Understanding Your Health Numbers: BMI, BMR, and Body Composition Explained
What BMI, BMR, and body fat percentage formulas actually measure, their limitations, and how to interpret your results.
Health calculators reduce complex biology to a few numbers, which makes them useful for tracking trends but risky to over-interpret in isolation. Here's what each one actually measures.
BMI: a screening tool, not a diagnosis
Body Mass Index is calculated as BMI = weight(kg) / height(m)². For someone who weighs 70kg and is 1.75m tall, that's 70 / (1.75)² = 70 / 3.0625 = 22.9, comfortably inside the "healthy weight" range of 18.5–24.9.
BMI was designed in the 1830s as a population-level statistical tool, not an individual diagnostic measure. It doesn't distinguish muscle from fat, which is why a muscular athlete can show as "overweight" despite having low body fat. It's a useful starting point, not a verdict.
BMR: how many calories your body burns doing nothing
Basal Metabolic Rate estimates the calories your body burns at complete rest, just keeping your organs running. The Mifflin-St Jeor equation is the most widely used version:
BMR = 10 × weight(kg) + 6.25 × height(cm) − 5 × age + (5 for men, −161 for women)
A 30-year-old man weighing 70kg at 175cm has a BMR of (10×70) + (6.25×175) − (5×30) + 5 = 700 + 1093.75 − 150 + 5 = 1,648.75 kcal/day. That's the calorie floor before any activity is factored in — multiplying by an activity multiplier (1.2 to 1.9) gives Total Daily Energy Expenditure (TDEE), the number actually useful for calorie planning.
Body fat percentage: circumference vs. imaging
The BMI and scale-weight approach can't tell you what fraction of your weight is fat versus muscle. The U.S. Navy method estimates body fat using waist, neck, and (for women) hip circumference measurements plugged into a logarithmic formula. It's a reasonable field estimate, but DEXA scans and hydrostatic weighing remain more accurate — the circumference method trades precision for something you can measure with a tape measure.
Using these numbers together
No single number tells the whole story. BMI flags a starting point, BMR/TDEE inform calorie targets, and body fat percentage adds context BMI can't provide on its own. Track trends over weeks and months rather than reacting to any single day's number.
- The BMI calculator and ideal weight calculator give quick screening numbers
- The BMR calculator and TDEE calculator turn those numbers into an actual calorie target
- The body fat percentage calculator adds the composition context BMI is missing
None of these replace a conversation with a doctor or registered dietitian, especially if you have specific health conditions.