Body Mass Index with metric / imperial inputs and WHO category bands.
Updated
| Category | BMI range |
|---|---|
| Underweight | < 18.5 |
| Normal | 18.5 – 24.9 |
| Overweight | 25.0 – 29.9 |
| Obesity I | 30.0 – 34.9 |
| Obesity II | 35.0 – 39.9 |
| Obesity III | ≥ 40 |
Quick start
Enter weight and height in metric or imperial units, and read your BMI plus the WHO category band — all in your browser.
Choose metric (kg / cm) or imperial (lb / ft+in). The form switches between them instantly.
Type your weight and height. The result updates live — there's no Calculate button.
See your BMI, the WHO category (underweight / normal / overweight / obesity I–III), and the healthy weight range for your height.
In-depth guide
Body Mass Index is the most widely used screening number in healthcare: one division (weight ÷ height²) that puts you on a scale clinicians around the world recognise. It's also famously rough — it can't see muscle, fat distribution, age or genetics. This guide explains where BMI is useful, where it fails, and how to interpret the bands.
BMI was proposed by Belgian statistician Adolphe Quetelet in the 1830s. The math is identical for metric and imperial — only the conversion changes:
A 70 kg / 170 cm adult has a BMI of 70 ÷ 1.70² = 24.2 — comfortably inside the "normal weight" band. The same person at 80 kg would be at 27.7 (overweight); at 92 kg they'd cross into obesity class I (30).
The World Health Organisation defines six cut-points for adults (≥20 years):
The bands shift in some Asian-population guidelines — Indian and Chinese health ministries use lower overweight cutoffs (~23 instead of 25) because cardio-metabolic risk appears at lower BMIs in these populations.
BMI's appeal is its simplicity — but that simplicity is also its weakness. Things BMI doesn't know about:
If your BMI puts you outside the normal band, the next set of numbers worth knowing:
No. The BMI formula runs entirely in your browser as you type. Nothing is sent to a server, stored or logged.
Classic Quetelet BMI: weight in kilograms divided by the square of height in metres. Imperial inputs are converted to metric internally (1 lb = 0.45359237 kg, 1 in = 0.0254 m) before the calculation.
The WHO adult cutoffs: <18.5 underweight, 18.5–24.9 normal, 25–29.9 overweight, 30–34.9 obesity class I, 35–39.9 obesity class II, and ≥40 obesity class III.
BMI is a screening metric, not a diagnosis. It can over-estimate body fat in muscular individuals and under-estimate it in older or sedentary people who have lost lean mass. It also ignores body-fat distribution, which matters for cardio-metabolic risk. Use it as a starting point, not the final word.
Not really. Clinicians use BMI-for-age percentile charts for under-20s, which compare the BMI to peers of the same age and sex rather than to absolute cutoffs. If you enter an age below 20 we show a warning.
The kg range that, for your given height, would land you between BMI 18.5 and 24.9 — i.e. the normal-weight band. It's not a target, just a frame of reference.
For body composition, waist-to-height ratio and body-fat percentage (DEXA scan or skinfold) are more informative. For cardio-metabolic risk, blood markers (lipid panel, fasting glucose) matter more than BMI.
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