What BMI measures
Body mass index compares your weight with your height. It was designed for studying populations, and doctors use it as a quick screening number because it correlates reasonably well with body fat across large groups of adults and with the risk of conditions such as type 2 diabetes and heart disease.
The World Health Organization classifies adult BMI as follows. The same thresholds apply to men and women.
| BMI (kg/m²) | Class |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Healthy weight |
| 25.0 – 29.9 | Overweight |
| 30.0 – 34.9 | Obesity class I |
| 35.0 – 39.9 | Obesity class II |
| 40.0 and above | Obesity class III |
The calculator classifies the BMI rounded to one decimal place — the number you see — so the class always matches the displayed value.
BMI formula
In metric units, BMI is weight divided by the square of height:
BMI = weight (kg) ÷ height (m)²
With pounds and inches, a conversion factor of 703 turns the result into the same kg/m² scale:
BMI = 703 × weight (lb) ÷ height (in)²
The factor comes from the exact definitions 1 lb = 0.45359237 kg and 1 in = 0.0254 m (0.45359237 ÷ 0.0254² ≈ 703.07). This calculator converts to metric first, so both unit systems give identical results.
The healthy weight range for a height is found by reversing the formula:
weight = BMI × height (m)², for BMI 18.5 and 24.9
BMI Prime is BMI divided by 25, the upper limit of the healthy range: a value of 1.00 sits exactly on the boundary, 1.20 is 20% above it.
Worked example
Someone who is 5 ft 9 in tall and weighs 154 lb:
- Height: 69 in × 0.0254 = 1.7526 m
- Weight: 154 lb × 0.45359237 = 69.85 kg
- BMI: 69.85 ÷ 1.7526² = 69.85 ÷ 3.0716 = 22.7 — healthy weight
- Healthy range at this height: 18.5 × 3.0716 = 56.8 kg (125.3 lb) to 24.9 × 3.0716 = 76.5 kg (168.6 lb)
With the 703 shortcut: 703 × 154 ÷ 69² = 108,262 ÷ 4,761 = 22.74, which also rounds to 22.7.
Where BMI falls short
- Muscle counts as weight. Athletes and people with a lot of muscle can have a BMI in the overweight range with little body fat.
- Fat distribution matters. Fat around the waist carries more health risk than fat elsewhere, and BMI cannot see it. Waist circumference or the body fat calculator adds that information.
- Ethnicity. Health risks rise at lower BMI in some populations. WHO suggests additional action points of 23 and 27.5 for Asian populations, and some national guidelines use lower thresholds for people of South Asian, Chinese, Middle Eastern, Black African or African-Caribbean background.
- Older adults tend to lose muscle, so the same BMI can mean more body fat.
- Children and teens need BMI-for-age percentiles, because healthy body composition changes as they grow. These adult classes don’t apply under 20.
- Pregnancy changes weight for reasons unrelated to body fat; BMI isn’t meaningful during pregnancy.
A BMI outside the healthy range is a reason to look more closely, not a diagnosis. For energy needs, see the calorie calculator.
Frequently asked questions
What is a healthy BMI?
For adults, a BMI from 18.5 to 24.9 is classed as healthy weight by the World Health Organization. Some guidelines use lower thresholds for people of Asian and some other ethnic backgrounds, because health risks begin at a lower BMI.
Is BMI different for men and women?
No. The adult formula and classes are the same for men and women. At the same BMI, women on average carry more body fat than men, which is one reason BMI is treated as a screening number rather than a measure of fat.
Can I use this BMI calculator for a child?
No. For ages 2 to 19, BMI has to be compared with growth charts for the child’s age and sex, giving a percentile. Adult classes such as 25 for overweight don’t apply.
Why does my BMI say overweight when I’m muscular?
BMI only knows your weight, not what it is made of. Muscle is dense, so muscular people often score in the overweight range despite low body fat. A body fat estimate or waist measurement gives a better picture.
How accurate is the 703 formula?
The factor is 703.07 when calculated from the exact definitions of the pound and inch, so using 703 changes the result by about 0.01%. This calculator converts to metric with the exact factors.
Sources
Last reviewed September 15, 2026