BMI Calculator

What BMI does not measure

Before reading anything else on this page, an honest statement: Body Mass Index is a 19th-century population statistic that doctors still use because nothing as cheap and as easy has replaced it. It is not a measurement of body fat. It is not a measurement of health. It is a ratio of mass to squared height — that is it. The formula was published in 1832 by Adolphe Quetelet, a Belgian astronomer studying average measurements of a population, not individual health. It was renamed "Body Mass Index" in 1972 and adopted as a clinical shortcut because the alternatives (skinfold calipers, DEXA scans, hydrostatic weighing) cost more and take longer.

For most adults in the middle of the bell curve, BMI happens to track body-fat percentage well enough to be useful as a quick screen. For people outside the middle of that bell curve, it can be misleading enough to be actively unhelpful. The calculator above gives you the number; this page tells you when to trust it.

Who BMI works less well for

Four well-documented categories where BMI's number-to-actual-health correlation breaks down:

• Muscular athletes. A 6'0", 200-lb rugby player at 12% body fat has the same BMI (27.1) as a sedentary 6'0", 200-lb office worker at 28% body fat. One is in excellent condition; the other has metabolic risk. BMI cannot tell them apart. The body-fat calculator (U.S. Navy circumference method) and the FFMI calculator are better tools for muscular individuals.
• Older adults. Muscle mass declines roughly 3-8% per decade after 30. A 70-year-old with a "healthy" BMI may have proportionally less muscle and more fat than the number implies. Sarcopenia (low muscle mass) is its own risk factor that BMI misses entirely.
• People with high or low fat distribution asymmetry. Two people with identical BMI can have very different cardiometabolic risk depending on where the fat is stored. Visceral fat (around the organs) carries far higher risk than subcutaneous fat (under the skin). Waist-to-hip ratio captures this; BMI does not.
• Some ethnic groups. The WHO's standard BMI cutoffs were derived primarily from European populations. South Asian, East Asian, and some other populations carry elevated cardiometabolic risk at lower BMI values; the WHO has published alternative cutoffs (overweight ≥ 23, obese ≥ 27.5) for screening in these groups.

The categories, with caveats

With the caveats above in mind, the standard WHO adult classifications:

• Under 18.5 — Underweight. May indicate undernutrition, eating disorder, or chronic illness. Worth a conversation with a clinician.
• 18.5–24.9 — "Normal" range. Lowest statistical risk on cardiometabolic markers, on average, in the populations the cutoffs were derived from.
• 25.0–29.9 — Overweight. Increased average risk; many individuals in this range are perfectly healthy.
• 30.0 and above — Obesity. Sub-divided into Class I (30–34.9), Class II (35–39.9), and Class III (≥ 40). Risk rises steeply across these.

The formula, as a footnote

Metric: BMI = mass (kg) / height (m)². Imperial: BMI = (mass (lb) / height (in)²) × 703. Both produce the same number for the same body. The squaring of height is the part that surprises people — it's not weight divided by height, it's weight divided by height squared, which is why two people of different heights but the same body composition do not have the same BMI.

Questions worth asking, instead of the standard FAQ

My BMI is "overweight" but I am in good shape. Should I care?

Probably not, on that information alone. Pair the BMI number with body-fat percentage (Navy method via the body-fat calculator or DEXA if available), waist measurement, and resting-state blood markers (lipid panel, fasting glucose) before drawing a conclusion. If body-fat is in range and waist-to-height ratio is below 0.5, BMI is the noisy signal — the other measurements are the signal.

My BMI is "normal" but I have a high waist measurement. Should I care?

Yes — more than you might think. "Normal-weight central obesity" (normal BMI, elevated waist circumference) has been associated with cardiometabolic risk comparable to overweight BMI in several studies. The general rule of thumb: waist circumference under half your height. If BMI is reassuring but waist measurement is not, trust the waist measurement.

Should I track BMI over time?

For most people, tracking it monthly is more useful than the absolute number on any given day. Trends are more informative than instantaneous values, especially since BMI lumps muscle and fat together — a strength-training program can move the number "up" while making everything underlying it better. Once you have a number, use the calorie calculator and TDEE calculator to plan intake, or the dedicated BMI calculator for men / for women / for kids for sex- and age-specific guidance.

What if my BMI is in a concerning category?

Talk to a clinician. The calculator on this page is a screening number, not a diagnosis. A real evaluation accounts for body composition, family history, blood markers, fitness, and a dozen other things this page cannot see.