Calculate an adult BMI
BMI is a screening measure, not a diagnosis. It places a weight-to-height ratio into a band that public-health bodies associate with different average levels of risk across a population; it says nothing about one person's body composition or health. Treat it as a starting point for a conversation with a clinician. Measurements stay in your browser.
Enter Your Measurements
Your calculated BMI
How it works
BMI is weight divided by the square of height. In metric units that is kilograms ÷ metres², so a centimetre entry is divided by 100 first. In imperial units, feet and inches are added into total inches and the result is pounds ÷ inches² × 703 — the 703 converts the ratio into the metric scale, so both paths land on the same number for the same body.
Metric: BMI = kg ÷ (cm ÷ 100)²
Imperial: BMI = lb ÷ (total inches)² × 703
Rounding and band boundaries
The number is displayed to one decimal place, but the band is chosen from the unrounded value. The two can therefore disagree at a boundary. At 72.13 kg and 170 cm the exact BMI is 24.958, which displays as 25.0 yet is still reported as healthy weight. The reverse happens at 52 kg and 170 cm: 17.993 displays as 18.0 but is reported as underweight.
Each boundary belongs to the band above it. A BMI of exactly 25 — 81 kg at 180 cm — is overweight, not healthy weight, and exactly 30 is class 1 obesity. Printed tables reading "18.5 to 24.9" and "25.0 to 29.9" describe the same scheme to one decimal place.
Accepted inputs
- Metric: weight and height must both be numbers greater than zero.
- Imperial: weight and feet must be greater than zero, and additional inches from 0 up to but not including 12. Leave the inches field blank and it is treated as 0.
- Switching between metric and imperial clears the result rather than converting it, so re-enter the measurements in the new units.
- Editing any field clears the previous result, so a number on screen always matches the figures beside it.
Worked examples
| Measurements | Exact value | Shown as | Band |
|---|---|---|---|
| 70 kg, 175 cm | 22.857 | 22.9 | Healthy weight |
| 180 lb, 5 ft 9 in | 26.578 | 26.6 | Overweight |
| 72.13 kg, 170 cm | 24.958 | 25.0 | Healthy weight |
| 81 kg, 180 cm | 25.000 | 25.0 | Overweight |
Follow the second row by hand. Five feet nine inches is 69 inches, and 69² is 4,761. Then 180 ÷ 4,761 = 0.037807, and × 703 = 26.578. The third and fourth rows display the same 25.0 in different bands, which is the rounding behaviour described above rather than an error.
Who the bands do not apply to
The categories on this page come from adult population research and are meaningful only for adults aged 20 and older who are not pregnant. In particular:
- Children and adolescents aged 2 to 19. The same arithmetic applies, but the result is read against age- and sex-specific growth percentiles, because a healthy ratio changes throughout growth. A fixed adult band would misclassify most children.
- Pregnancy and breastfeeding. Weight gain is expected and the adult bands are not designed for it. Clinical guidance normally refers to pre-pregnancy BMI.
- People with high muscle mass. Muscle is denser than fat, so trained athletes and heavy manual workers can reach an overweight or obesity figure with low body fat. The number is not wrong; it is measuring the wrong thing for that person.
- Older adults and anyone with muscle loss. A BMI inside the healthy band can mask reduced muscle mass, and height loss with age raises BMI without any change in body composition.
- Across populations. The World Health Organization has noted that the same cut-off points do not represent the same level of risk in every population, and several national guidelines use lower thresholds for people of Asian descent.
Common mistakes
- Entering height in metres in the centimetre field. A value of 1.75 gives a BMI in the thousands.
- Putting a full height in the feet field, such as 5.9 for five feet nine inches. Nine inches is 0.75 of a foot, so use 5 feet and 9 inches in the separate fields.
- Treating a one-decimal difference as meaningful. Everyday weight fluctuates by one to two kilograms, which moves BMI by several tenths, and shoes or posture change a height reading.
- Using a child's measurements and reading the adult band.
- Reading a band as a diagnosis. It is a screening signal that may prompt further measurement, not a finding about one person's health.
Limits
BMI uses two numbers, so it cannot distinguish fat from muscle or bone, and it says nothing about where body fat sits — which is why waist circumference, waist-to-height ratio, blood pressure, blood lipids and blood glucose carry information that BMI does not. Two people with the same BMI can have very different health profiles.
This page also does not store or chart results over time, calculate child or teen percentiles, estimate body fat percentage, suggest a target weight, or give dietary or exercise guidance. For an individual assessment, speak to a qualified healthcare professional.
Questions
Should men and women use different bands?
The adult bands used here are the same for both. Body composition does differ by sex at a given BMI, which is one reason the number is treated as a population screening measure rather than an individual assessment.
Why do metric and imperial give slightly different numbers?
Usually because the measurements are not exactly equivalent. The 703 factor is itself a rounded conversion, so a tiny difference is normal; a difference of more than about a tenth means one of the entries is not the same height or weight as the other.
Does a figure just above 25 mean something is wrong?
Not on its own. The bands mark changes in average risk across large groups, not thresholds at which anything happens to an individual. Where you sit relative to the boundary matters far less than trend, waist measurement and the other markers a clinician would check.
Are my measurements sent anywhere?
No. The calculation runs in your browser and the code contains no request that transmits the values you enter.
Sources and review
The boundaries and stated limitations follow public guidance from the United States Centers for Disease Control and Prevention and the World Health Organization. The CDC gives adult categories as underweight below 18.5, healthy weight 18.5 to 24.9, overweight 25.0 to 29.9 and obesity 30 or above, split into class 1 (30 to 34.9), class 2 (35 to 39.9) and class 3 (40 or above), and describes BMI as a screening tool rather than a diagnosis. The WHO fact sheet uses the same 25 and 30 adult thresholds.
CDC: adult BMI categories · WHO: obesity and overweight
Bands are applied to the unrounded value, as described above.