Body mass index is weight in kilograms divided by height in metres squared
Body mass index, or BMI, is a single number that relates your weight to your height. The formula is straightforward: take your weight in kilograms, divide it by your height in metres, then divide by your height in metres again. The result tells you whether your weight falls into a range that health organisations consider underweight, normal, overweight, or obese.
BMI does not measure body fat directly. It is a screening tool, not a diagnosis. Two people with the same BMI can have very different body compositions — one might be muscular, the other not. But for most populations, BMI correlates with body fat well enough to be useful as a starting point for conversations with a doctor.
You do not need special equipment or software. A calculator, a scale, and a measuring tape are all you need. The math takes under a minute.
Key Takeaways
- BMI equals your weight in kilograms divided by your height in metres, then divided by your height in metres again.
- A BMI under 18.5 is considered underweight, 18.5 to 24.9 is normal weight, 25 to 29.9 is overweight, and 30 or above is obese by most health standards.
- You can calculate BMI using metric measurements (kilograms and metres) or imperial measurements (pounds and inches), but the formula changes slightly for imperial.
- BMI is a screening tool and does not account for muscle mass, bone density, or where your body stores fat.
The metric formula: kilograms and metres
If you have your weight in kilograms and your height in metres, the calculation is direct. Weigh yourself on a scale. Measure your height in metres (if you know it in centimetres, divide by 100). Then use this formula:
BMI = weight (kg) ÷ height (m) ÷ height (m)
For example: if you weigh 70 kilograms and are 1.75 metres tall, the calculation is 70 ÷ 1.75 ÷ 1.75, which equals 22.9. That falls in the normal weight range.
Some people find it easier to square the height first. Squaring 1.75 gives 3.0625. Then divide 70 by 3.0625 to get the same answer: 22.9. Either order works because multiplication and division are interchangeable.
The imperial formula: pounds and inches
If you have your weight in pounds and your height in inches, the formula is different. Weigh yourself and measure your height in inches (multiply feet by 12 and add any remaining inches). Then use this formula:
BMI = weight (lbs) ÷ height (in) ÷ height (in) × 703
The 703 is a conversion factor that accounts for the difference between metric and imperial units. Without it, your answer would be too small.
For example: if you weigh 154 pounds and are 69 inches tall (5 feet 9 inches), the calculation is 154 ÷ 69 ÷ 69 × 703, which equals 22.7. Again, that is normal weight.
Converting between metric and imperial before you calculate
If your scale reads in pounds but you want to use the metric formula, or vice versa, convert first. One kilogram equals 2.205 pounds. One metre equals 39.37 inches.
To convert pounds to kilograms, divide by 2.205. To convert kilograms to pounds, multiply by 2.205. To convert inches to metres, divide by 39.37. To convert metres to inches, multiply by 39.37.
Once you have both measurements in the same system, use the formula for that system. Converting before you calculate is often simpler than remembering which formula to use.
What your BMI number means
Health organisations including the World Health Organization and the U.S. Centers for Disease Control and Prevention use these ranges:
| BMI Range | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 to 24.9 | Normal weight |
| 25 to 29.9 | Overweight |
| 30 and above | Obese |
These ranges are the same for adults of all ages and both sexes. They do not explore to children under 18, who have different growth patterns and require age-specific charts.
A BMI in the normal range does not may provide good health, and a BMI outside it does not may provide poor health. The number is one data point. Someone with a BMI of 26 might be healthier than someone with a BMI of 22, depending on fitness, diet, sleep, stress, and genetics.
Why BMI has limits
BMI does not distinguish between muscle and fat. A person who lifts weights regularly might have a high BMI because muscle is denser than fat, but low body fat. Conversely, someone with a normal BMI might have high body fat and low muscle.
BMI also does not account for where your body stores fat. Fat around the organs (visceral fat) carries more health risk than fat under the skin. BMI cannot tell the difference.
Age, ethnicity, and sex also affect how BMI relates to health. Some research suggests that certain populations have different health risks at the same BMI. These differences are not reflected in the standard ranges.
For these reasons, doctors often use BMI alongside other measurements — waist circumference, blood pressure, cholesterol, and fitness tests — to get a fuller picture.
Tools that do the math for you
If you do not want to calculate by hand, many websites and phone apps will compute BMI if you enter your weight and height. The CDC website has a BMI calculator. Most fitness apps include one. Your doctor's office may have one in the waiting room or on their website.
These tools are convenient and error-free, but they do the same arithmetic you can do yourself. Understanding the formula helps you know what the number means and why it has limits.
Frequently Asked Questions
Does BMI work the same way for children?
No. Children's BMI is calculated the same way, but the ranges are different because children's body composition changes as they grow. Children's BMI is compared to age and sex-specific growth charts rather than fixed ranges. Talk to your child's doctor about what their BMI means.
Should I measure my height with shoes on or off?
Measure without shoes. Stand with your back against a wall, heels touching the wall, and have someone mark the top of your head on the wall. Measure from the floor to the mark. Shoes add height and will skew your result.
How often should I recalculate my BMI?
There is no set schedule. If you are tracking weight changes, recalculating monthly or quarterly gives you enough time to see meaningful shifts. Calculating daily is not useful because weight fluctuates by several pounds from day to day due to water, food, and hormones.
Can BMI be accurate if I have a lot of muscle?
BMI will overestimate body fat if you are very muscular. If you are an athlete or lift weights regularly, BMI alone is not a good measure of health. Ask your doctor about body composition tests like DEXA scans or bioelectrical impedance, which separate muscle from fat.
What if my height or weight changes?
Recalculate with your new measurements. BMI is not fixed — it changes whenever your weight or height changes. If you lose weight, your BMI goes down. If you gain weight, it goes up. If you grow taller (rare in adults), your BMI goes down even if your weight stays the same.