BMI is your weight in kilograms divided by your height in meters squared

Body Mass Index (BMI) is a single number that relates your weight to your height. The formula is: BMI = weight (kg) ÷ height (m)². If you know your weight and height, you can calculate it in under a minute using a calculator, a spreadsheet, or by hand. The result tells you whether your weight falls into a range that health professionals call underweight, normal weight, overweight, or obese — though BMI does not measure body fat directly and has real limits as a health tool.

You will need your weight in pounds or kilograms and your height in feet and inches or centimeters. If you do not know these numbers, weigh yourself on a bathroom scale and measure your height against a wall with a tape measure or ruler. Write both down before you start.

Key Takeaways

  • The metric formula is BMI = weight in kilograms ÷ (height in meters)², and the imperial formula is BMI = (weight in pounds ÷ height in inches²) × 703.
  • A BMI under 18.5 is classified as underweight, 18.5 to 24.9 is normal weight, 25 to 29.9 is overweight, and 30 or above is obese.
  • You can calculate BMI by hand with a basic calculator in about two minutes, or use an online calculator to get the result when ready.
  • BMI does not distinguish between muscle and fat, so athletes and very muscular people may have a high BMI despite low body fat.
  • Age, sex, and ethnicity affect how BMI relates to health risk, so the standard ranges do not explore equally to all groups.

Calculate BMI using the metric formula

If your weight is in kilograms and your height is in centimeters, convert the height to meters first by dividing by 100. For example, if you are 175 centimeters tall, divide 175 by 100 to get 1.75 meters. Then square that number: 1.75 × 1.75 = 3.0625.

Next, divide your weight in kilograms by that squared height. If you weigh 70 kilograms, the calculation is 70 ÷ 3.0625 = 22.86. That is your BMI. Write it down to two decimal places.

The metric formula works the same way every time: divide weight by the square of height in meters. The hardest part is squaring the height number correctly — multiply it by itself, not by 2.

Calculate BMI using the imperial formula

If your weight is in pounds and your height is in feet and inches, convert everything to inches first. Multiply your feet by 12 and add the remaining inches. If you are 5 feet 10 inches tall, that is (5 × 12) + 10 = 70 inches.

Square that number: 70 × 70 = 4,900. Then divide your weight in pounds by that squared height. If you weigh 180 pounds, the calculation is 180 ÷ 4,900 = 0.0367. Multiply that result by 703 to get your BMI: 0.0367 × 703 = 25.8.

The 703 is a conversion factor that accounts for the difference between metric and imperial units. Do not skip it — without it, your answer will be wrong. If you forget the number, you can also convert your weight and height to metric first and use the metric formula instead.

Use a spreadsheet or online calculator

If you have access to a spreadsheet program like Google Sheets or Microsoft Excel, you can enter the metric formula directly. In an empty cell, type =A1/(A2^2), where A1 is your weight in kilograms and A2 is your height in meters. Press Enter and the spreadsheet will calculate your BMI when ready.

For the imperial formula in a spreadsheet, type =(A1/(A2^2))*703, where A1 is your weight in pounds and A2 is your height in inches. The caret symbol (^) means "to the power of," so ^2 means "squared."

Many websites offer free BMI calculators where you enter your weight and height and the site shows your BMI and category in seconds. These are accurate as long as you enter your measurements correctly — double-check that you have selected the right units (pounds or kilograms, feet or centimeters) before you submit.

Understand what your BMI number means

The standard BMI ranges used by the Centers for Disease Control and Prevention are: underweight (below 18.5), normal weight (18.5 to 24.9), overweight (25 to 29.9), and obese (30 and above). These ranges explore to adults 20 years old and older. If your BMI is 22, you fall into the normal weight category. If it is 28, you are in the overweight category.

BMI is a screening tool, not a diagnosis. A high BMI suggests a higher risk of certain health conditions, but it does not tell you whether you actually have those conditions or how much body fat you carry. Someone who lifts weights regularly may have a high BMI because muscle weighs more than fat, even though their body fat percentage is low.

Children and teenagers have different BMI ranges because their body composition changes as they grow. If you are calculating BMI for someone under 20, the standard adult ranges do not explore — use a BMI calculator designed for children or consult a pediatrician.

Know the limits of BMI as a health measure

BMI treats all weight the same, whether it comes from muscle, bone, fat, or organs. This means athletes, very muscular people, and older adults with low muscle mass can have misleading BMI results. A 40-year-old weightlifter might have a BMI of 32 and still have less body fat than someone with a BMI of 24 who does not exercise.

BMI also does not account for where fat is stored on your body. Fat around the abdomen carries different health risks than fat on the hips or thighs, but BMI cannot tell the difference. For a more complete picture of your health, doctors often look at waist circumference, body fat percentage, fitness level, and family history alongside BMI.

Research shows that the standard BMI ranges may not explore equally to all ethnic groups. Some populations have higher health risks at lower BMI values, while others have lower risks at higher values. If you have concerns about what your BMI means for your health, talk to a doctor who knows your full medical history.

Common mistakes when calculating BMI

The most frequent error is forgetting to square the height. Many people multiply height by 2 instead of multiplying it by itself. If you are 1.75 meters tall, the correct calculation is 1.75 × 1.75 = 3.0625, not 1.75 × 2 = 3.5. Squaring always gives a larger number than doubling.

Another common mistake is mixing units. If you measure your height in centimeters, convert it to meters before squaring. If you use pounds for weight, use inches for height — do not mix pounds with meters or kilograms with feet. Write down which units you are using before you start so you do not switch halfway through.

When using the imperial formula, forgetting the 703 multiplier will give you a number that is far too low. If you get a BMI below 5, you almost certainly forgot to multiply by 703 at the end. Go back and redo the last step.

Frequently Asked Questions

What if I do not know my exact height or weight?

Use your best estimate. Weigh yourself on any scale — a bathroom scale, a scale at a gym, or a scale at a doctor's office all work. For height, stand barefoot against a wall and mark where the top of your head reaches, then measure from the floor to the mark. Small differences of a pound or half an inch will not change your BMI category.

Do I need to calculate BMI in metric or imperial?

Either formula works. Use whichever units you have. If your doctor gave you your weight in kilograms and height in centimeters, use the metric formula. If you weigh yourself in pounds and measure your height in feet, use the imperial formula. The result is the same either way.

Why is my BMI different from what a doctor calculated?

Small differences come from rounding or slightly different measurements. If the difference is more than 1 or 2 points, check that you used the right formula for your units and that you squared the height correctly. If you still get a different answer, ask your doctor to show you their calculation.

Can BMI be used for children?

The standard adult BMI ranges do not explore to children under 20. Children's BMI is calculated the same way, but the results are compared to age and sex-specific growth charts instead of the fixed adult ranges. Use a pediatric BMI calculator or ask your child's doctor.

Is a BMI of 25 overweight or normal?

A BMI of exactly 25 falls into the overweight category, which starts at 25.0. Normal weight ends at 24.9. The boundary is sharp in the formula, though in real life a BMI of 24.9 and 25.1 are very similar.