How to Calculate BMI (and What It Really Tells You)

Quick Answer

Body Mass Index is your weight in kilograms divided by the square of your height in metres. A person who is 70 kg and 1.75 m tall has a BMI of 70 / (1.75 × 1.75), which is about 22.9. For adults the World Health Organization reads that number against four bands: under 18.5 is underweight, 18.5 to 24.9 is healthy, 25 to 29.9 is overweight, and 30 or above is obesity. This calculator works it out in metric or imperial, names your band, and shows the actual weight range that would keep you in the healthy zone at your height. Also known as a body mass index calculator, a BMI checker, or a weight status calculator, it measures where you sit today and how the BMI is calculated so the number is never a black box. Understanding a BMI figure quoted at a doctor visit or on an insurance form is the most common reason people look it up.

How BMI Works

BMI exists to turn two easy measurements, height and weight, into a single number that scales fairly across tall and short people. Weight alone is misleading because a taller person is expected to weigh more. Dividing by height squared cancels most of that height effect, so the resulting figure can be compared against the same fixed bands for everyone. That is the whole idea: one ratio, four categories, no equipment beyond a scale and a tape measure. The BMI formula is weight divided by height squared, and it is what the tool uses to convert between metric and imperial. If you enter imperial units, the calculator first converts pounds to kilograms and inches to metres, then applies the identical formula, which is why 154 lb at 69 inches and 69.9 kg at 175 cm land on the same BMI.

Reading the WHO Categories

The WHO weight categories, also called the BMI categories, mean the same four fixed bands for all adults: 18.5, 25, and 30. They come from large population studies linking BMI bands to health risk on average. A BMI of 24.9 is healthy and 25.0 is overweight, but the boundary is a statistical line, not a cliff, so treat a figure near an edge as "close to the line" rather than a hard verdict. Alongside your category the tool shows the healthy weight range for your exact height, so instead of an abstract score you see the concrete weights that would place you inside the 18.5 to 24.9 band.

Examples

70 kg at 175 cm. For example, BMI is 70 / (1.75)² = 22.9, comfortably inside the healthy 18.5 to 24.9 range. To find the healthy weight range for a given height, the tool shows roughly 56.7 kg to 76.3 kg here.

90 kg at 170 cm. For example, BMI is 90 / (1.70)² = 31.1, which falls in the obesity band; the tool shows how many kilograms separate you from the healthy range.

154 lb at 69 inches. For example, converted internally that is a BMI of about 22.7, matching the metric result for the same body.

What BMI Cannot See

BMI uses only height and weight, so it cannot tell muscle from fat or where fat sits on your body. That is why BMI can mislabel very muscular or older bodies: a muscular athlete can score as overweight while carrying very little fat, and an older person who has lost muscle can score as healthy while carrying too much. It also was not designed for children, for whom age and sex specific percentile charts are used instead. Read BMI as a fast, free screening ratio that flags cases warranting a closer look, not as a diagnosis. For a fuller picture, weigh BMI versus body fat percentage as a health measure: pair this screen with a body fat percentage estimate and a look at waist size. In short, BMI is a screening ratio, not a diagnosis, and cannot tell muscle from fat.

Common Mistakes