Free tool
BMI calculator
A free adult BMI calculator in metric or imperial units, with the category, the healthy weight range for your height, and a plain account of what BMI cannot tell you.
Adult BMI categories only apply from age 20.
Enter your height, weight, and age to see your BMI. Nothing you type here leaves your browser.
A general screening estimate for adults, not a diagnosis. Nothing you enter is sent anywhere or stored.
The maths
One division, shown in full
The calculator converts imperial entries first, which is arithmetically the same as the 703 shortcut printed on most imperial charts.
Metric
BMI = weight (kg) ÷ height (m)²
70 kg at 1.75 m → 70 ÷ 3.0625 = 22.9
Imperial
BMI = 703 × weight (lb) ÷ height (in)²
The 703 factor is the metric conversion folded into the formula.
How it is read
Adult categories
The same cut points regardless of age or sex, once you are twenty or older. Published by the CDC and the World Health Organization.
| Category | BMI |
|---|---|
| Underweight | Below 18.5 |
| Healthy weight | 18.5 – 24.9 |
| Overweight | 25.0 – 29.9 |
| Obesity, class 1 | 30.0 – 34.9 |
| Obesity, class 2 | 35.0 – 39.9 |
| Obesity, class 3 | 40.0 and above |
The age limit
Why this one stops at twenty
Under twenty, BMI is plotted against age- and sex-specific growth percentiles, not fixed cut points. Handing a teenager an adult category would be wrong, so this calculator declines instead of adding a footnote.
For coaches
A screening number, not a coaching target
A cheap first look. The numbers that move week to week tell you more, and sooner.
- Weight trend and measurements
- Read across weeks, not a single morning.
- Photos on one timeline
- Beside the numbers, from every check-in.
- Volume and adherence
- What they did, alongside what it produced.
Limits
What this does not tell you
- It cannot tell muscle from fat. A lifter and a sedentary person of the same height and weight get the same number, and it means something different for each of them.
- It says nothing about where body fat sits, and distribution is a large part of what makes body fat a health risk.
- It was built on population averages, so it reads differently across ethnic groups, older adults, and highly trained athletes.
- It is a screening measure. A number outside the healthy band is a reason to look further, not a diagnosis of anything.
- It is not interpretable during pregnancy or breastfeeding.
Sources
- CDC — Frequently asked questions about BMI
- CDC — Adult BMI categories
- World Health Organization — Obesity and overweight
- Ages supported
- adults aged 20 and over
- Formulas and sources last reviewed
Keep reading
- Calorie intake calculatorEstimated maintenance plus a conservative starting range for losing, maintaining, or gaining, with the maths shown.
- BMR calculatorEstimate resting energy expenditure with Mifflin–St Jeor, and see the difference between that and what you burn in a day.
- Check-insBuild the form once, schedule it, and get each response back with the changes laid out and a draft reply to edit.
- NutritionMeal options that all hit the same targets, switched on per client, logged against a 300,000-food library.
A number is a start. Coaching is the rest.
Straton keeps a client's targets, their logs, and their check-ins in one record, so an estimate turns into something you can adjust week to week.