How to Calculate BMI

Calculating your Body Mass Index (BMI) takes less than a minute once you know the formula. You need two numbers — your weight and your height — and one division. This page shows you the metric formula and the imperial formula, works through examples in both unit systems, and explains what your result means. Use the BMI calculator above to check your own number as you read, or follow the steps below with a pen and paper.

BMI was designed as a screening tool, not a diagnosis. Doctors and health organisations around the world use it as a quick first check of whether an adult is underweight, at a healthy weight, overweight, or living with obesity. It does not measure body fat directly, and it cannot tell muscle apart from fat — but as a starting point, it is fast, free, and surprisingly useful.

The BMI Formula (Metric)

The metric formula is the original and simplest version of BMI. It divides your weight in kilograms by the square of your height in metres:

BMI = weight (kg) ÷ height (m)²

Height is squared because bodies grow in three dimensions, and squaring keeps the number comparable between short and tall people. The formula dates back to the 1830s, when Belgian statistician Adolphe Quetelet noticed that adult weight tends to grow with the square of height. The World Health Organization later adopted the same formula with the category cut-offs still used today.

Step 1 — Measure Your Height in Metres

Measure your height barefoot, standing straight against a wall, and convert centimetres to metres by dividing by 100. A height of 175 cm is 1.75 m. If you only know your height in feet and inches, convert it first: multiply feet by 30.48 and inches by 2.54, then add the two results. Five feet 9 inches is 175.26 cm, or 1.75 m.

Step 2 — Square Your Height

Multiply your height in metres by itself. For 1.75 m, that is 1.75 × 1.75 = 3.0625. This is the step people most often skip or get wrong — dividing by height once instead of height squared gives a meaningless number, so square first and then divide.

Step 3 — Divide Your Weight by That Number

Divide your weight in kilograms by the squared height. For a person who weighs 70 kg at 1.75 m, the calculation is 70 ÷ 3.0625 = 22.9. That is the BMI, and it falls inside the healthy range of 18.5 to 24.9. The BMI calculator above does all three steps instantly, but doing it once by hand shows you exactly what the number means.

The Imperial Formula (Pounds and Feet/Inches)

If you weigh yourself in pounds and measure height in feet and inches, the formula needs a conversion factor of 703:

BMI = 703 × weight (lb) ÷ height (in)²

The 703 is not a magic number. It is the factor that converts pounds per square inch into kilograms per square metre, so the result matches the metric formula exactly. Without it, the imperial calculation would give a number about 703 times too small. Because the factor is built into the formula, you can work entirely in pounds and inches and still get the same BMI as the metric version.

1. Convert your height to inches only. Multiply feet by 12 and add the remaining inches. Five feet 9 inches is 5 × 12 + 9 = 69 inches.
2. Square your height in inches. For 69 inches, that is 69 × 69 = 4,761.
3. Multiply your weight in pounds by 703. For 154 lb, that is 703 × 154 = 108,262.
4. Divide the result by the squared height. For this example, 108,262 ÷ 4,761 = 22.7. The BMI is 22.7, inside the healthy range — and it agrees with the metric calculation for the same person (70 kg at 175 cm gives 22.9, with the small difference coming from rounding 154 lb and 5 ft 9 in).

A second imperial example: a person who weighs 180 lb and stands 5 feet 10 inches tall. Height in inches is 70, squared is 4,900. Multiply 703 × 180 = 126,540, then divide: 126,540 ÷ 4,900 = 25.8. That is just inside the overweight band, which starts at 25.0. And a person at 140 lb and 5 feet 4 inches: height is 64 inches, squared is 4,096; 703 × 140 = 98,420; 98,420 ÷ 4,096 = 24.0, at the top of the healthy range.

Worked Examples

The table below works through five people in both unit systems. Each row shows the inputs, the arithmetic, and the category, so you can check your own calculation against a worked case that matches your size.

BMI Categories (WHO Classification)

Once you have your number, read it against the World Health Organization bands. The healthy band runs from 18.5 to 24.9, and the bands on either side describe how far outside it you sit.

The bands are conventions for population statistics, not cliffs in your body. The risk at a BMI of 25.0 is only slightly above the risk at 24.9, and the risk rises smoothly as BMI climbs. Some populations use lower action points: for many people of South or East Asian descent, health risks start rising at a BMI of 23 rather than 25, so treat 23 to 27.5 as the overweight band if that applies to you, and discuss it with your doctor. The World Health Organization’s obesity fact sheet explains how these thresholds are used worldwide.

How to Measure Height and Weight Correctly

A BMI result is only as good as the two numbers fed into it. At 175 cm, misreading your weight by 3 kg shifts BMI by about 1.0, and misreading your height by 2 cm shifts it by about 0.5. That is enough to move someone across a category boundary, so it is worth taking two careful minutes to measure properly rather than guessing from memory.

1. Place the scale on a hard, level floor and check it reads zero before you step on.
2. Weigh yourself in the morning, after using the bathroom and before eating or drinking.
3. Wear light clothing, or the same clothing each time, and no shoes.
4. Stand barefoot against a wall, heels together, looking straight ahead.
5. Have someone mark the top of your head with a level object, then measure from the floor to the mark.
6. Record both numbers and enter them exactly — do not round your height up or your weight down.

Common Measuring Mistakes

• Guessing height from memory. Most adults overestimate their height by 1 to 3 cm, which flatters the BMI result.
• Weighing after a large meal. Food and drink can add 1 to 2 kg temporarily, pushing BMI up by nearly a full point.
• Measuring height in shoes. Trainers and heels add 2 to 4 cm, which makes BMI look lower than it really is.
• Using a scale on carpet. Soft flooring absorbs weight and gives a falsely low reading.
• Comparing readings from different scales. Home scales disagree by a kilogram or more, so stick to one scale for trends.

Common Mistakes When Calculating BMI

Most wrong BMI results come from the same short list of errors. Check yours against it before trusting the number.

• Forgetting to square the height. Dividing weight by height once, instead of height squared, gives a number far too large to be a BMI.
• Mixing centimetres and metres. Dividing kilograms by centimetres squared instead of metres squared gives a BMI around 0.002 — always convert height to metres first.
• Forgetting the 703 in imperial units. Pounds divided by inches squared without the 703 gives about 0.03, not a BMI at all.
• Using height in feet as a decimal. Five feet 9 inches is 5.75 feet, not 5.9 feet. Convert to total inches (69) before squaring.
• Rounding too early. Round only at the end. Rounding height to 1.8 m when it is really 1.75 m shifts the result by more than a full BMI point.
• Using adult cut-offs for children. Children and teenagers need age- and sex-specific percentile charts, not the fixed adult bands on this page.

BMI Prime: A Second Number Worth Knowing

The calculator above also shows BMI prime, which is your BMI divided by 25 — the top of the healthy range. It answers the question “how far am I from the healthy ceiling?” in one tidy number. A prime below 1.00 means you are inside the healthy band, 1.00 means you sit exactly on the boundary, and anything above it tells you by what fraction you exceed it.

BMI prime is best for tracking progress toward the healthy band, because the goal is always the same: get the prime under 1.00. The calculator also shows the ponderal index, which divides weight by height cubed instead of squared. It stays steadier for very short or very tall people, where ordinary BMI can mislead. For the default example on this page — 70 kg at 175 cm — the ponderal index is 13.1.

What to Do After You Calculate Your BMI

A number on its own changes nothing. What matters is the next step, and it depends on where your number landed.

1. Inside the healthy band (18.5–24.9). Keep doing what works. Recheck monthly, keep active, and watch your waist as well as your weight — a waist under half your height is a good second check.
2. Just above the band (25.0–29.9). Find your daily calorie needs with the calorie calculator, then eat 250 to 500 kcal below maintenance and add a 30-minute daily walk. Recheck in a month.
3. Well above the band (30.0 and over). The same steps apply, but involve your doctor as well, especially if you have high blood pressure, high blood sugar, or joint pain. A plan built with a clinician beats a plan built alone.
4. Below the band (under 18.5). Being underweight carries its own risks, including nutritional deficiency and bone loss. If you are trying to gain weight, add calorie-dense foods and strength training, and talk to a doctor if the low weight is unintentional.
5. Check body fat too. Pair your BMI with your body fat percentage and your ideal weight range before acting on the number, because BMI cannot tell muscle from fat.

When BMI Misleads

BMI is a screening tool, and like every screening tool it has blind spots. Knowing them keeps you from acting on a misleading number.

• Muscular athletes. A rugby player weighing 100 kg at 1.80 m has a BMI of 30.9, technically obese — but if a body fat check shows 14%, the player is clearly not carrying excess fat, and the BMI label is simply wrong for them.
• Older adults. Muscle loss with age means an older person can sit inside the healthy BMI band while carrying an unhealthy amount of fat. Strength, mobility, and waist size matter more here than the number alone.
• Very short or very tall people. BMI tends to overstate weight for shorter people and understate it for taller people, because it divides by height squared while bodies scale more like volume. The ponderal index in the result above is steadier at the extremes.
• Pregnancy. BMI bands do not apply during pregnancy, when weight gain is expected and healthy. Use pregnancy-specific guidance instead.
• Children and teenagers. Growing bodies need BMI-for-age percentile charts, not adult cut-offs. Never judge a child’s weight with the adult bands.

Related tools on this site: the main BMI calculator for a quick check, the body fat calculator for fat versus lean mass, the ideal weight calculator for your target range, and the calorie calculator for the daily intake behind any weight plan.

Frequently Asked Questions

What is the formula to calculate BMI?

In metric units, BMI = weight in kilograms divided by height in metres squared. In imperial units, BMI = 703 × weight in pounds divided by height in inches squared. Both formulas give the same result for the same person.

How do I calculate BMI without a calculator?

Measure your height in metres and your weight in kilograms, square the height, then divide the weight by that number. For 70 kg at 1.75 m: 1.75 × 1.75 = 3.0625, then 70 ÷ 3.0625 = 22.9. A phone calculator handles the division in seconds.

What is a good BMI?

For most adults, a BMI between 18.5 and 24.9 is considered healthy. That band is linked to the lowest risk of weight-related disease in large population studies, though individual health depends on much more than one number.

Why is there a 703 in the imperial BMI formula?

The 703 converts pounds per square inch into kilograms per square metre, so the imperial calculation gives the same BMI as the metric one. Without it, the imperial result would be about 703 times too small.

Can I calculate BMI for my child?

Not with the adult formula and bands. Children and teenagers need age- and sex-specific BMI percentile charts, because healthy weight changes constantly as they grow. Ask your paediatrician for the right assessment.

How accurate is BMI?

As a population screening tool, BMI is well validated. As an individual verdict, it is limited: it cannot tell muscle from fat, ignores where fat sits, and does not adjust for age, sex, or ethnicity. Pair it with waist size and body fat percentage for a fairer picture.

Disclaimer: This calculator is for general information only and is not medical advice. BMI is a screening measure, not a diagnosis. Talk to a healthcare professional about your weight and health.