BMI Calculator

What BMI Measures — and How

Body Mass Index reduces the relationship between your height and weight to a single number, and this calculator computes it from the height and weight you enter in either metric or imperial units. It then places that number on the standard scale so you can see which category it falls into. BMI was devised in the 19th century by Belgian mathematician Adolphe Quetelet and remains the most widely used quick screen for weight-related health risk at the population level. The results here are general estimates for educational purposes and are not medical advice.

The Formula

The core calculation is BMI = kg / m², meaning weight in kilograms divided by height in meters squared. If you prefer imperial units, the equivalent is BMI = 703 x weight(lb) / height(in)². Both formulas produce the same result; the 703 factor simply converts pounds and inches into the metric relationship. Because height is squared in the denominator, BMI is quite sensitive to height, which is part of why the same weight maps to very different scores for a short versus a tall person.

The Standard BMI Categories

For adults, the widely used World Health Organization categories split the scale into four bands:

• Underweight: BMI below 18.5
• Normal weight: BMI from 18.5 to 24.9
• Overweight: BMI from 25.0 to 29.9
• Obesity: BMI of 30.0 and above

These cutoffs apply to adults aged 20 and older. They are not used the same way for children and teens, who are assessed against age-and-sex-specific percentile charts rather than these fixed numbers.

Example: 70 kg at 1.75 m

Consider a hypothetical adult who is 1.75 meters tall and weighs 70 kilograms. Squaring the height gives 1.75 x 1.75, or about 3.06. Dividing 70 by 3.06 produces a BMI of roughly 22.9, which sits within the normal band. If that same person weighed 85 kilograms, the BMI would rise to about 27.8, landing in the overweight band. These figures are illustrative and meant only to show how the formula behaves as weight changes.

What Moves the Number

Only height and weight enter the BMI formula itself, and they pull in opposite directions:

• Weight: sits in the numerator, so BMI rises and falls in direct proportion to it. Add weight and the number goes up; lose weight and it goes down.
• Height: sits in the denominator and is squared, which makes BMI especially sensitive to it. A small difference in height produces a noticeably different score for the same weight.
• Units: switching between metric and imperial does not change your BMI; it only changes how you enter the same measurements.

How to Read Your Result

Find which band your number lands in, then hold it loosely. A single BMI reading is a snapshot, and where you sit near a boundary matters less than the overall trend in your health over time. Because the categories are general and built for adults, treat a result near a cutoff as a prompt to look at the fuller picture rather than a hard line you have crossed.

It also helps to remember what BMI cannot see. Two people with the same score can have very different amounts of muscle, fat, and bone, and very different health. That is why the number works best as a conversation starter: it flags that a closer look might be worthwhile, while leaving the actual assessment to measurements and professional judgment that account for your individual body and history.

Limitations of BMI

BMI does not measure body fat directly, and it cannot tell muscle apart from fat or show where weight is carried on the body. It is an inexpensive proxy that works reasonably well across large groups but can mislabel individuals. Common cases where it can be misleading include:

• Muscular athletes, whose dense muscle can push BMI into the overweight range despite low body fat.
• Older adults who have lost muscle, where a normal BMI may understate body fat.
• Growing children and teens, who need percentile-based assessment rather than the adult cutoffs.
• People with very different builds, since BMI ignores fat distribution, which matters for health.

Treat BMI as one starting data point among many. Measures like waist circumference, body-fat percentage, blood pressure, blood sugar, fitness, and sleep round out the picture, and individual needs vary. For anything health-related, use this number as a prompt for a conversation with a qualified clinician rather than a verdict on your own. If you want to look at energy needs instead of weight classification, the Calorie Calculator estimates daily calories from your activity level.