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

Body Mass Index (BMI) is a single number that relates your weight to your height. The formula is straightforward: take your weight in kilograms, divide it by your height in metres, then divide by your height in metres again. The result tells you whether your weight falls into a range that health professionals use as a screening tool.

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 gives a useful starting point for conversations with a doctor. You can do it by hand in under a minute, or use an online calculator if you prefer.

Key Takeaways

  • BMI = weight (kg) ÷ height (m) ÷ height (m), and the result falls into four standard ranges that health professionals recognize.
  • If you know your weight in pounds and height in inches, multiply weight by 703, divide by height, then divide by height again to get the same result.
  • A BMI under 18.5 is classed as underweight, 18.5 to 24.9 as normal weight, 25 to 29.9 as overweight, and 30 or above as obese by standard medical definitions.
  • BMI does not measure body fat directly and can be misleading for athletes, older adults, or people with high muscle mass.

Calculate BMI using the metric formula

If you know your weight in kilograms and height in metres, the calculation takes three steps. First, write down your weight in kilograms. Second, write down your height in metres (for example, 1.75 m). Third, multiply your height by itself — so 1.75 × 1.75 = 3.0625. Then divide your weight by that number.

Example: A person who weighs 70 kg and is 1.75 m tall would calculate: 70 ÷ 3.0625 = 22.9. That BMI of 22.9 falls in the normal weight range.

Write down each step as you go. The most common mistake is forgetting to multiply height by itself before dividing — if you divide weight by height only once, you will get a number that is far too large.

Calculate BMI using pounds and inches

If you know your weight in pounds and height in inches, use this formula: multiply your weight by 703, divide by your height in inches, then divide by your height in inches again.

Example: A person who weighs 154 pounds and is 69 inches tall would calculate: (154 × 703) ÷ 69 ÷ 69 = 108,262 ÷ 69 ÷ 69 = 22.7. That BMI of 22.7 is in the normal weight range.

The number 703 is a conversion factor that accounts for the difference between metric and imperial units. You do not need to understand why it is 703 — just use it as written. Again, make sure you divide by height twice, not once.

What your BMI number means

Health professionals use four standard BMI ranges. A BMI under 18.5 is classed 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 adults of all ages, though some doctors adjust them slightly for older adults.

These ranges are screening categories, not diagnoses. A BMI in the overweight or obese range does not mean you are unhealthy, and a BMI in the normal range does not mean you are healthy. They are a signal to talk to a doctor if you have other health concerns.

When BMI is misleading

BMI does not distinguish between muscle and fat. A person who lifts weights regularly may have a high BMI because muscle weighs more than fat, even though their body composition is healthy. Athletes, bodybuilders, and people who do heavy physical work often fall into the overweight or obese BMI range despite having low body fat.

BMI also does not account for where you carry weight. Carrying extra weight around your abdomen is associated with higher health risk than carrying it in your hips or thighs, but BMI treats all weight the same. Older adults may have lost height due to compression of the spine, which can inflate their BMI without any change in actual weight.

If your BMI is high but you are physically active, have good blood pressure and cholesterol, or have a lot of muscle, talk to a doctor about whether BMI is the right tool for your situation. They may suggest other measurements like waist circumference or body fat percentage.

Use an online BMI calculator to check your work

If you want to verify your calculation or prefer not to do the math by hand, many websites offer free BMI calculators. You enter your weight and height, and the calculator does the division for you. The result should match what you calculated yourself if you did the steps in order.

Online calculators are reliable — they all use the same formula — but they are not more accurate than doing it yourself. The advantage is speed and the chance to catch arithmetic errors. Some calculators also show you which BMI range you fall into and explain what that range means.

Frequently Asked Questions

Does BMI change if I lose or gain weight?

Yes. BMI is calculated from your current weight, so it changes whenever your weight changes. If you lose 5 kg, recalculate and your BMI will be lower. If you gain weight, your BMI goes up. Height does not change in adults, so the only variable that shifts your BMI is weight.

Is BMI different for children?

The formula is the same, but the ranges are different. Children's BMI is compared to growth charts for their age and sex, not to the adult ranges. If you are calculating BMI for a child, talk to their doctor about what the number means for them specifically.

What if I am very tall or very short?

The formula works the same way regardless of height. Very tall or very short people may find that BMI is less useful as a screening tool — a doctor can tell you whether it makes sense in your case. Some research suggests BMI is less reliable at the extremes of height.

Can I use BMI to track weight loss progress?

Yes, BMI will change as you lose weight, so you can recalculate it monthly or quarterly to see the trend. However, other measurements like how your clothes fit, your energy level, or your strength may be more meaningful to you personally than a number on a chart.

Do I need to know my exact weight and height?

Close estimates work fine for a rough BMI. If you are within a pound or half a centimetre of your actual measurements, the BMI will be nearly the same. For a more precise number, use a scale and measuring tape, but for general screening purposes, estimates are sufficient.