BMI Calculator

Body Mass Index is the most widely used screening tool for weight-related health risk. It's also widely misunderstood. BMI is a population-level signal, not a precise individual diagnosis — but for most adults it's a reasonable starting point.

The BMI formula

Metric: BMI = weight (kg) / height (m)². Imperial: BMI = (weight (lbs) × 703) / height (inches)². The result is a unitless number compared against WHO categories: under 18.5 is underweight, 18.5–24.9 is normal weight, 25.0–29.9 is overweight, 30.0+ is obese.

When to use this calculator

Use it as a quick health screening check, especially when paired with waist circumference and a blood pressure reading. Be skeptical of the result if you're an athlete with significant muscle mass — BMI doesn't distinguish muscle from fat, so muscular bodies often score 'overweight' despite low body fat. For older adults (65+), the optimal range tends to skew slightly higher than the standard categories.

Worked example

A 5'10" person weighing 175 lbs: BMI = (175 × 703) / 70² = 25.1. That puts them in the 'overweight' category by exactly 0.1 — a useful illustration of why BMI categories shouldn't be treated as rigid clinical lines.

Limits and assumptions

The calculator takes height in centimetres or in feet and inches, and weight in kilograms or pounds. Imperial inputs use the standard 703 constant, and the result is shown to one decimal place. A height of zero or less produces no result, but there is no upper bound on either field, so a typo gives a wildly wrong BMI rather than an error.

There is no age or sex input. The adult WHO cut-offs (18.5, 25 and 30) are applied to everyone, so the categories are not valid for children or teenagers, who need age- and sex-specific percentile charts. The tool also knows nothing about muscle mass, waist size or anything else that changes what a given BMI means for one person.

Frequently asked questions