What is BMI?
Body mass index (BMI) is a single number that compares your weight to your height. It is calculated by dividing your weight in kilograms by the square of your height in metres. Health bodies, including the World Health Organization and Health Canada, use BMI to sort adults into weight categories: underweight, healthy weight, overweight, and obese. It is a quick screening measure, not a direct measure of body fat or health.
BMI = weight (kg) ÷ height (m)²
How BMI is calculated
The same index can be worked out in metric or imperial units. Both routes give the identical number; the imperial version just folds in a conversion factor of 703.
Take your height and weight
Measure height without shoes and weight in light clothing for the most consistent reading.
Convert height to the right unit
Metric uses metres (centimetres ÷ 100). Imperial uses total inches (feet × 12, plus inches).
Apply the formula
Metric: kg divided by height in metres squared. Imperial: 703 times pounds, divided by height in inches squared.
Read the category
Compare the result to the standard cut-offs at 18.5, 25, and 30 to find your category.
Worked example: a person 1.70 m tall weighing 68 kg has a BMI of 68 ÷ (1.70 × 1.70) = 23.5, which falls in the healthy-weight range.
Adult BMI categories
For adults aged 20 and over, BMI is read against fixed cut-offs that are the same for men and women. The obese band is sometimes split into three classes to flag higher levels.
| Category | BMI range (kg/m²) | What it flags |
|---|---|---|
| Underweight | Below 18.5 | Weight may be low for height; worth discussing with a clinician. |
| Healthy weight | 18.5 to 24.9 | Weight sits in the range linked to lower risk for most adults. |
| Overweight | 25.0 to 29.9 | Above the healthy range on this screening measure. |
| Obese, class 1 | 30.0 to 34.9 | First obesity class. |
| Obese, class 2 | 35.0 to 39.9 | Second obesity class. |
| Obese, class 3 | 40.0 and above | Third obesity class. |
Healthy weight range by height
A healthy weight is not a single number but a range that scales with height. The values below are the weights that produce a BMI of 18.5 to 24.9 at each height, rounded to the nearest unit.
| Height (cm) | Height (ft/in) | Healthy range (kg) | Healthy range (lb) |
|---|---|---|---|
| 150 | 4'11" | 42 to 56 | 92 to 124 |
| 155 | 5'1" | 44 to 60 | 98 to 132 |
| 160 | 5'3" | 47 to 64 | 104 to 141 |
| 165 | 5'5" | 50 to 68 | 111 to 149 |
| 170 | 5'7" | 53 to 72 | 118 to 159 |
| 175 | 5'9" | 57 to 76 | 125 to 168 |
| 180 | 5'11" | 60 to 81 | 132 to 178 |
| 185 | 6'1" | 63 to 85 | 140 to 188 |
| 190 | 6'3" | 67 to 90 | 147 to 198 |
| 195 | 6'5" | 70 to 95 | 155 to 209 |
What BMI does not tell you
BMI is useful because it is simple, but the same simplicity means it misses things. These are the cases where a BMI reading needs extra context.
Muscle counts as weight
BMI cannot tell muscle from fat. Very muscular people, such as athletes, can read as overweight while carrying little fat.
Age changes the picture
Older adults can lose muscle and bone, so a normal BMI may hide low muscle mass. Ranges can shift with age.
Children use percentiles
For anyone under 20, BMI is plotted against age and sex percentiles, not the adult categories on this page.
Ethnicity can shift risk
Health risk can rise at lower BMI levels in some populations, so the WHO notes lower cut-offs may be considered for certain groups.
Pregnancy is different
BMI categories are not designed for pregnancy. Healthcare providers track weight change differently during this time.
Fat location matters
Where fat sits affects risk. Waist measures add information BMI alone cannot give, which is why both are often used together.
BMI versus other body measures
BMI is one tool among several. Each measures something slightly different, and they are most useful read together rather than alone.
| Measure | What it estimates | Strength | Limitation |
|---|---|---|---|
| BMI | Weight relative to height | Fast, free, no tools beyond a scale and tape | Cannot separate fat from muscle |
| Waist circumference | Fat carried around the middle | Flags abdominal fat linked to higher risk | Needs correct tape placement |
| Waist-to-height ratio | Waist divided by height | Simple, and adds shape information to BMI | Still an estimate, not a fat measurement |
| Body fat percentage | Share of body that is fat | Closest to what BMI is used to approximate | Accurate methods need special equipment |