What is BMI?
Body mass index (BMI) is a simple number that relates your weight to your height. Doctors and public health agencies such as the CDC, the NHS and the World Health Organization use it as a quick screening tool for weight categories in adults. It does not measure body fat directly, but for most people it gives a reasonable first indication.
The BMI formula
Metric: BMI = weight (kg) ÷ height (m)²
Imperial: BMI = 703 × weight (lb) ÷ height (in)²
Imperial example: you weigh 165 lb and are 5 ft 10 in (70 inches) tall. BMI = 703 × 165 ÷ 70² = 115,995 ÷ 4,900 = 23.7, which is in the healthy range.
Metric example: 72 kg and 178 cm. BMI = 72 ÷ 1.78² = 72 ÷ 3.1684 = 22.7.
BMI categories for adults
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Healthy weight |
| 25.0 – 29.9 | Overweight |
| 30.0 – 34.9 | Obesity class 1 |
| 35.0 – 39.9 | Obesity class 2 |
| 40 and above | Obesity class 3 |
The calculator also shows the healthy weight range for your height: the weights that give a BMI between 18.5 and 24.9. For someone 5 ft 10 in tall that is roughly 129 to 174 lb (58.6 to 78.9 kg for 178 cm).
Limitations of BMI
BMI is a population-level tool, not a diagnosis. Keep these limitations in mind:
- Muscle versus fat. Athletes and people with a lot of muscle can have a high BMI while carrying little body fat.
- Fat distribution. Fat around the waist carries more health risk than fat on the hips. Waist circumference is a useful extra check: a waist above about 40 in (102 cm) for men or 35 in (88 cm) for women is associated with higher risk.
- Age. Older adults naturally lose muscle, so a slightly higher BMI may be fine for them.
- Ethnicity. For people of South Asian, Chinese and some other backgrounds, health risks rise at a lower BMI. Some guidelines use 23 and 27.5 as thresholds.
- Children and teens. BMI for under-18s is interpreted with age- and sex-specific percentile charts, not with the adult table. This calculator is meant for adults.
- Pregnancy. BMI is not meaningful during pregnancy; use your pre-pregnancy weight when discussing weight gain with your midwife or doctor.
What next?
If your BMI is outside the healthy range, a doctor can look at your full picture, including blood pressure, blood sugar and lifestyle. For a sustainable change, small steps work best: an energy deficit of about 500 kcal a day leads to roughly 1 lb (0.5 kg) of weight loss per week. Use the calorie calculator to estimate how much energy you need, and the kg to lbs converter if your scale and your doctor use different units.
This calculator gives general information and is not medical advice.
BMI for children and teenagers
For people under 18, the adult categories don’t apply. Children’s body composition changes as they grow, so their BMI is compared with growth charts for their age and sex, giving a percentile instead of a fixed category. A child at the 85th percentile, for example, has a higher BMI than 85% of children of the same age and sex. Ask your pediatrician or school nurse to interpret a child’s BMI.
Frequently asked questions
What is a healthy BMI?
For adults, a BMI between 18.5 and 24.9 is considered healthy weight. Below 18.5 is underweight, 25 to 29.9 is overweight and 30 or more is obesity.
How do I calculate BMI in pounds and inches?
Multiply your weight in pounds by 703 and divide by your height in inches squared. 150 lb at 5 ft 6 in (66 in): 703 × 150 ÷ 4,356 = 24.2.
Is BMI accurate for athletes?
Not always. Muscle is denser than fat, so muscular people may get an overweight BMI despite a low body-fat percentage. Waist size or a body-fat measurement gives extra insight.
Is BMI the same for men and women?
The adult BMI formula and categories are the same for men and women, although women naturally carry a little more body fat at the same BMI.
Can children use this BMI calculator?
No. For children and teens, BMI is compared to growth charts for their age and sex. Ask your pediatrician or use a dedicated child BMI percentile tool.
How often should I check my BMI?
There is no need to check it daily. Weight fluctuates by a few pounds from day to day. Checking once a month at the same time of day gives a better trend.
Last reviewed: 2026-10-06. Results are estimates for information only.