Body Mass Index is weight (kg) divided by height squared (m²). It's a fast screening number, not a diagnosis — it doesn't separately account for muscle mass, bone density, or body composition. For anyone under 20, raw BMI thresholds don't apply the same way — bodies are still growing — so the calculator automatically switches to the CDC's BMI-for-age growth-chart method: your BMI is compared against thousands of children of the same age and sex to produce a percentile, which is what pediatricians actually use.
| Adults (20+) | BMI range |
|---|---|
| Underweight | Below 18.5 |
| Healthy weight | 18.5 – 24.9 |
| Overweight | 25.0 – 29.9 |
| Obesity (Class I) | 30.0 – 34.9 |
| Obesity (Class II) | 35.0 – 39.9 |
| Obesity (Class III) | 40.0 and above |
Ages under 20 use CDC BMI-for-age percentiles instead: under 5th percentile (underweight), 5th–85th (healthy), 85th–95th (overweight), 95th and above (obesity).
A BMI calculator is a tool that works out your Body Mass Index — a quick screening number for whether your weight is in a healthy range for your height — by dividing your weight by your height squared. This one goes further than most: it's a single calculator that automatically switches to CDC age-and-sex growth-chart percentiles for anyone under 20, instead of applying adult cutoffs to a still-growing body.
Behind the single number, this calculator does a few things automatically: it converts whatever units you type into a common internal format, checks your age, and then applies one of two methods — the standard WHO adult BMI categories (20 and up) or the CDC's BMI-for-age growth-chart percentiles (under 20), comparing your result to thousands of others of the same age and sex. The result is plotted on the live gauge instantly as you type, with no page reload.
Choose metric or imperial units, then enter your height, weight, and age. If you enter an age under 20, a sex selector appears automatically since the CDC growth-chart method needs it — adults don't need to fill that in. Your BMI, category, and secondary stats update live as you type; from there you can save the result to your local history or export it as a shareable image.
BMI is calculated as weight in kilograms divided by height in meters, squared: BMI = kg ÷ m². For example, someone weighing 70 kg at 1.70 m tall has a BMI of 70 ÷ (1.70 × 1.70) = 24.2. In pounds and inches, the equivalent formula is (lb ÷ in²) × 703 — this calculator handles that conversion automatically when you switch to imperial units.
A height of 170 cm and a weight of 70 kg gives a BMI of 24.2 (70 ÷ 1.70²), which falls in the healthy weight range for an adult. Enter those numbers above to see the healthy weight range, BMI Prime, and Ponderal Index alongside it.
5 ft 7 in is about 170.2 cm, so at 70 kg that works out to a BMI of roughly 24.2 — the same healthy-weight result as 170 cm and 70 kg, just entered in mixed units. This calculator accepts height in feet and inches with weight in either kilograms or pounds, so nothing needs to be converted by hand.
A normal (healthy) adult BMI falls between 18.5 and 24.9. Below 18.5 is classified as underweight, 25 to 29.9 as overweight, and 30 and above as obesity, split into three classes. For anyone under 20, "normal" is defined differently — as a BMI-for-age percentile between the 5th and 85th for their age and sex, rather than a fixed number, since healthy body composition changes as kids grow.
Yes, for what BMI is designed to measure. The math itself is exact — this calculator uses the precise WHO formula for adults and the CDC's actual published BMI-for-age growth-chart data, not a rough approximation, for anyone under 20, so the number and percentile you get match what a clinician's chart would show. What BMI can't do is measure body composition directly — see the next question for that limitation.
No — it's a quick screening tool, not a diagnosis. It doesn't distinguish muscle from fat, so athletes and very muscular people can show a high BMI despite low body fat. Use it as a starting point, not a diagnosis, and talk to a clinician for the full picture.
No. Everything is computed in your browser. Saved history lives only in this browser's local storage — nothing is uploaded, and there's no account or login.
Yes. BMI uses the same formula and the same WHO weight categories for adult men and women — a BMI calculator for women doesn't need different math than a BMI calculator for men. The one place sex matters is for children and teens, where CDC growth charts are sex-specific.
CDC growth charts are sex-specific for ages 2–19, since typical growth patterns differ. Adult BMI categories from the WHO don't split by sex, so the field only appears when it's actually used.
Yes. Switch between metric (kg/cm) and imperial (lb/ft-in) at any time without losing your numbers, and enter your age — the calculator automatically applies adult BMI categories at 20 and up, or CDC age-and-sex percentiles below that.
The BMI chart with all adult weight categories (underweight through Obesity Class III) is above on this page. For children and teens, the equivalent reference is the CDC's BMI-for-age percentile chart, which this calculator applies automatically based on age and sex.
BMI Prime is your BMI expressed as a ratio to the upper healthy limit (25) — 1.0 or below means you're within the healthy range. Ponderal Index is a height-cubed alternative to BMI that's less distorted for very tall or very short people.
Body weight can genuinely shift by a kilogram or two during the luteal phase due to hormonal water retention, which will nudge your BMI slightly — but that's fluid, not a change in body fat or muscle. It's one more reason a single BMI reading is only ever a snapshot; saving results over time with this calculator's local history feature shows your actual trend instead of one day's fluctuation.
