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BMI Calculator

Calculate adult body mass index using metric or imperial measurements, with category, healthy weight range, and medical context.

Use this field when Metric is selected.

Use this field when Metric is selected.

Use this field when Imperial is selected.

Use 0 when there are no additional inches.

Use this field when Imperial is selected.

Status: initial

Results

Awaiting calculation

Health guide

Introduction

Body mass index, or BMI, is a simple screening calculation that compares weight with height. This calculator supports metric and imperial inputs and returns BMI, adult BMI category, healthy weight range, calculation breakdown, interpretation, and a medical disclaimer.

Purpose

Use this calculator as a fast adult BMI screening tool. It can help users understand a height-weight relationship, but it should not replace clinical assessment, body composition review, or advice from a qualified healthcare professional.

Formula explanation

BMI is calculated from weight divided by height squared. Metric BMI uses kilograms and metres. Imperial BMI uses pounds and inches with the standard 703 conversion factor.

Variable explanations

Measurement system

Choose metric for centimeters and kilograms, or imperial for feet, inches, and pounds.

Height

Height is squared in the BMI formula, so small height-entry mistakes can noticeably change the result.

Weight

Weight is compared against height squared to produce a screening number, not a direct body-fat measurement.

Formula and category guide

Metric BMI

BMI = weight kg / (height m x height m)

  • Weight is entered in kilograms.
  • Height is entered in centimeters and converted to metres.

Convert centimeters to metres, square the height, then divide weight by the squared height.

Imperial BMI

BMI = 703 x weight lb / (height in x height in)

  • Weight is entered in pounds.
  • Height is entered as feet and inches, then converted to total inches.

Convert feet and inches to total inches, square that height, multiply weight by 703, then divide.

Adult BMI classification

Underweight < 18.5; healthy 18.5-24.9; overweight 25.0-29.9; obesity >= 30.0

  • BMI is the calculated body mass index.

The calculator uses common adult BMI categories used by major public-health references.

Healthy weight range

Weight range = BMI boundary x height squared

  • The lower BMI boundary is 18.5.
  • The upper BMI boundary is 24.9.

The calculator reverses the BMI formula for the entered height to estimate the adult healthy weight range.

Worked examples

Metric healthy range

  1. Choose Metric.
  2. Enter height as 175 cm.
  3. Enter weight as 70 kg.
  4. The BMI is 22.9, which is in the healthy weight range.

Imperial healthy range

  1. Choose Imperial.
  2. Enter height as 5 ft 9 in.
  3. Enter weight as 154 lb.
  4. The BMI is about 22.7, which is in the healthy weight range.

Underweight boundary

  1. Enter height and a lower weight.
  2. Calculate BMI.
  3. A result below 18.5 is classified as underweight for adults.

Overweight boundary

  1. Enter height and weight.
  2. Calculate BMI.
  3. A result from 25.0 to 29.9 is classified as overweight for adults.

Obesity boundary

  1. Enter height and weight.
  2. Calculate BMI.
  3. A result of 30.0 or above is classified as obesity for adults.

Healthy weight range

  1. Enter height and weight in either unit system.
  2. Calculate BMI.
  3. Review the healthy weight range calculated from the entered height.

Medical limitations and common misconceptions

Treating BMI as a diagnosis

BMI is a screening tool. It does not diagnose health, disease, or body fat percentage.

Ignoring muscle mass

People with high muscle mass may have a higher BMI without having high body fat.

Using adult categories for children

Children and teenagers need age- and sex-specific BMI percentiles, not the adult category table.

Entering mixed units

Use centimeters and kilograms for metric, or feet, inches, and pounds for imperial.

Skipping waist and clinical context

Waist circumference, medical history, metabolic markers, and clinical judgment can change how BMI should be interpreted.

FAQs

What is BMI?
BMI stands for body mass index. It compares weight with height squared to provide a simple adult screening number.
What is the metric BMI formula?
Metric BMI is weight in kilograms divided by height in metres squared.
What is the imperial BMI formula?
Imperial BMI is 703 multiplied by weight in pounds, divided by height in inches squared.
What BMI is considered a healthy weight?
For most adults, a BMI from 18.5 to 24.9 is commonly classified as the healthy weight range.
What BMI is underweight?
For adults, a BMI below 18.5 is commonly classified as underweight.
What BMI is overweight?
For adults, a BMI from 25.0 to 29.9 is commonly classified as overweight.
What BMI is obesity?
For adults, a BMI of 30.0 or above is commonly classified as obesity.
Does BMI measure body fat directly?
No. BMI uses height and weight. It does not directly measure body fat, muscle mass, or fat distribution.
Is BMI accurate for athletes?
BMI can overestimate risk for some athletes or muscular people because it does not distinguish muscle from fat.
Is BMI accurate for older adults?
BMI can be less informative for older adults because body composition, muscle loss, and health context matter.
Can children use this BMI calculator?
This calculator uses adult BMI categories. Children and teenagers should use age- and sex-specific BMI percentile tools.
Does BMI account for pregnancy?
No. BMI categories are not designed to interpret weight during pregnancy. Clinical guidance is more appropriate.
Why does the calculator show a healthy weight range?
The healthy weight range reverses the BMI formula for the entered height using the 18.5 and 24.9 adult BMI boundaries.
Should I act on BMI alone?
No. BMI is a starting point. Health decisions should consider symptoms, medical history, waist measurement, lab results, and professional guidance.
Are formulas calculated in the page component?
No. The page renders a registered calculator specification and the Formula Engine performs the calculation.

References

Related calculators

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  • Calorie Calculator
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  • Pregnancy Weight Gain Calculator
  • TDEE Calculator
  • Macro Calculator

Last updated and version history

Last updated: 2026-07-03

  • 1.0.0 (2026-07-03): Initial CAL-0003 BMI Calculator implementation using platform engines.