What This BMI Calculator Does and Who It's For

This BMI calculator estimates your Body Mass Index from your height and weight, then tells you which weight category you fall into and what weight range would be considered healthy for your height. Enter your measurements in either metric (kilograms and centimeters) or imperial (pounds, feet, and inches) units and the tool handles the conversion for you.

Body mass index is a quick screening number used by clinicians, fitness professionals, and individuals to flag whether a person's weight may be too low or too high relative to their height. It's most useful for adults aged 20 and over. For children and teens, BMI is interpreted using age- and sex-specific percentile charts instead, so this calculator's adult categories don't apply to them.

How BMI Is Calculated: The Formula

BMI divides your weight by the square of your height. The two standard versions are:

Both formulas give the same number; the imperial version just includes the 703 factor to convert pounds and inches into the metric result. The output is a single value, usually shown to one decimal place.

Metric: BMI = weight (kg) / [height (m)]²
Imperial: BMI = 703 × weight (lb) / [height (in)]²

BMI Categories for Adults

Once your BMI is calculated, it's matched against standard adult weight-status ranges. These thresholds are the same regardless of age or sex for adults:

Under 18.5 — underweight
18.5 to 24.9 — normal (healthy) weight
25.0 to 29.9 — overweight
30.0 and above — obese

Worked Example With Real Numbers

Suppose you are 1.75 m tall and weigh 70 kg. First square the height: 1.75 × 1.75 = 3.0625. Then divide the weight by that figure: 70 / 3.0625 = 22.9. A BMI of 22.9 falls in the 18.5–24.9 band, so this person is in the normal range.

The same person measured in imperial units (about 154 lb and 69 inches tall) gives: 703 × 154 / (69 × 69) = 108,262 / 4,761 = 22.7. The small difference comes from rounding the unit conversions, but both land squarely in the healthy category.

To find your own healthy weight range, multiply 18.5 and 24.9 by your height in meters squared. For a height of 1.75 m, that's 18.5 × 3.0625 = 56.7 kg up to 24.9 × 3.0625 = 76.3 kg.

Limitations and Common Mistakes

BMI measures weight relative to height, not body composition. It cannot tell muscle from fat, so it has real blind spots. Keep these factors in mind when reading your result:

Muscular people — athletes, weightlifters, manual laborers — often score as overweight or obese despite low body fat, because muscle is dense.
BMI can underestimate fat in older adults who have lost muscle mass, and in people who carry weight around the abdomen.
It is not validated for pregnant women, and uses different percentile-based criteria for anyone under 20.
A common entry error is mixing units (for example, entering height in centimeters but selecting meters), which throws the result off dramatically.

Using Your Result Sensibly

Treat BMI as a starting point, not a diagnosis. It works well for tracking trends in a population or watching your own number change over time, but it doesn't capture where fat is stored or your overall fitness.

If your BMI sits outside the normal range, consider pairing it with other measures such as waist circumference, waist-to-height ratio, or a body-fat assessment, and talk to a healthcare professional before making major changes to diet or exercise. For most people, a BMI in the 18.5–24.9 band combined with a healthy waist measurement is a reasonable target.