Health & Fitness

What Body Mass Index Is: Formula, Categories and Limits

Calculate adult BMI with consistent units, interpret category boundaries correctly, and distinguish a screening measure from body-fat percentage or a diagnosis.

By Uddhav Gopani, FounderPublished 25 September 2026Reviewed 29 September 2026

What Body Mass Index Is: Formula, Categories and Limits

Define the number before interpreting it

Body mass index, or BMI, is body weight in kilograms divided by the square of height in metres. Its unit is kg/m². The result describes weight relative to height; it is not a percentage of body fat and is not a diagnosis by itself. CDC describes adult BMI as a screening measure to consider alongside other health information. The local calculator takes weight and height and applies standard adult category thresholds. It does not measure body composition, ask about medical history or determine whether a particular person should change their weight.

Work a complete metric example

For a weight of 70 kg and height of 175 cm, first convert height to 1.75 m by dividing centimetres by 100. Square the converted height: 1.75×1.75 equals 3.0625 m². Divide 70 by 3.0625 to obtain approximately 22.8571 kg/m², displayed as BMI 22.86. In the local form, enter 70 in Weight (kg) and 175 in Height (cm); the tool performs the centimetre-to-metre conversion internally. Entering 1.75 in a field labeled centimetres would supply the wrong unit even though 1.75 is the correct height in metres.

See why the squared unit matters

Using centimetres directly in the denominator would give 70 divided by 175², or about 0.0022857, which is ten thousand times smaller than the correctly calculated BMI. The factor is ten thousand because the length conversion of 100 is squared. Likewise, doubling height while keeping weight fixed would divide the mathematical index by four, not two. These are checks on the formula, not realistic growth predictions. Write the weight and height units alongside each value when checking a spreadsheet so a unit mismatch cannot be hidden by a plausible-looking rounded number.

Read adult categories at their boundaries

CDC's standard categories for adults aged 20 and older use BMI below 18.5 for underweight, 18.5 to less than 25 for healthy weight, 25 to less than 30 for overweight, and 30 or above for obesity. The inequalities matter: 25 belongs to the next band, while 24.99 is below that boundary. A displayed BMI is rounded, so the tool's category uses the unrounded calculation. A value close to a boundary can round to the boundary on screen. That rounding should not be interpreted as a sudden clinical change between otherwise nearly identical measurements.

Separate an inverse calculation from a target

If a height is fixed at 1.75 m, the weight corresponding mathematically to BMI 25 is 25×1.75², or 76.5625 kg. The weight corresponding to BMI 18.5 is 56.65625 kg. This rearrangement follows weight equals BMI multiplied by height squared. It does not establish a personal target or show that every weight inside two numerical cutoffs is equally appropriate for an individual. The calculator's example weight values are arithmetic illustrations from category numbers. Clinical interpretation should take other relevant measurements and circumstances into account rather than relying on an isolated inverse formula.

Recognize what BMI does not measure

CDC explains that BMI does not distinguish fat from lean body mass or capture where body fat is distributed. Two people with the same height and weight have the same BMI even if their muscle mass and fat distribution differ. Children and teenagers require age- and sex-specific interpretation using BMI-for-age information; the adult categories on this local form do not supply that assessment. A formula can produce a number for many inputs, but that does not make an adult screening label suitable for every population or clinical circumstance.

Keep the useful measurement record

Record the measured weight, measured height, their units and the date, then retain the unrounded calculation when checking a category boundary. Avoid comparing a carefully measured value with an old guessed height without acknowledging the different inputs. If the number raises a health question, discuss it with a healthcare professional who can consider the wider picture. The useful role of the local calculator is to reproduce the stated adult BMI arithmetic and explain its category convention. It cannot assess fitness, nutritional status or a person's overall health from two numbers alone.