 218-calorie spread, which over a week is the difference between losing weight and staying exactly the same. Here's why this happens, and which number you should actually use.
Bottom line up front: Mifflin-St Jeor (1990) is the most accurate for ~80% of people. Harris-Benedict (1984) tends to overestimate by 5–15%. Katch-McArdle is the best if you know your body fat percentage. When in doubt, use the median of all three — that's what our calculator below does.
Your Basal Metabolic Rate (BMR) is the number of calories your body burns at complete rest — just to keep your heart beating, lungs breathing, and brain running. It's 60–70% of your total daily calorie burn, so getting it right matters.
But here's the thing: there's no single "BMR formula." There are at least a dozen, each developed by different researchers studying different populations at different times. The three you'll encounter most often:
| Formula | Year | Based On | Best For | Known Issue |
|---|---|---|---|---|
| Mifflin-St Jeor | 1990 | 498 people (normal, overweight, obese) | General population, weight loss planning | Slightly underestimates very muscular people |
| Harris-Benedict | 1919 / revised 1984 | 239 young, healthy subjects in 1919 | Historical reference, still widely used | Overestimates by 5–15% in modern populations |
| Katch-McArdle | 1996 | Lean body mass (not total weight) | Athletes, people who know their body fat % | Requires accurate body fat input; poor without it |
Let's use a concrete example: a 30-year-old woman, 65 kg, 165 cm. Here's what each formula gives her:
| Formula | BMR Result | Difference from Median |
|---|---|---|
| Mifflin-St Jeor | 1,404 kcal | −1.3% |
| Harris-Benedict (1984) | 1,489 kcal | +4.7% |
| Katch-McArdle | 1,423 kcal | +0.1% |
| Median (recommended) | 1,423 kcal | — |
That's an 85-calorie gap between the lowest and highest. Over a month, using the wrong number means a 2,550-calorie error — about 0.33 kg (0.7 lbs) of unexpected weight change.
For a larger person — say a 100 kg man — the spread can exceed 200 calories. That's a full meal's worth of error every week.
The original Harris-Benedict study in 1919 measured 239 people — 136 men, 103 women — all young, healthy, and mostly white. Their metabolisms don't represent a 2026 population that's heavier, more sedentary, and more diverse. The 1984 revision improved things but still carries the bias of its original data.
Mifflin-St Jeor, published in 1990, studied 498 people across normal-weight, overweight, and obese categories — a much better representation of today's population. That's why the Academy of Nutrition and Dietetics recommends it as the standard.
Both Mifflin-St Jeor and Harris-Benedict use total body weight. But fat tissue burns far fewer calories than muscle — about 4.5 kcal/kg vs 13 kcal/kg per day. A 90 kg person with 15% body fat has a very different BMR from a 90 kg person with 35% body fat, even though both formulas give them the same number.
Katch-McArdle solves this by using lean body mass instead of total weight. That's why it's more accurate for muscular individuals — and less accurate for people who don't know their actual body fat percentage.
Each formula weights age, height, and weight differently:
This means a 60-year-old and a 20-year-old with the same weight and height will get very different gaps between formulas.
Quick decision guide:
Don't know your body fat %? Use Mifflin-St Jeor. It's the modern standard.
Know your body fat % from a DEXA scan or reliable measurement? Use Katch-McArdle.
Want the safest single number? Use the median of all three. It averages out each formula's bias.
Tracking trends over time? Pick one formula and stick with it. Consistency matters more than absolute accuracy.
Even the most accurate BMR formula only tells you your resting burn. Your actual daily calorie needs (TDEE) depend on how much you move:
| Activity Level | Multiplier | Real-Life Example |
|---|---|---|
| Sedentary | × 1.2 | Desk job, walking <30 min/day |
| Light exercise | × 1.375 | Walking or yoga 1–3 days/week |
| Moderate exercise | × 1.55 | Gym or running 3–5 days/week |
| Heavy exercise | × 1.725 | Intense training 6–7 days/week |
| Athlete | × 1.9 | Daily training + physical job |
Most people overestimate their activity level. If you're unsure, pick one level below what you think you are — the data consistently shows that self-reported activity is inflated by 20–30%.
Get a free personalized report with your BMR (all 3 formulas), TDEE at every activity level, body fat distribution metrics, and ideal weight range — all in one page, with a downloadable PDF.
Get Your Free ReportMyFitnessPal uses Mifflin-St Jeor exclusively. If our recommended number is the median of three formulas, it may differ slightly from MFP's single-formula result. The difference is usually small (under 50 calories), but if you have higher muscle mass, MFP may underestimate your needs.
Yes, but mostly because people lose muscle as they age — not because metabolism inherently "slows down." A major 2021 study in Science found that metabolism stays stable from age 20 to 60 when adjusted for lean mass. The age penalty in BMR formulas is a proxy for the muscle loss that typically happens with aging. If you maintain muscle through resistance training, your actual BMR may be higher than age-adjusted formulas predict.
TDEE. Subtract 300–500 calories from your TDEE (not your BMR) for sustainable weight loss. Eating below BMR for extended periods can trigger metabolic adaptation — your body slows down to conserve energy, making weight loss harder. The exception is medically supervised very-low-calorie diets, which are a different category entirely.