BMI Calculator

When BMI is a fair guide, and when it is not

BMI earns its place as the most common screening number because it needs only a tape and a scale and it works for large averages. Use it as a trend line: the direction of your own BMI across months says more than one reading. It is also a shared language doctors and insurers understand, which is why forms ask for it.

Be sceptical in four situations. Children and teens grow into their numbers and use age-adjusted percentile charts, not adult bands. Athletic builds carry dense muscle that tips BMI high without excess fat. Older adults lose muscle as they age, so a "normal" BMI can hide reduced reserves. Pregnancy changes weight by design, making the pre-pregnancy BMI the relevant one. In all four cases a professional adds context: waist circumference, body fat percentage from the Body Fat Estimate, or simply clinical judgement.

A worked example, step by step

Someone weighs 68 kg and is 172 cm tall:

1. Convert height to metres: 172 cm = 1.72 m.
2. Square it: 1.72 × 1.72 = 2.9584.
3. Divide weight by the square: 68 ÷ 2.9584 = 22.98…
4. Rounded to one decimal: BMI ≈ 23.0, inside the 18.5–24.9 "normal weight" band.

Notice how much the squaring matters: a 9% taller person at the same weight already shows a 17% lower BMI. Size, not just weight, drives the number.

Weight bands for a person 1.70 m tall

Standard adult cut-off points, converted to kilograms at this height:

The same bands scale with height squared, which is exactly what the calculator does for you.

Mistakes that distort BMI

• Entering centimetres where metres are expected. 180 instead of 1.8 produces a BMI near zero and a misleading "underweight" label.
• Reading single readings as verdicts. 24.9 versus 25.1 is measurement noise, not a health cliff.
• Comparing across body types. A rugby second row and a swimmer can share a BMI while sharing almost nothing else.
• Ignoring the waist. Central fat tracks risk more closely than BMI alone; the two measurements belong together.
• Using imperial numbers in the metric form. Pounds and inches need either conversion first or the imperial version of this tool.

Keep exploring

• BMI Calculator (Imperial) — the same index in pounds and inches.
• Body Fat Estimate (US Navy Method) — the Navy tape method as a second view.
• Daily Calorie Estimate — what daily energy needs look like at your size.
• Ideal Weight Calculator — formula-based weight bands for comparison.
• Guide: BMI, Calories and Water Intake.