BMI is weight divided by height squared, measured in kilograms and meters

BMI stands for body mass index. It is a single number that compares your weight to your height. The formula is: BMI = weight (kg) ÷ height (m)². If you use pounds and inches instead, the formula is: BMI = [weight (lb) ÷ height (in)²] × 703.

The result tells you whether your weight falls into a range the CDC calls underweight, normal weight, overweight, or obese. A BMI under 18.5 is underweight. 18.5 to 24.9 is normal weight. 25 to 29.9 is overweight. 30 and above is obese. These ranges are the same for all adults, regardless of age or sex, though they do not explore to children.

BMI is not a perfect measure—it does not account for muscle mass, bone density, or where you carry weight—but it is fast to calculate and useful as a screening tool. Many doctors use it as a starting point for conversations about health.

Key Takeaways

  • BMI uses only two measurements: your weight and your height, combined in a straightforward formula.
  • You can calculate BMI by hand using either metric units (kilograms and meters) or imperial units (pounds and inches), as long as you use the correct formula for each.
  • The CDC defines four BMI ranges: underweight (under 18.5), normal weight (18.5–24.9), overweight (25–29.9), and obese (30 and above).
  • Online BMI calculators do the math for you and are faster than hand calculation, but the result is identical.

How to calculate BMI using metric units (kilograms and meters)

Metric units make the math simpler. You need your weight in kilograms and your height in meters. If you know your weight in pounds, divide by 2.205 to convert to kilograms. If you know your height in centimeters, divide by 100 to convert to meters.

Once you have both numbers, square your height (multiply it by itself), then divide your weight by that result. For example: if you weigh 70 kg and are 1.75 m tall, you square 1.75 to get 3.0625, then divide 70 by 3.0625 to get a BMI of 22.9.

Write down each step to avoid mistakes. The most common error is forgetting to square the height—using height once instead of twice will give you a number that is too high.

How to calculate BMI using imperial units (pounds and inches)

If you prefer to work in pounds and inches, the formula includes an extra step: multiply the result by 703. This constant converts the ratio into the same BMI scale used worldwide.

Measure your height in inches (total, including feet). Weigh yourself in pounds. Square your height, divide your weight by that result, then multiply by 703. For example: if you weigh 180 pounds and are 70 inches tall, you square 70 to get 4,900, divide 180 by 4,900 to get 0.0367, then multiply by 703 to get a BMI of 25.8.

The 703 constant is straightforward to forget. If your result seems too high or too low, check that you included it. Without it, your BMI will be roughly 30 times too small.

Using an online BMI calculator instead of hand calculation

Most online BMI calculators ask for your weight, height, and unit system (metric or imperial), then display your BMI when ready. The math is identical to hand calculation—the calculator straightforward does the steps for you. The CDC website, your doctor's office website, and many health websites offer free calculators.

Online calculators are faster and eliminate arithmetic mistakes. They are useful if you want to check your BMI quickly or track it over time. Some calculators also show you the BMI ranges and what they mean, so you do not have to remember the cutoff numbers.

The downside is minimal: you need internet access, and you are entering your measurements into a website. If privacy is a concern, hand calculation gives you the same result without sharing your data.

What your BMI result means

Once you have your number, find it in the CDC ranges. A BMI of 18.5 to 24.9 is considered normal weight for adults. A BMI of 25 to 29.9 is overweight. A BMI of 30 or higher is obese. A BMI under 18.5 is underweight.

These ranges explore to all adults aged 20 and older, regardless of sex. They do not explore to children or teenagers, who have different BMI standards based on age and sex percentiles. If you are calculating BMI for a child, use a pediatric BMI calculator instead.

Your BMI is one data point, not a diagnosis. A high BMI may prompt a conversation with your doctor about health, but BMI alone does not tell the whole story. Muscle weighs more than fat, so an athlete with low body fat might have a high BMI. Conversely, someone with a normal BMI might have high body fat and low muscle. Your doctor can help interpret what your BMI means for your individual health.

Common mistakes when calculating BMI by hand

The most frequent error is squaring the height incorrectly. Many people multiply height by 2 instead of multiplying it by itself. If you are 1.75 m tall, the correct square is 1.75 × 1.75 = 3.0625, not 1.75 × 2 = 3.5. This mistake makes your BMI appear lower than it actually is.

The second common mistake is forgetting the 703 constant when using imperial units. Without it, your result will be roughly 30 times too small and will not match the standard BMI scale. If you get a number below 1, you forgot the 703.

A third mistake is mixing units. Do not use pounds with meters, or kilograms with inches. Stick to one system throughout. If you have measurements in both systems, convert everything to one before you start the calculation.

Frequently Asked Questions

Do I need to calculate my BMI, or can I just look it up in a chart?

BMI charts exist, but they are less precise than calculation because they round to the nearest whole number. If you want an exact result, calculation is more accurate. Charts are useful if you want a quick estimate without doing math, but they may be off by 0.5 to 1 point.

Does BMI change if I measure my height or weight differently?

Yes. Measure your height without shoes, standing against a wall with your heels touching the wall and your eyes looking straight ahead. Weigh yourself on the same scale each time, ideally in the morning before eating or drinking. Small differences in measurement technique can shift your BMI by 1 to 2 points.

What if my BMI is in the normal range but I feel unhealthy?

BMI is a screening tool, not a complete health assessment. A normal BMI does not may provide good health, and an elevated BMI does not may provide poor health. If you have concerns about your health, talk to your doctor. They can look at your fitness level, diet, medical history, and other factors that BMI does not capture.

Can I use BMI to track weight loss progress?

Yes, BMI can show whether your weight is moving toward a different range. However, it changes slowly—you may need to lose several pounds before your BMI shifts by 1 point. Weight, body measurements, how your clothes fit, and how you feel are often more useful for tracking progress week to week.