Calculator
BMI Calculator
Calculate adult BMI from height and weight using metric or imperial units. See the category, BMI range, BMI-based healthy weight range, formula, worked example, and limitations.
Calculate your adult BMI
Your result
Understanding your BMI calculation
What BMI is designed to do
Body mass index (BMI) is a ratio of body weight to height. It is useful because the calculation is simple, reproducible, and easy to compare across large groups. For adults, BMI is used as a screening measure rather than a diagnosis. A calculated value can help start a conversation about weight-related health, but it cannot tell you how much of your weight is muscle, bone, water, or fat.
For adults 20 and older, the standard categories are underweight below 18.5, healthy weight from 18.5 to less than 25, overweight from 25 to less than 30, and obesity at 30 or greater. These are screening categories, not a complete description of an individual's health.
Formula and variables
Imperial: BMI = 703 × weight (lb) ÷ [height (in)]²
- Weight: body mass in kilograms for the metric equation.
- Height: standing height in metres.
- 703: conversion factor used with pounds and inches.
Worked example
Suppose a hypothetical adult weighs 70 kg and is 1.75 m tall. First square height: 1.75 × 1.75 = 3.0625. Then divide weight by squared height: 70 ÷ 3.0625 = about 22.9 kg/m². The calculator displays the BMI rounded to one decimal place.
How to interpret the result
The category tells you where the calculated value falls within the adult screening ranges. It should not be read as a statement that someone is healthy or unhealthy. A person near a category boundary may have a very similar risk profile to someone just across the boundary, and other measures can add important context.
Why BMI can be misleading
BMI does not directly measure body fat. People with substantial muscle mass may have a higher BMI without having the same body-fat pattern as someone with less muscle. Older adults can also have changes in muscle and fat distribution that are not captured by the ratio. Waist circumference, medical history, blood pressure, laboratory results, activity, and other factors may provide additional context.
Age, pregnancy, and special situations
Adult BMI categories are intended for adults. Children and teenagers are interpreted using BMI-for-age references that account for age and sex because they are still growing. Pregnancy also requires different clinical considerations; standard adult BMI results should not be used as a pregnancy health assessment.
Limitations
- BMI is a screening measure, not a diagnosis.
- It does not measure body-fat percentage directly.
- It does not describe fat distribution or muscle mass.
- Adult categories should not be applied to children and teens.
- Individual health decisions require more information than height and weight.
Frequently asked questions
What is BMI?
BMI is a calculated measure of weight relative to height. For adults it is commonly used as a screening measure.
How is BMI calculated?
Metric BMI is weight in kilograms divided by height in metres squared. The imperial equation uses pounds and inches with a factor of 703.
What is a healthy adult BMI?
The standard adult screening category called healthy weight is 18.5 to less than 25 kg/m².
Does BMI measure body fat?
No. BMI uses only height and weight. It does not directly measure body-fat percentage or fat distribution.
Can BMI be inaccurate for athletes?
Yes. Higher muscle mass can raise BMI without producing the same body-fat level implied by the category.
Can children use adult BMI categories?
No. Children and teens require age- and sex-specific BMI-for-age references.
Does pregnancy change how BMI should be interpreted?
Pregnancy requires different clinical assessment. This adult BMI calculator should not be used as a pregnancy health assessment.
How often should BMI be checked?
There is no universal schedule. If you track it, use consistent measurement conditions and focus on meaningful trends rather than small changes.