What BMI actually tells you
Body Mass Index is a sixty-second screening tool: it compares your weight to your height and places the result on a scale that large population studies have linked to health risk. Doctors, insurers and researchers use it because it needs no equipment beyond a scale and a tape measure — and because, across millions of people, the categories track real outcomes like diabetes and cardiovascular risk reasonably well.
What BMI is not is a body-fat measurement. It cannot tell muscle from fat, and it cannot see where fat sits — which matters, because weight carried at the waist is riskier than weight carried at the hips. Treat your number as a starting point for a conversation, not a verdict.
The formula
BMI = 703 × weight (lb) ÷ height² (in²)
Both versions give the same answer — the 703 factor simply converts US units to metric.
Worked example
Say you're 170 cm and 70 kg. Height in metres is 1.70, squared is 2.89. Then 70 ÷ 2.89 = 24.2 — inside the healthy band, close to its upper edge. In US units the same person is about 5'7" and 154 lb: 703 × 154 ÷ 67² = 703 × 154 ÷ 4,489 = 24.1 (the tiny difference is rounding in the unit conversion).
How to use this calculator
- Units — flip between metric and US any time; your numbers convert automatically.
- Height & weight — measure without shoes, ideally in the morning. Small errors matter: 2 cm of height error shifts BMI by roughly half a point.
- Waist (optional) — open the advanced section and add your waist measurement to get your waist-to-height ratio. Keeping your waist under half your height is a widely used complementary target, because it catches central fat that BMI misses.
- Healthy weight range — we invert the formula (18.5–24.9 × height²) to show the weight band considered healthy for your height, and how far you are from it.
- Export — download your numbers as CSV, print a clean PDF report, or copy a link that reopens the calculator with your values filled in.
Reading the extra numbers
BMI Prime is your BMI divided by 25, the upper healthy limit. A Prime of 1.0 means you're exactly at that limit; 1.12 means 12% above it; 0.9 means 10% below. It makes "how far past the line am I?" instantly readable.
Ponderal Index divides weight by height cubed instead of squared. Because of that, it stays fairer at the extremes of height — very tall people score slightly high on ordinary BMI and very short people score slightly low, a distortion the Ponderal Index largely removes. Typical healthy values run roughly 11–15 kg/m³.
Where BMI breaks down
- Athletes and gym-goers. Muscle is denser than fat, so a lean, muscular person can read "overweight" while carrying very little fat. Rugby players and sprinters routinely score 27+.
- Older adults. Muscle loss with age means an elderly person can hold a "healthy" BMI while carrying more fat than the number suggests. Some research favours a slightly higher target band (up to ~27) after 65.
- Ethnic differences. Health risks appear at lower BMIs in many Asian populations — the WHO notes action points of 23 (elevated) and 27.5 (high) rather than 25 and 30. If this applies to you, read your result against those thresholds.
- Children and teens. Adult cut-offs don't apply before age 20. Kids are assessed by BMI-for-age percentile against growth charts — this calculator is for adults.
- Pregnancy. BMI targets and weight-gain guidance during pregnancy are a separate, supervised topic — use pre-pregnancy weight and talk to your midwife or doctor.
A healthier way to use the number
Rather than obsessing over one reading, track direction: is your BMI drifting up over years? Pair it with your waist-to-height ratio, energy levels, blood pressure and, when you have them, blood results. If your BMI sits outside 18.5–24.9 and stays there, that's a good prompt for a check-up — not a diagnosis, and never a reason to crash-diet. Sustainable change of 0.25–0.5 kg per week is what the evidence supports.