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BMI Calculator

Find your body mass index from height and weight.

How it is calculated

BMI is computed as BMI = weight / height², with weight in kilograms and height in metres. In metric mode you type height in centimetres and weight in kilograms; the tool divides height by 100 first. In imperial mode height is read as inches and weight as pounds, converted with 1 in = 2.54 cm and 1 lb = 0.45359237 kg. The result is rounded to two decimals and placed in one of four adult bands: below 18.5 is Underweight, 18.5 up to (but not including) 25 is Normal, 25 up to 30 is Overweight, and 30 or more is Obese. Zero or negative inputs return no result. Children and teenagers are judged against age-based charts, so these adult bands do not apply to them.

Worked example

Take a height of 175 cm and a weight of 70 kg. Height in metres is 175 / 100 = 1.75, and squared that is 3.0625. Dividing 70 by 3.0625 gives about 22.857, which the tool rounds to 22.86. That value is at least 18.5 and below 25, so the category shown is Normal.

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Common mistakes

  • Typing height in metres (1.75) in metric mode: the tool expects centimetres, so the BMI comes out about 10,000 times too large.
  • Leaving the unit switch on metric while entering pounds and inches, or the reverse.
  • Treating the category as a diagnosis: BMI cannot tell muscle from fat, so very muscular people can land in Overweight.
  • Applying the adult bands to a child or teenager, who need age-and-sex-specific charts.
  • Comparing a BMI at 24.9 and 25.1 as if they were very different: the bands are cut-offs, not sharp health boundaries.

Sources

About this calculator

Body Mass Index takes a person's weight and divides it by the square of their height, producing a single number that places them into broad categories such as underweight, normal, overweight, and beyond. It was designed as a quick population-level screening figure rather than a precise measure of body fat or health, since it cannot distinguish muscle mass from fat mass.

People commonly check it out of general curiosity about where they fall relative to standard reference ranges, and it appears in fitness tracking, insurance forms, and routine health checkups as a starting point for further conversation rather than a diagnosis. Athletes and very muscular individuals in particular tend to see figures that don't reflect their actual body composition.

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Comments (1)

  • Editorial team

    BMI uses only height and weight, so it cannot tell muscle from fat. A muscular person can land in the overweight band with a healthy body, while someone with little muscle can sit in the normal band with a high fat share. The categories here are the standard adult cut-offs of 18.5, 25 and 30, and they are not meant for children. Which other body measurement would you like to see next to BMI?

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