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BMI Calculator

Check your Body Mass Index with WHO categories and a healthy weight range.

How the calculator works

BMI is weight in kilograms divided by the square of height in meters. Imperial inputs are converted first: pounds to kilograms and feet-plus-inches to meters, so both modes produce identical results for the same body. The resulting index is matched against the standard adult thresholds — below 18.5 is underweight, 18.5 to 24.9 is normal, 25 to 29.9 is overweight, and 30 and above is obesity, subdivided into classes I, II, and III at 35 and 40. The healthy weight range is found by reversing the formula: the boundaries 18.5 and 24.9 are multiplied by your height squared, giving the minimum and maximum kilograms your height can carry within the normal band.

Formula

BMI = weight (kg) ÷ height2 (m2)
Healthy range = 18.5 × height2 to 24.9 × height2

Where:

  • weight — body mass in kilograms (converted from pounds if needed)
  • height — stature in meters (converted from cm or ft/in)
  • 18.5 – 24.9 — the WHO normal-weight band

Example

Take an adult weighing 70 kg at 175 cm. Height squared is 1.75 × 1.75 = 3.0625, so BMI = 70 ÷ 3.0625 ≈ 22.9 — comfortably inside the normal band. The same person's healthy weight window runs from 18.5 × 3.0625 ≈ 56.7 kg to 24.9 × 3.0625 ≈ 76.3 kg, a spread of nearly 20 kg that shows how wide the “normal” range really is.

Frequently Asked Questions

Is BMI accurate for athletes?
Not always. Muscle is denser than fat, so muscular people can register as overweight despite low body fat. Athletes are usually better served by body-fat percentage or waist measurements, interpreted by a professional.
Do children and pregnant women use the same scale?
No. Children and teens are assessed with age- and sex-specific BMI percentiles, and pregnancy changes weight in a way the adult scale does not account for. This calculator applies to adults only.
Why do the thresholds differ in some countries?
Some Asian health authorities use lower overweight cut-offs (around 23) because health risks appear at lower BMIs in those populations. The WHO global bands used here are the international reference; local guidance may be stricter.
What is a quick way to improve my BMI?
Because BMI moves with weight, even modest, sustained changes shift it: a loss of 3–5% of body weight over a few months is enough to move many people down one category. Diet quality, activity, and sleep all matter — and any plan should be discussed with a healthcare provider.

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