BMI Calculator

About the BMI Calculator

Body Mass Index is a quick screening number that relates your weight to your height. It's the figure a GP or nurse often notes at a check-up, what insurers and clinical guidelines use as a first filter, and a common starting point when someone is tracking a weight goal. This calculator gives you the number, the adult category it falls into, and the approximate weight range that would put you in the "normal" band for your height — in either metric (centimetres and kilograms) or imperial (feet, inches and pounds) units.

It's designed for adults doing a self-check. It is not a diagnosis and doesn't replace advice from a health professional — see the accuracy note below for why the single number can mislead for some people.

How it works — the actual formula

BMI is weight divided by height squared:

• Metric: BMI = weight(kg) ÷ height(m)². Height entered in cm is divided by 100 first. Someone 170 cm and 70 kg is 70 ÷ 1.70² ≈ 24.2.
• Imperial: BMI = 703 × weight(lb) ÷ height(in)². Feet and inches are combined into total inches (ft × 12 + in). The 703 factor converts pounds-and-inches into the same scale as the metric formula.

The result is shown to one decimal place. The "healthy weight for your height" range is worked out by inverting the formula at the band edges — 18.5 × height(m)² to 24.9 × height(m)² — and rounded to whole kilograms.

The categories (WHO adult bands)

The category uses the World Health Organization cut-offs for adults:

• Under 18.5 — underweight
• 18.5 to under 25 — normal (healthy) weight
• 25 to under 30 — overweight
• 30 and above — obese

These thresholds are fixed values, the same worldwide in the standard WHO scheme.

Assumptions and rounding

• Uses the standard WHO adult thresholds above; it doesn't apply the lower cut-offs some health bodies recommend for South and East Asian populations (where risk rises at a lower BMI).
• The BMI value is rounded to one decimal place; the suggested healthy-weight range is rounded to whole kilograms, so it's an approximate guide rather than a to-the-gram target.
• The imperial path assumes the US convention where 1 inch = 0.0254 m and uses the 703 conversion factor.

Limitations — where BMI misleads

BMI only knows your height and weight. It can't tell muscle from fat or see where weight sits, so it's a rough screen, not a body-composition measurement:

• Muscular people (athletes, regular lifters) can read as "overweight" or "obese" despite low body fat, because muscle is dense. If you want an estimate that accounts for this, the Body Fat Calculator uses the US Navy method — waist, neck (and hip for women) measurements — instead of just weight and height.
• Older adults may lose muscle and carry more fat at the same BMI, so a "normal" number can understate risk.
• It's adult-only. For children and teenagers, BMI must be read against age-and-sex percentile charts, not these fixed bands — this tool doesn't do that.
• It isn't valid during pregnancy, and doesn't account for sex, frame size, or ethnicity.

Treat the number as one input among several (waist measurement, blood markers, how you feel), and talk to a clinician for anything that matters.

Privacy

Your height and weight are used only for the on-screen calculation, in your browser. Nothing is uploaded, saved, or shared.