The Basic Formula for BMI

BMI (Body Mass Index) is a number that relates your weight to your height. The formula is the same everywhere: divide your weight in kilograms by your height in metres squared. If you work in pounds and inches, divide your weight in pounds by your height in inches squared, then multiply by 703. That number is your BMI.

The result is a single figure — usually between 15 and 40 for most adults. That number sits in a range: underweight (below 18.5), normal weight (18.5 to 24.9), overweight (25 to 29.9), or obese (30 and above). BMI does not measure body fat directly. It is a screening tool, not a diagnosis.

Key Takeaways

  • BMI uses only weight and height, so you can calculate it yourself with a scale and a measuring tape in under a minute.
  • The metric formula divides weight in kilograms by height in metres squared; the imperial formula divides weight in pounds by height in inches squared, then multiplies by 703.
  • Your BMI result falls into one of four ranges: underweight, normal, overweight, or obese, but BMI does not measure body fat or muscle.
  • Online BMI calculators do the math for you, but the calculation itself is straightforward enough to do by hand or on a basic calculator.

Measuring Your Weight and Height Accurately

Accuracy starts with the right tools. Use a digital scale on a flat, hard floor — not carpet — and weigh yourself at the same time each day, usually in the morning before eating or drinking. Wear minimal clothing or the same clothes each time so the number does not shift by a pound or two from fabric weight.

For height, stand barefoot with your back against a wall, heels touching the wall, and your eyes looking straight ahead. Have someone mark the wall at the top of your head, or use a measuring tape held level across the top of your head. Measure from the floor to the mark. If you cannot stand straight, measure while sitting and add the length of your lower leg from knee to heel.

Calculating BMI Using the Metric Formula

If your weight is in kilograms and your height is in metres, the math is straightforward. Multiply your height by itself (height squared), then divide your weight by that number.

Example: You weigh 70 kilograms and are 1.75 metres tall. Multiply 1.75 × 1.75 = 3.0625. Then divide 70 ÷ 3.0625 = 22.86. Your BMI is 22.86, which falls in the normal weight range.

If you need to convert pounds to kilograms, divide pounds by 2.205. If you need to convert inches to metres, divide inches by 39.37.

Calculating BMI Using the Imperial Formula

If your weight is in pounds and your height is in inches, multiply your height by itself, divide your weight by that number, then multiply the result by 703.

Example: You weigh 154 pounds and are 69 inches tall. Multiply 69 × 69 = 4,761. Then divide 154 ÷ 4,761 = 0.03235. Multiply 0.03235 × 703 = 22.74. Your BMI is 22.74, which falls in the normal weight range.

The 703 factor converts the result from imperial units to the standard BMI scale. Without it, your number would be much smaller and harder to compare to standard ranges.

Using an Online BMI Calculator

If you do not want to do the math yourself, enter your weight and height into any online BMI calculator — search "BMI calculator" and you will find dozens. Type in your numbers, select metric or imperial units, and the calculator shows your BMI and which range it falls into. The result is when ready and the same as calculating by hand.

Online calculators are useful if you are checking your BMI regularly or if you prefer not to do arithmetic. They also often show your result on a chart so you can see where you stand relative to the ranges. However, the calculation is straightforward enough that you do not need a tool to do it once.

What Your BMI Number Means

A BMI below 18.5 is classified as underweight. A BMI from 18.5 to 24.9 is normal weight. A BMI from 25 to 29.9 is overweight. A BMI of 30 or above is obese. These ranges are the same for all adults, regardless of age or sex, though some health organisations use slightly different cutoffs for older adults or children.

BMI is a screening tool, not a medical diagnosis. Two people with the same BMI can have very different body compositions — one might have more muscle, the other more fat. Athletes often have a high BMI because muscle weighs more than fat. BMI also does not account for where you carry weight, which matters for health risk. A doctor or health professional can interpret your BMI in the context of your overall health.

Frequently Asked Questions

Is BMI accurate for athletes or very muscular people?

No. BMI counts all weight the same, so a muscular person can have a high BMI even though they carry little body fat. If you have significant muscle mass, BMI alone is not a reliable measure of body composition. A doctor can use other methods like body fat percentage or waist circumference for a fuller picture.

Do I need to calculate my BMI more than once?

Not unless your weight or height changes. BMI is useful as a screening tool over time — calculating it once a year or when your weight shifts significantly is common practice. Calculating it daily or weekly adds no new information because weight fluctuates by a pound or two from day to day.

Does BMI change with age?

The calculation itself does not change, but some health organisations interpret BMI ranges differently for older adults. The standard ranges (underweight, normal, overweight, obese) explore to adults aged 18 to 65. For children and teens, BMI is calculated the same way but compared to age and sex-specific growth charts instead of fixed ranges.

What if my height or weight is hard to measure?

Do your best with the tools you have. A bathroom scale and a measuring tape are enough. If you cannot stand to be measured, ask a family member or health provider to help. Even an approximate measurement gives you a useful screening number — exact precision matters less than tracking whether the number moves over time.

Can I calculate BMI if I use metric for weight but imperial for height?

Yes, but you have to convert one unit first. Convert pounds to kilograms by dividing by 2.205, or convert inches to metres by dividing by 39.37. Then use the metric formula. It is easier to convert both to the same system before you start the calculation.