What is BMI?
Body Mass Index (BMI) is a number calculated from your height and weight. It is the most widely used screening tool in medicine for assessing whether a person's weight falls within a healthy range for their height.
BMI does not directly measure body fat, but research shows it correlates reasonably well with more accurate measures of body composition for most adults. A BMI in the normal range is associated with lower risk of conditions like type 2 diabetes, cardiovascular disease, sleep apnea, and certain cancers.
The formula was developed by Belgian mathematician Adolphe Quetelet in the 1830s. The World Health Organization adopted it as a global standard for adult weight classification, and it remains the most common screening metric used in clinical settings today.
BMI formula
There are two versions of the BMI formula — one for metric units and one for imperial. Both produce the same result.
Metric formula (kg and cm)
Note that height must be in metres, not centimetres. Convert by dividing centimetres by 100. For example, 175 cm = 1.75 m.
Imperial formula (lb and inches)
Height must be in total inches. Convert feet and inches by multiplying feet by 12 then adding the remaining inches. For example, 5'9" = (5 × 12) + 9 = 69 inches.
The constant 703 is an adjustment factor that makes the imperial formula produce the same result as the metric formula.
How to calculate BMI step by step
Metric — step by step
- Measure your weight in kilograms.
- Measure your height in centimetres, then divide by 100 to get metres.
- Square your height in metres (multiply it by itself).
- Divide your weight by the squared height.
Imperial — step by step
- Measure your weight in pounds.
- Convert your height to total inches (feet × 12 + remaining inches).
- Square your height in inches (multiply it by itself).
- Divide your weight by the squared height.
- Multiply the result by 703.
Worked examples
Example 1 — Metric
A person who weighs 72 kg and is 175 cm tall:
Example 2 — Imperial
A person who weighs 160 lb and is 5'9" tall:
Example 3 — Overweight range
A person who weighs 155 lb and is 5'6" tall:
BMI categories explained
The World Health Organization defines four main BMI ranges for adults aged 18 and over. These categories apply regardless of age, sex, or ethnicity, though some guidelines suggest adjusted thresholds for certain populations.
| BMI range | Category | Health context |
|---|---|---|
| Below 18.5 | Underweight | May indicate malnutrition, eating disorders, or underlying health conditions |
| 18.5 – 24.9 | Normal weight | Associated with lowest risk of weight-related health conditions |
| 25.0 – 29.9 | Overweight | Elevated risk for some conditions; diet and activity changes often effective |
| 30.0 and above | Obese | Higher risk for type 2 diabetes, cardiovascular disease, joint problems |
Within the obese category, some guidelines further divide into Class I (30–34.9), Class II (35–39.9), and Class III (40 and above). Higher classes are associated with progressively greater health risk.
When BMI is misleading
BMI is a useful screening tool but has well-documented limitations. Understanding these helps you interpret your result more accurately.
- Athletes and muscular individuals: Muscle weighs more than fat. A rugby player or bodybuilder may have a BMI in the overweight or obese range despite very low body fat. BMI cannot distinguish muscle mass from fat mass.
- Older adults: As people age, muscle mass tends to decrease while fat increases — often without any change in body weight. An older adult may have a "normal" BMI while actually having excess fat relative to muscle.
- Biological sex: Women typically carry more body fat than men at the same BMI. The same BMI number can represent different health risks depending on sex.
- Ethnicity: Research suggests that people of Asian descent tend to have higher health risks at lower BMI values. Some guidelines recommend a lower threshold of 23 as the start of the overweight range for East Asian populations.
- Fat distribution: BMI does not indicate where fat is stored. Abdominal fat carries significantly higher cardiovascular risk than fat stored in the hips or thighs. Waist circumference or waist-to-hip ratio captures this risk better than BMI alone.
For most non-athletic adults, BMI gives a reasonable first estimate. For a more complete picture, combine BMI with waist circumference, body fat percentage, and routine blood work.
How to find your healthy weight range from BMI
To find the weight range that corresponds to a BMI of 18.5–24.9 for your height, rearrange the formula:
For example, for a height of 5'9" (69 inches):
The BMI calculator does this automatically — it shows your healthy weight range alongside your BMI result so you can see how far you are from the normal range and in which direction.
Does the same BMI formula apply to children?
No. The standard adult BMI categories do not apply to children and teenagers. For people under 18, the appropriate measure is BMI-for-age percentile, which compares a child's BMI against CDC growth charts for their age and sex.
A child at the 85th–94th percentile is considered overweight. At or above the 95th percentile is considered obese. Below the 5th percentile is underweight. These thresholds differ from adult ranges because children's bodies change significantly as they grow.
Frequently asked questions
What is a healthy BMI for adults?
The WHO defines 18.5 to 24.9 as the normal range for adults. This is associated with the lowest risk of weight-related conditions. However, BMI is a screening tool — not a complete picture of health on its own.
Is BMI the same for men and women?
The formula is the same, but the health interpretation can differ. Women typically have more body fat than men at the same BMI. Some clinical guidelines suggest slightly different risk thresholds by sex, though the WHO standard categories remain the same.
How often should I calculate my BMI?
For general monitoring, once every few months is sufficient for most adults. More frequent checks are useful when actively working toward a weight goal. Daily tracking is not recommended — weight fluctuates by 1–4 lbs day to day due to water, food timing, and other factors.
My BMI is in the normal range but I feel unfit — is that possible?
Yes. A person can be "skinny fat" — normal BMI but high body fat and low muscle mass. This condition carries real metabolic health risks despite a normal BMI reading. Body fat percentage and fitness testing give a more complete picture in this case.
What should I do if my BMI is outside the normal range?
If your BMI is above 25 or below 18.5 and you are not a highly muscular athlete, it is worth discussing with a doctor or registered dietitian. Small, consistent lifestyle changes tend to be more effective than drastic measures. The Calorie Deficit Calculator can help you understand what a realistic path to a healthier weight looks like.