How it works
Plug in your stature and body mass, pick a measurement system, and this tool computes your Body Mass Index — a single dimensionless figure built only from those two physical quantities. It then maps that figure onto the standard adult thresholds the World Health Organization has used since the 1990s: 18.5, 25.0, and 30.0. Those three cut-points carve the spectrum into four labeled zones: underweight, normal, overweight, and obese.
The index itself is a height-normalized mass ratio. By dividing mass by the square of stature, it answers a simple screening question: relative to skeletal size, is this person's weight low, typical, or elevated? It produces a quick population-level signal, not a body-composition measurement. Two individuals can share an identical BMI yet differ substantially in lean tissue versus adipose tissue, which is the single biggest critique of the metric.
This is a general-population screening instrument for adults aged 20 and older. Pediatric assessments require BMI-for-age percentile plotting because children's body proportions shift as they grow, making the fixed adult thresholds inappropriate.
The formula
BMI = mass / (stature × stature)
Metric inputs feed straight into the equation. US inputs require a scaling factor of 703 because pounds and inches are not coherent SI units:
BMI (US units) = (mass in lb / (stature in in × stature in in)) × 703
| BMI Band | WHO Classification |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Normal |
| 25.0 – 29.9 | Overweight |
| 30.0 and above | Obese |
Worked example
Consider an adult standing 5 feet 9 inches tall and weighing 165 pounds, using US units.
Stature in inches: (5 × 12) + 9 = 69 in
Stature squared: 69 × 69 = 4,761
Mass ÷ squared stature: 165 / 4,761 = 0.034658
Apply conversion factor: 0.034658 × 703 = 24.36
Rounded to one decimal, the result is 24.4. That value sits inside the 18.5–24.9 band, so the classification is Normal.
The figure is close to the 25.0 boundary, though. Gaining roughly four pounds would push this person into the overweight zone; losing about twenty pounds would approach the underweight threshold at 18.5.
Common mistakes
Mixing unit systems. Entering pounds with centimeters, or kilograms with inches, produces a nonsensical figure. The conversion factor 703 exists specifically because pounds and inches are not SI-compatible; metric inputs need no such scaling.
Confusing feet-and-inches formatting. A stature entry of "5.9" reads as 5.9 inches, not 5 feet 9 inches. The correct total is 69 inches. Always convert composite measurements to a single unit before inputting.
Treating the output as a body-fat reading. The index measures mass relative to stature — nothing else. A 6-foot linebacker at 240 pounds lands in the obese band despite minimal body fat, because dense muscle tissue registers identically to adipose tissue in the numerator. Conversely, an older adult with sarcopenia may show a normal figure while carrying a high fat-to-lean ratio.
BMI is a population-level screening signal, not a clinical diagnosis. It cannot assess metabolic health, visceral fat distribution, cardiovascular risk, or fitness. Use it as one data point alongside waist circumference, blood panels, and a clinician's evaluation. This calculator provides an estimate, not professional medical advice.
Rounding the square. Truncating 69 × 69 to 4,700 instead of 4,761 shifts the final figure by roughly 0.2. Small errors compound when you are near a classification boundary, so carry full precision through each step.