How BMI is calculated
Body mass index compares your weight with your height. It is the same formula for men and women aged 20 and over.
The calculator converts imperial measurements with the exact definitions (1 inch = 2.54 cm, 1 pound = 0.45359237 kg), so both versions give the same answer.
Worked examples
Metric: 70 kg and 175 cm. Height in metres is 1.75, and 1.75² = 3.0625. BMI = 70 ÷ 3.0625 = 22.9, which is in the healthy range.
Imperial: 154 lb and 5 ft 9 in (69 inches). 69² = 4,761. BMI = 703 × 154 ÷ 4,761 = 22.7. For someone 5 ft 9 in tall, a BMI of 18.5 to 24.9 is a weight of about 125 to 169 lb.
BMI categories for adults
The World Health Organization and the US Centers for Disease Control and Prevention use the same main cut-offs. WHO also splits underweight into three grades of thinness.
| BMI | Category |
|---|---|
| Below 16.0 | Underweight: severe thinness |
| 16.0–16.9 | Underweight: moderate thinness |
| 17.0–18.4 | Underweight: mild thinness |
| 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 |
BMI is normally reported to one decimal place, and the category is read from that reported number.
BMI chart by height
This chart turns the three main cut-offs into weights. For each height, it shows the weight at which BMI reaches 18.5 (the bottom of the healthy range), 25 (where overweight starts) and 30 (where obesity starts). Weights are worked out with the formula above and rounded: pounds to the nearest whole pound, kilograms to one decimal.
| Height | cm | BMI 18.5 | BMI 25 | BMI 30 |
|---|---|---|---|---|
| 4 ft 10 in | 147.3 | 89 lb / 40.2 kg | 120 lb / 54.3 kg | 144 lb / 65.1 kg |
| 5 ft 0 in | 152.4 | 95 lb / 43.0 kg | 128 lb / 58.1 kg | 154 lb / 69.7 kg |
| 5 ft 2 in | 157.5 | 101 lb / 45.9 kg | 137 lb / 62.0 kg | 164 lb / 74.4 kg |
| 5 ft 4 in | 162.6 | 108 lb / 48.9 kg | 146 lb / 66.1 kg | 175 lb / 79.3 kg |
| 5 ft 6 in | 167.6 | 115 lb / 52.0 kg | 155 lb / 70.3 kg | 186 lb / 84.3 kg |
| 5 ft 8 in | 172.7 | 122 lb / 55.2 kg | 164 lb / 74.6 kg | 197 lb / 89.5 kg |
| 5 ft 10 in | 177.8 | 129 lb / 58.5 kg | 174 lb / 79.0 kg | 209 lb / 94.8 kg |
| 6 ft 0 in | 182.9 | 136 lb / 61.9 kg | 184 lb / 83.6 kg | 221 lb / 100.3 kg |
| 6 ft 2 in | 188.0 | 144 lb / 65.4 kg | 195 lb / 88.3 kg | 234 lb / 106.0 kg |
| 6 ft 4 in | 193.0 | 152 lb / 68.9 kg | 205 lb / 93.2 kg | 246 lb / 111.8 kg |
How to read it: at 5 ft 6 in (167.6 cm), the healthy range runs from about 115 lb (52.0 kg) up to just under 155 lb (70.3 kg). From 155 lb, BMI is 25 or more; from 186 lb (84.3 kg), it is 30 or more. For a height between two rows, enter your exact numbers in the calculator above.
Lower cut-offs for Asian populations
In 2004 a WHO expert consultation, published in The Lancet, reviewed evidence that many Asian populations have a higher risk of type 2 diabetes and heart disease at a lower BMI than European populations. It kept the international categories but added public health action points, with 23 marking increased risk and 27.5 high risk. Several countries in South and East Asia use these lower thresholds in their own guidance. Tick the option under the inputs to see your result against them.
BMI for women vs men
For adults aged 20 and over, the formula and the categories are the same for women and men. A woman and a man of the same height and weight have the same BMI and fall in the same category.
What differs is what sits behind the number. On average, women carry more body fat than men at the same BMI, and men carry more muscle. So the same BMI can mean a different body composition. BMI does not adjust for this, which is one reason doctors also look at waist size: the NIH’s heart and lung institute notes that a waist over 35 inches (89 cm) for women or over 40 inches (102 cm) for men raises health risk.
During pregnancy BMI does not apply in the usual way, because weight gain is expected. Pregnancy weight is tracked against your BMI from before you were pregnant, with a midwife or doctor.
BMI for children and teens
For anyone aged 2 to 19, the adult categories do not apply. Children’s body fat changes as they grow, and it differs between boys and girls. So the CDC compares a child’s BMI with growth charts for children of the same age and sex, and reports it as a percentile.
| BMI-for-age percentile (CDC) | Category |
|---|---|
| Below the 5th percentile | Underweight |
| 5th to below the 85th | Healthy weight |
| 85th to below the 95th | Overweight |
| 95th percentile or above | Obesity |
| 120% of the 95th percentile or above | Severe obesity |
A percentile tells you how a child compares with others. At the 70th percentile, a child’s BMI is higher than that of 70% of children of the same age and sex in the reference data. The same BMI number can be healthy for a 17-year-old and high for an 8-year-old, which is why you cannot read a child’s result from the adult table. This page does not calculate child percentiles; use the CDC child and teen BMI calculator, and talk to your child’s doctor about the result.
BMI vs body fat percentage
BMI uses only height and weight, so it estimates whether weight is high or low for a height. It does not measure fat. Body fat percentage tells you how much of your weight is fat, which is closer to what matters for health but harder to measure.
| BMI | Body fat percentage | |
|---|---|---|
| What you need | Height and weight | Tape measurements, scales or a scan |
| Tells muscle from fat | No | Yes, roughly |
| Same scale for women and men | Yes (adults) | No; healthy ranges are higher for women |
| Best use | Quick screening | Tracking body composition |
To estimate it at home with a tape measure, try the body fat calculator. Home methods are estimates too, so watch the trend over time rather than a single reading.
What the extra numbers mean
- Healthy weight for your height is the weight that gives a BMI of 18.5 to 24.9 at your height (18.5 to 22.9 with the Asian cut-offs).
- To reach the healthy range shows how far you are from the nearest edge of that range. It is not a target that suits everyone.
- BMI Prime is your BMI divided by 25, the top of the healthy range. A value of 0.92 means you are 8% below that limit; 1.20 means 20% above it.
Limits of BMI
BMI is cheap and quick, which is why doctors use it for screening, but it is only one number:
- It cannot tell muscle from fat. A muscular athlete can have a BMI over 25 with little body fat.
- Older adults often lose muscle, so a normal BMI can hide extra fat.
- It ignores where fat sits. Fat around the waist carries more risk than fat on the hips; waist measurement adds useful information.
- Pregnancy changes weight for reasons BMI does not account for.
- Children and teens need age- and sex-specific percentiles, as explained above.
A BMI outside the healthy range is a reason to talk to a doctor, not a diagnosis. Our guide BMI explained goes further into what your number does and does not tell you.
Next steps
If you want to change your weight, start by finding how many calories you burn each day with the TDEE calculator, then set a safe daily target with the calorie deficit calculator. To switch a weight between kilograms, pounds and stone, use the weight converter.