BMI Calculator

BMI Calculator — Complete Guide

Body mass index (BMI) is a single number that relates your weight to your height. It is a quick screening measure, not a diagnosis: it shows how your weight compares with a statistical range for your height and flags whether further assessment may be worthwhile.

What this calculator does

Enter your height and weight in metric (cm, kg) or imperial (ft + in, lb) units and switch between them freely. The tool returns your BMI, the standard weight category, the healthy weight range for that height, and roughly how much you would need to lose or gain to reach the healthy band.

The BMI formula

The imperial version carries the 703 conversion factor so both forms agree. BMI is proportional to mass and inversely proportional to the square of height, which is why a small change in height shifts the number far more than the same relative change in weight.

A worked example

Someone 170 cm tall weighing 70 kg has a height of 1.70 m, and 1.70² = 2.89.

That sits in the "normal" band (18.5–24.9). The healthy weight range for 170 cm runs from 18.5 × 2.89 ≈ 53.5 kg up to 24.9 × 2.89 ≈ 72.0 kg. The imperial route agrees: 5 ft 7 in is 67 in, and 703 × 154 ÷ 67² ≈ 24.1 for a 154 lb person.

Limitations and edge cases

• BMI cannot tell muscle from fat. A muscular athlete may read "overweight" while carrying very little body fat.
• It ignores where fat sits. Waist circumference adds useful context, since abdominal fat carries more health risk.
• Children and teenagers are assessed with age- and sex-specific percentile charts, not the adult bands above.
• Pregnancy, very tall or very short builds, and some ethnic groups need adjusted thresholds.
• Treat the number as one signal among several — the trend over time matters more than a single reading.