Utilities Center

Guide

How BMI Is Calculated (and How to Read It)

Understand the BMI formula, what the categories mean, and the limitations of BMI as a health measure.

Open the BMI Calculator

A BMI calculator divides weight by height squared to give a single number that classifies weight status. It is a quick screening tool, not a full health assessment.

The BMI formula

Metric: BMI = weight(kg) ÷ height(m)²

Imperial: BMI = 703 × weight(lb) ÷ height(in)²

What the categories mean

  • Under 18.5 — underweight.
  • 18.5–24.9 — healthy weight.
  • 25–29.9 — overweight.
  • 30 and above — obese.

These ranges are for adults aged 20 and over. Children and teens use age- and sex-specific percentile charts instead.

How to use the calculator

  1. Enter your height and weight (metric or imperial).
  2. Read your BMI and weight category.
  3. Use the category as a prompt, not a diagnosis.

Examples

A person 1.70 m tall weighing 70 kg has BMI = 70 ÷ (1.70 × 1.70) = 24.2 — in the healthy range. The same person at 85 kg has BMI 29.4 — overweight, just under the obese threshold.

Limitations of BMI

BMI does not distinguish muscle from fat, so very muscular athletes can register as "overweight" while carrying little fat. It also does not account for fat distribution, age, or sex differences. Treat BMI as a rough screen and pair it with waist measurement and body composition for a fuller picture.

When a BMI calculator is useful

  • A quick screen of whether your weight is in a healthy range.
  • Tracking changes in weight status over time.
  • A starting point for a conversation with a healthcare provider.
  • Estimating BMI for fitness or insurance forms.

Put it into practice

Use the BMI Calculator right now — free, in your browser, no sign-up required.

Try the BMI Calculator

More guides

Back to all tools

Related free tools

More free online utilities that pair well with this one — same-category tools for the same task, plus handy extras from other categories.