BMI Classification Reference

Body Mass Index is the most widely used screening number for weight relative to height. This tool calculates your BMI from metric or imperial measurements and maps it onto the World Health Organization’s adult classification, from severe underweight through to obese class III, with the exact numeric cut-offs.

How BMI is calculated

BMI is defined as weight in kilograms divided by the square of height in metres:

BMI = weight_kg / (height_m * height_m)

For imperial measurements the equivalent formula multiplies pounds by 703 and divides by height in inches squared, which reproduces the same metric result. The tool then compares the value against the WHO bands — underweight below 18.5, normal 18.5 to 24.9, overweight 25 to 29.9, and obese class I, II and III at 30, 35 and 40 — and highlights the matching row.

Full WHO adult BMI classification table

The tool highlights the row that matches your result and shows the exact numeric cut-offs above and below your score.

Two worked examples

A person who is 170 cm tall and weighs 70 kg:

BMI = 70 / (1.70 × 1.70) = 70 / 2.89 = 24.2

That falls in the Normal weight band, just under the 25.0 pre-obesity threshold.

The same height at 90 kg:

BMI = 90 / 2.89 = 31.1

That lands in Obese class I — above the 30.0 threshold.

What the bands mean in kilograms, by height

Because BMI divides by height squared, the same category spans very different weights at different heights. Applying the formula in reverse gives the weight window for each band (values rounded to 0.1 kg):

Two things stand out. The normal band is wide — roughly 15–23 kg depending on height — so BMI is far less sensitive to small weight changes than day-to-day scale readings suggest. And each extra 10 cm of height raises the obesity threshold by about 10 kg, which is why comparing raw weights between people of different heights is meaningless.

Notes and limitations

BMI is a population screening tool, not a body-composition measurement. It treats muscle and fat identically, so strength athletes and bodybuilders often score as overweight despite having low body fat. Conversely, older adults who have lost muscle may have a “normal” BMI while carrying excess fat. The formula is not validated for children, pregnant people, or the frail elderly.

The WHO notes that for many Asian populations, cardiometabolic risk rises at lower BMI values. Proposed action points for these populations are 23 (instead of 25) for overweight and 27.5 (instead of 30) for obesity class I. The standard table shown here applies to the general adult population.

Treat the result as a starting point for a conversation with a clinician, not a diagnosis. BMI is a fast, free screen — but waist circumference, body-fat percentage, blood pressure, and blood glucose together tell a much fuller story.

Sources and references

The classification bands used here are the World Health Organization adult cut-offs; the Asian-population action points come from a WHO expert consultation:

• WHO — A healthy lifestyle: WHO recommendations (BMI classification)
• WHO expert consultation, The Lancet (2004) — appropriate BMI for Asian populations
• CDC — About Adult BMI

Reviewed by the Gera Tools editorial team. BMI is a population screening tool, not a diagnosis, and is not validated for children, pregnant people, athletes, or the frail elderly. This page is health information, not medical advice — consult a healthcare professional to interpret your result. Last reviewed 2026-07-02.