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    UnitConvertLab — Unit conversion laboratory

    BMI Calculator

    Calculate your Body Mass Index using metric or imperial units. Get WHO classification and health insights.

    Enter Your Measurements

    Your Result

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    Enter your measurements to see your BMI

    Underweight< 18.5
    Normal18.5 – 24.9
    Overweight25 – 29.9
    Obese≥ 30

    How BMI is calculated

    Body Mass Index divides body mass by the square of height: BMI = kg / m². For imperial inputs the same ratio is applied with a 703 conversion factor, so BMI = (lb / in²) × 703. This calculator converts your entered height and weight to SI units first, then rounds the result to one decimal place, which is the precision used in clinical charts. Because the formula depends on the square of height, small measurement errors in height affect the result roughly twice as much as the same relative error in weight — measure height without shoes and against a flat wall for the most reliable figure.

    Reading the WHO categories

    The World Health Organization classifies adult BMI as underweight below 18.5, healthy weight from 18.5 to 24.9, overweight from 25.0 to 29.9, and obese at 30.0 and above, with obesity further split into class I (30.0–34.9), class II (35.0–39.9) and class III (40.0 and over). These cut-off points describe population-level risk, not an individual diagnosis. Some national health bodies apply lower thresholds for people of South Asian, Chinese and Japanese descent, where cardiometabolic risk rises at a lower BMI, commonly using 23.0 as the overweight boundary.

    Limits of the measure

    BMI does not distinguish muscle from fat, nor does it describe where fat is stored. Trained athletes frequently score in the overweight range while carrying low body fat, and older adults can lose muscle while their BMI stays flat. Waist circumference, waist-to-height ratio and body-fat measurement add information BMI cannot provide. BMI is also not valid for children and teenagers, who are assessed against age- and sex-specific percentile charts instead, nor for pregnant people. Treat the number here as one screening signal and discuss any concern with a clinician.

    Frequently asked questions

    What is a healthy BMI for an adult?
    The World Health Organization defines the healthy adult range as 18.5 to 24.9. Below 18.5 is underweight, 25.0 to 29.9 is overweight and 30.0 or above is obese. Several national guidelines lower the overweight boundary to 23.0 for people of South Asian, Chinese and Japanese descent.
    What is the BMI formula in pounds and inches?
    BMI = (weight in pounds ÷ height in inches²) × 703. The 703 factor converts the imperial ratio to the metric kg/m² scale, so an imperial and a metric calculation of the same body give the same number.
    Is BMI accurate for athletes?
    Often not. BMI weighs mass without asking what the mass is made of, so a muscular athlete can land in the overweight or obese band while carrying low body fat. Body-fat percentage, waist circumference or a DEXA scan describe such a body far better.
    Can I use this BMI calculator for a child?
    No. Children and teenagers are assessed against age- and sex-specific percentile charts, because healthy body composition changes throughout growth. A raw adult BMI number is not meaningful under 18. Use a paediatric BMI-for-age chart or ask a clinician.
    How much does measurement error change my BMI?
    Height matters roughly twice as much as weight, because the formula squares it. A 2 cm height error shifts BMI by about 0.6 for an average adult, while a 1 kg weight error shifts it by about 0.35. Measure height barefoot against a flat wall.

    Cite this page

    UnitConvertLab. (2026). BMI Calculator. UnitConvertLab. Retrieved from https://unitconvertlab.com/tools/bmi-calculator