Guide
How to Calculate BMI
BMI is one simple division. Here's the formula in metric and imperial units, worked examples, and the category chart — or skip the math and use the calculator below.
The metric formula
BMI = weight (kg) ÷ height (m)². Example: if you weigh 70 kg and are 1.75 m tall, your height squared is 1.75 × 1.75 = 3.0625, and 70 ÷ 3.0625 = 22.9. That's a BMI of 22.9, comfortably in the healthy range.
The imperial formula
With pounds and inches, multiply by 703: BMI = 703 × weight (lb) ÷ height (in)². Example: 155 lb at 5 ft 9 in (69 inches) gives 703 × 155 = 108,965, divided by 69² = 4,761, which equals 22.9 — the same person, the same result.
Reading your result
For adults, the WHO categories are: under 18.5 underweight, 18.5–24.9 healthy weight, 25–29.9 overweight, and 30 or above obesity. The healthy weight range for your height is simply the formula run in reverse: 18.5 × height² to 24.9 × height².
Common mistakes to avoid
The most common error is forgetting to square your height — divide by height twice, not once. Second, convert units fully first: centimeters to meters (divide by 100), and feet plus inches to total inches (feet × 12 + inches). Third, don't mix systems — kilograms with inches or pounds with meters will give a nonsense result.
When the formula doesn't apply
The standard formula and categories are for adults aged 20 and over. Children and teens need BMI-for-age percentile charts, pregnant women shouldn't use BMI at all, and very muscular people may get a misleadingly high reading. Older adults over 65 may also be healthiest slightly above the standard range.
Key facts about BMI
Once you've done the math, compare your number against the World Health Organization's adult classification — the same bands used by the CDC for adults 20 and older.
Adult BMI categories
How BMI is read by age group
| Group | How it's assessed | Healthy / lowest-risk |
|---|---|---|
| Children & teens (2–19) | BMI-for-age percentile, by sex | 5th to under 85th percentile |
| Adults (20–64) | Fixed adult bands | 18.5 – 24.9 |
| Older adults (65+) | Adult bands, interpreted with care | Lowest risk seen around 23 – 29.9 |
| Asian adults | Lower action points suggested | 18.5 – 22.9 (risk rises from 23) |
Children's results use CDC BMI-for-age growth charts. A meta-analysis of 32 studies in adults over 65 found the lowest mortality at BMI 23–29.9, and a WHO expert consultation published in The Lancet recommended lower action points for Asian populations.
Limitations of BMI
- It doesn't measure body fat. In a study of over 13,000 adults, BMI ≥30 missed about half of people with excess body fat.
- Muscle counts as weight. Athletes can score "overweight" while lean.
- Fat location matters. Belly fat carries more risk; waist-to-height ratio predicts heart and metabolic risk better than BMI. Aim for a waist under half your height.
- Age, sex and ethnicity change the picture, as the table above shows.
- Not for pregnancy or people with major fluid shifts or amputations.
Even so, large studies such as the Global BMI Mortality Collaboration (10.6 million adults) confirm BMI is a useful population-level screening tool. Use it as a starting point and discuss results with a health professional.
Frequently asked questions
What is the BMI formula?
BMI = weight in kilograms ÷ height in meters squared. In pounds and inches: BMI = 703 × weight (lb) ÷ height (in)².
How do I calculate BMI without a calculator?
Square your height in meters, then divide your weight in kilograms by that number. For a rough check: at 1.75 m, height² is about 3.06, so every 3 kg of weight is roughly 1 BMI point.
What is a good BMI?
For most adults, 18.5–24.9 is considered healthy by the WHO. Context matters — muscle mass, age, and where fat is stored all affect what your number means.
Why does the imperial formula use 703?
703 is the conversion factor that makes pounds and inches produce the same result as kilograms and meters (it's 0.45359237 ÷ 0.0254², rounded). Without it, the imperial result would be far too small.