How to Calculate BMI: Your Complete Guide

If you want to learn how to calculate BMI, the good news is that it takes one formula and about two minutes. You need only two numbers: your weight and your height.

The confusion starts because there are two versions of the formula, one for metric units and one for imperial units, and most articles show only one. This guide shows you both, with a full worked example for a man and a woman in each system, a complete BMI chart, and the five mistakes that give people the wrong number. By the end, you will know exactly how to calculate BMI for yourself, and what to do with the number once you have it.

Quick Answer: How to Calculate BMI?

Body mass index, or BMI, is your weight divided by your height squared. In metric units, that is weight in kilograms divided by height in meters squared. In imperial units, it is 703 times weight in pounds, divided by height in inches squared.

System	BMI formula	Example
Metric	Weight in kg divided by height in m squared	80 kg, 1.80 m: 80 divided by 3.24 = 24.7
Imperial	703 times weight in lb, divided by height in inches squared	176.4 lb, 70.9 in: 703 times 176.4, divided by 5,026.8 = 24.7

Both versions answer how to calculate BMI the same way for the same person, as the worked examples below prove. If you want to skip the arithmetic, our BMI calculator does the same math instantly. The NHLBI states the same formulas on its Calculate Your BMI page.

What Is BMI and What Does It Measure?

BMI stands for body mass index. It is a ratio of your weight to your height, used as a quick screening tool to sort adults into underweight, healthy, overweight, and obesity categories. Doctors and researchers use it because it needs only two measurements anyone can take at home.

BMI estimates body fatness from weight and height, but it does not measure body fat directly. It cannot tell muscle from fat, and it says nothing about where you carry fat. That is why it is a screening tool, not a diagnosis.

BMI was designed for adults. Children and teens use the same arithmetic but a different interpretation, covered below. The formula is the same for men and women.

How to Calculate BMI in Metric Units?

The metric version is the simplest form of the BMI formula, and the one used in most of the world. You need your weight in kilograms and your height in meters.

BMI = weight in kg divided by (height in m times height in m)

If your height is in centimeters, divide it by 100 first: 180 cm is 1.80 m, and 168 cm is 1.68 m. Skipping this conversion is where most metric mistakes happen.

Short example: a man weighs 80 kg and is 1.80 m tall. Square the height (3.24), then divide: 80 divided by 3.24 equals 24.7, a healthy BMI. The full step-by-step version, plus the imperial check, is in the worked examples below.

How to Calculate BMI in Imperial Units?

The imperial version is the one used in the United States. You need your weight in pounds and your height in inches. If your height is in feet and inches, convert it to total inches first: 5 feet 11 inches is 71 inches, and 5 feet 6 inches is 66 inches.

BMI = 703 times weight in lb, divided by (height in inches times height in inches)

The 703 is a unit-conversion constant. It puts pounds and inches on the same scale as kilograms and meters, so the imperial formula gives the same BMI as the metric one. Without it, the answer comes out about 700 times too small.

Short example: a woman weighs 149.9 lb and is 66.1 inches tall. Square the height (4,369.2), divide (149.9 divided by 4,369.2 equals 0.0343), then multiply by 703 to get 24.1, a healthy BMI. The full step-by-step version, plus the metric check, is in the worked examples below.

How Do I Measure BMI Without a Calculator?

To see how to calculate BMI without a calculator, weigh yourself on a scale, measure your height against a wall without shoes, then do the arithmetic by hand. In metric, square your height in meters and divide your weight in kilograms by that number. In imperial, square your height in inches, divide your weight in pounds by that number, and multiply by 703.

Worked Example 1: How to Calculate BMI for a Man?

Meet Alex: 80 kg, 1.80 m tall (about 5 feet 11 inches), or 176.4 lb and 70.9 inches. Here is how to calculate BMI for him in both systems.

Metric calculation:

1. Height: 1.80 m (already in meters).
2. Square it: 1.80 times 1.80 equals 3.24.
3. Divide: 80 divided by 3.24 equals 24.7.
4. Result: BMI 24.7, healthy weight.

Imperial calculation:

1. Convert weight: 80 kg times 2.20462 equals 176.4 lb.
2. Convert height: 1.80 m times 39.3701 equals 70.9 inches.
3. Square the height: 70.9 times 70.9 equals 5,026.8.
4. Divide: 176.4 divided by 5,026.8 equals 0.0351.
5. Multiply by 703: 0.0351 times 703 equals 24.7.
6. Result: BMI 24.7, healthy weight.

Both methods give 24.7. If your two answers disagree by more than 0.2, one has a unit error. The mistakes section below will help you find it.

Worked Example 2: How to Calculate BMI for a Woman?

Meet Sara: 68 kg, 1.68 m tall (about 5 feet 6 inches), or 149.9 lb and 66.1 inches. Here is how to calculate BMI for her in both systems.

Metric calculation:

1. Height: 1.68 m (already in meters).
2. Square it: 1.68 times 1.68 equals 2.8224.
3. Divide: 68 divided by 2.8224 equals 24.1.
4. Result: BMI 24.1, healthy weight.

Imperial calculation:

1. Convert weight: 68 kg times 2.20462 equals 149.9 lb.
2. Convert height: 1.68 m times 39.3701 equals 66.1 inches.
3. Square the height: 66.1 times 66.1 equals 4,369.2.
4. Divide: 149.9 divided by 4,369.2 equals 0.0343.
5. Multiply by 703: 0.0343 times 703 equals 24.1.
6. Result: BMI 24.1, healthy weight.

Again, both methods agree: 24.1. The formula is exactly the same for a woman as for a man. What differs is interpretation, covered in the men-versus-women section below.

How Do You Read Your BMI Number?

Once you know how to calculate BMI, the next step is reading the result.

BMI	Category	What it means
Below 18.5	Underweight	Below the healthy range
18.5 to 24.9	Healthy weight	Linked with the lowest health risk
25.0 to 29.9	Overweight	Above the healthy range
30.0 to 34.9	Obesity class 1	Obesity range
35.0 to 39.9	Obesity class 2	Higher-risk obesity range
40.0 or higher	Obesity class 3	Severe obesity range

A BMI chart shows these cutoffs in a visual grid, with height on one side and weight on the other. The table in the next section is the most useful version of a BMI chart, because it tells you the actual weight range for your height.

One thing to notice when you learn how to calculate BMI is that the healthy range is wide. For a person who is 1.70 m tall, it spans about 53 to 72 kg. There is no single ideal BMI, and the middle of the band is not healthier than the edges.

What Is Your Healthy Weight Range? A BMI Chart by Height

This BMI chart turns the formula around: it shows the weight range that puts you in the healthy BMI band for your height, plus the weights where overweight and obesity begin.

Height	Healthy weight range (BMI 18.5 to 24.9)	Overweight starts (BMI 25)	Obesity starts (BMI 30)
1.50 m (4 ft 11 in)	41.6 to 56.0 kg	56.3 kg	67.5 kg
1.55 m (5 ft 1 in)	44.4 to 59.8 kg	60.1 kg	72.1 kg
1.60 m (5 ft 3 in)	47.4 to 63.7 kg	64.0 kg	76.8 kg
1.65 m (5 ft 5 in)	50.4 to 67.8 kg	68.1 kg	81.7 kg
1.70 m (5 ft 7 in)	53.5 to 72.0 kg	72.3 kg	86.7 kg
1.75 m (5 ft 9 in)	56.7 to 76.3 kg	76.6 kg	91.9 kg
1.80 m (5 ft 11 in)	59.9 to 80.7 kg	81.0 kg	97.2 kg
1.85 m (6 ft 1 in)	63.3 to 85.2 kg	85.6 kg	102.7 kg
1.90 m (6 ft 3 in)	66.8 to 89.9 kg	90.3 kg	108.3 kg

How to use this chart: find your height row, then compare your weight. Inside the healthy range means a BMI of 18.5 to 24.9. Above the overweight line means 25 or higher. If your height falls between two rows, your range falls between them too. The chart also answers how to calculate BMI in reverse: pick a target BMI, multiply it by your squared height, and you get the weight for that BMI.

For a single target instead of a range, our ideal weight calculator gives you a weight range for your height using several standard formulas.

What Is a Good BMI to Aim For?

A good BMI target is the healthy range, 18.5 to 24.9. Anywhere inside that band counts. If your BMI is above 25, a realistic first goal is to get it below 30, then below 25, in stages. If it is below 18.5, aim to bring it up with guidance from your doctor. Pair your BMI goal with a waist and body fat check, since those tell you what BMI cannot.

How Do You Calculate BMI for Children and Teens?

For children and teens aged 2 to 19, how to calculate BMI uses the same arithmetic, but the interpretation is completely different. You never compare a child’s BMI to the adult cutoffs. A BMI of 22 is healthy for an adult but could be obesity for a young child, because children grow and their body composition changes with age.

Instead, a child’s BMI is plotted as a percentile on CDC growth charts for the same age and sex, showing how the child’s BMI compares to other children of the same age and sex.

Percentile	Category
Below the 5th percentile	Underweight
5th percentile to below the 85th percentile	Healthy weight
85th percentile to below the 95th percentile	Overweight
95th percentile or higher	Obesity

To find a child’s percentile, calculate BMI with the same formula, then look it up on a BMI-for-age percentile chart. A pediatrician should interpret the result, since growth patterns change fast. One reading matters less than the trend: steady tracking along one percentile curve is usually fine, while crossing percentiles quickly deserves a closer look.

Babies under age 2 do not use BMI at all. Doctors use weight-for-length charts for them instead. For teens, puberty changes body composition quickly, so a percentile that looks high during a growth spurt may settle on its own.

Is BMI Calculated Differently for Men and Women?

No. When you learn how to calculate BMI, the formula is identical for men and women, and so are the adult categories. A man and a woman with the same height and weight get the same BMI.

What differs is interpretation. At the same BMI, women typically carry more body fat than men, because women need more essential fat for hormonal health. So a BMI of 24 can mean a different body composition in a woman than in a man.

The same point applies to athletes and muscular people. Muscle weighs more than fat, so a muscular person can land in the overweight or obese range with low body fat. If you lift weights and your BMI says overweight, the next check is your body fat percentage, not a diet. Our guide to what is a healthy body fat percentage explains the ranges.

What Are the 5 Most Common BMI Calculation Mistakes?

Most wrong results when people learn how to calculate BMI come from five errors.

1. Mixing units. The most common mistake is putting kilograms with inches, or pounds with meters. Metric goes with metric, imperial goes with imperial. If your answer looks absurd, like 0.03 or 15,000, a unit mix is the cause.
2. Forgetting to square the height. BMI divides by height squared, not by height. Divide by height once and you get a number in the hundreds. Always multiply height by itself before dividing.
3. Using centimeters instead of meters. In the metric formula, height must be in meters. If you plug in 180 instead of 1.80, your BMI comes out 10,000 times too small. Sanity check: your squared height in meters should be between about 2 and 4. If it is in the thousands, you used centimeters.
4. Forgetting the 703 in the imperial formula. Without the 703, the imperial calculation gives a tiny decimal like 0.035 instead of a BMI like 24.7. If your imperial answer is below 1, you forgot the constant.
5. Rounding too early or using stale measurements. Rounding height or weight before calculating can shift your BMI by half a point, which matters near a cutoff like 25 or 30. Measure height without shoes, weigh yourself in the morning on the same scale, and keep one decimal place until the final step. Also recheck your height: many adults are shorter than the height on their driver’s license.

If two calculators disagree, it is almost always rounding or unit conversion, not a different formula. Run the hand calculation from the worked examples above to find which one rounded wrong.

Why Do Different BMI Calculators Give Different Answers?

Different BMI calculators usually disagree because of rounding or unit conversion, not because they use different formulas. Common causes are rounding height to whole inches, converting kilograms to pounds with a rounded factor, or measuring height with shoes on. The differences are usually under half a point.

What Should You Do After You Calculate Your BMI?

Once you learn how to calculate BMI, your number is a starting point, not a plan. What you do next depends on which category you landed in.

If your BMI is in the healthy range (18.5 to 24.9): your job is maintenance. Find your maintenance calories by learning how to calculate BMR and adding your activity, or read our guide to how many calories a day to size your daily intake. Recheck your BMI once or twice a year, and watch your waist and body fat too.

If your BMI is in the overweight range (25.0 to 29.9): aim for a small, steady calorie deficit of about 300 to 500 calories a day, plus two or three strength sessions a week. A realistic first target is a 5 to 10 percent weight loss, which already improves blood pressure, blood sugar, and joint load. Our guide to how many calories a day shows how to set the deficit.

If your BMI is in the obesity range (30 or higher): talk to your doctor before starting a major weight loss plan, especially with blood pressure, blood sugar, or joint problems. A doctor can check for conditions that affect weight and set a safe pace. The approach is the same, a steady deficit plus strength training, but medical guidance matters more at this range.

If your BMI is below 18.5: talk to your doctor too. Being underweight can come from a fast metabolism, a medical condition, or simply not eating enough, and the right fix depends on the cause. A doctor or dietitian can check for causes and build a plan around nutrient-dense food and strength training.

Whatever your category, recheck your BMI every four to eight weeks, not daily. Daily weight swings from water, salt, and meals can move your BMI by half a point. Track the trend, and judge progress by waist and body fat too.

What Are the Limits of BMI?

BMI is a useful first screen, and a poor final verdict. Once you know how to calculate BMI, remember it does not measure body fat, it does not see where you carry fat, and the same number means different things at different ages and body types.

This article is the how-to guide, so it does not repeat the full accuracy discussion. For when a high BMI is a false alarm and when a normal BMI hides a problem, read our guide to is BMI accurate. Use that article to judge your number, and this one to calculate it.

Frequently Asked Questions