BMI Calculator

How BMI is calculated

BMI (body mass index) is your weight in kilograms divided by the square of your height in metres: BMI = weight ÷ (height × height). Height is the key variable, and because it is squared, small height differences move the result more than you might expect. If you work in imperial units, the equivalent formula is BMI = 703 × weight in pounds ÷ (height in inches × height in inches). For example, someone who is 5 ft 9 in (69 in) and 160 lb gets 703 × 160 ÷ (69 × 69) ≈ 23.6, the same figure you would get after converting to metric. BMI does not measure body fat directly; it estimates whether your weight is roughly proportionate to your height, which is why it works as a quick screen rather than a diagnosis.

A worked example, step by step

Say you are 175 cm (1.75 m) and weigh 85 kg. First square the height: 1.75 × 1.75 = 3.0625. Then divide: 85 ÷ 3.0625 ≈ 27.8, which falls in the overweight band. The calculator also shows the healthy weight range for your height, which is the weight that corresponds to a BMI of 18.5 to 24.9. For 175 cm that is 18.5 × 3.0625 ≈ 56.7 kg at the low end and 24.9 × 3.0625 ≈ 76.3 kg at the high end. So this person would need to lose about 8.7 kg to reach the top of the healthy band. Seeing the target as an actual weight range, rather than just a category label, makes the number far easier to act on.

Categories and why the cut-offs differ by region

• WHO standard: underweight below 18.5, normal 18.5 to 24.9, overweight 25 to 29.9, obese 30 and above.
• Many Asian populations carry more metabolic risk at a lower BMI, so several Asian guidelines treat 23 and above as increased risk and 25 or 27.5 and above as high risk.
• This is why the same BMI of 24 can be labelled 'normal' under one guideline and 'overweight' under another. The number is identical; only the threshold applied to it changes.
• The bands are population screening tools, not personal health verdicts. Treat a result near a boundary (for example 24.8 vs 25.1) as essentially the same picture.

When BMI is genuinely useful

• Tracking your own trend over time. Comparing your BMI this month with six months ago, at the same height, is more informative than the single number, because it shows direction.
• Setting a concrete weight goal. Convert your target BMI back into kilograms for your height so you have a real number to aim for instead of a vague category.
• A first-pass screen before deeper checks. If your BMI sits well outside 18.5 to 24.9, it is a reasonable prompt to look at waist size, blood pressure, and blood markers with a professional.
• Comparing groups or populations, which is what BMI was originally designed for, rather than judging any one athlete or individual.

Common mistakes and misconceptions

• Entering height in centimetres where metres are expected. 175 typed as 175 m instead of 1.75 m produces a nonsense result. Check that the units match the field.
• Treating BMI as a body-fat percentage. It is not. A very muscular person at 183 cm and 95 kg scores about 28.4 and looks 'overweight' on paper while carrying little fat.
• Assuming a 'normal' BMI means no risk. Someone can sit at BMI 23 with a large waist and poor metabolic markers, sometimes called normal-weight obesity.
• Using adult categories for children or teens. Their BMI is read against age-and-sex percentile charts, not the fixed adult bands.
• Chasing the lowest possible number. Being underweight carries its own risks; the aim is the healthy range, not the minimum.

The limits of BMI, and what to pair it with

BMI ignores how weight is made up and where it sits. Muscle weighs more than fat for the same volume, so athletes are often overestimated, while older adults who have lost muscle can be underestimated. It also says nothing about fat distribution, and abdominal fat is more strongly linked to health risk than fat on the hips or limbs. For a fuller picture, pair BMI with a waist measurement: a rule of thumb is to keep your waist under half your height, and commonly cited high-risk thresholds are roughly 102 cm (40 in) for men and 88 cm (35 in) for women. BMI is not validated for pregnancy. Use it as one reference among several, not the last word on your health.