BMI Calculator - Check Your Body Mass Index

Your Details

Calculate your BMI (Body Mass Index) from height and weight. See if you are at a healthy weight, with your ideal range and waist-to-height risk context.

Choose your units

Enter height, weight and age

View your BMI and category

Add waist context (optional)

This calculator gives a general screening estimate, not a medical diagnosis. BMI does not directly measure body fat and may be less reliable for athletes, older adults, and children and teens; in pregnancy, guidance uses the BMI from before pregnancy. Consult a healthcare provider for personalized guidance.

Formulas and sources

Each formula this calculator uses, where it comes from, and what it leaves out. "How it works" shows the same formula with your own numbers.

Body mass index

Calculate BMI mode

BMI = weight (kg) ÷ height (m)²
BMI
body mass index
weight
weight in kilograms
height
height in meters

In Imperial units the calculator first converts pounds to kilograms (× 0.453592) and inches to meters (× 0.0254). The result matches the CDC's pounds × 703 ÷ inches² to within 0.01%.

Source: Uses the CDC's formula [1]. The pounds-and-inches form is in the CDC's BMI FAQ [2], and the conversion factors are NIST's [3].

Adult BMI categories

Calculate BMI and Target Weight modes, ages 20 and over

Underweight
below 18.5
Healthy weight
18.5 to 24.9
Overweight
25.0 to 29.9
Obesity
30.0 or more

The category is read on the BMI rounded to one decimal, the value the calculator shows. From ages 2 to 19, BMI is read on age- and sex-specific percentile charts instead, which this calculator does not compute.

Source: The categories and the starting age follow the CDC's adult BMI categories [1].

Weight for a target BMI

Target Weight mode, and for a Goal BMI in Calculate BMI mode

weight (kg) = BMI × height (m)²
weight
the weight that gives the target BMI
BMI
the target BMI
height
height in meters

Source: The CDC's formula [1], solved for weight.

Healthy weight range for your height

Calculate BMI and Target Weight modes, ages 20 and over

18.45 × height (m)² ≤ weight (kg) < 24.95 × height (m)²
weight
a weight in the healthy range
height
height in meters

These are the weights whose BMI, rounded to one decimal, is 18.5 to 24.9. The range shown is rounded inward to one decimal place, so both ends are inside it.

Source: Based on the CDC's healthy weight category [1].

Time to reach a goal BMI

Calculate BMI mode, with a Goal BMI

weeks = |goal weight - current weight| ÷ weekly change

Uses the weekly change you enter, the same every week.

Source: Arithmetic on your own assumption; no publisher.

Waist-to-height ratio

Calculate BMI and Target Weight modes, when waist context is on

ratio = waist ÷ height
ratio
waist-to-height ratio
waist
waist, in the same unit as height
height
height, in cm or inches
Healthy
0.40 to 0.49
Increased
0.50 to 0.59
High
0.60 or more

The ratio is banded on its value rounded to two decimals, for adults with a BMI under 35. Below 0.40 no band is shown, and in Calculate BMI mode none is shown at a BMI of 35 or more.

Source: The bands follow NICE guideline NG246 [4]. The 0.5 boundary is supported by Browning and colleagues, 2010 [5].

Sources

  1. Centers for Disease Control and Prevention. Adult BMI Categories. Page dated March 19, 2024.
  2. Centers for Disease Control and Prevention. BMI Frequently Asked Questions. Page dated June 28, 2024.
  3. National Institute of Standards and Technology. NIST Guide to the SI, Appendix B.9: Factors for units listed by kind of quantity or field of science, length and mass. Special Publication 811.
  4. National Institute for Health and Care Excellence. Overweight and obesity management (NG246), recommendation 1.9.14 and Box 1. Published January 14, 2025, last updated January 8, 2026.
  5. Browning LM, Hsieh SD, Ashwell M. A systematic review of waist-to-height ratio as a screening tool for the prediction of cardiovascular disease and diabetes: 0.5 could be a suitable global boundary value. Nutrition Research Reviews, 2010; 23(2): 247-269.

What this calculator does not cover

  • It does not compute BMI-for-age percentiles, which ages 2 to 19 need.
  • With Age left blank, it applies the adult categories.
  • It does not split obesity into classes.
  • It does not measure body fat, and it does not adjust for muscle, age or pregnancy.

Sources checked on .

Help

Quick Start Guide

  1. Choose your units: Select Metric (kg, cm) or Imperial (lbs, ft/in). Switching later converts what you have entered.
  2. Enter height, weight and age: Height and weight give your BMI; age decides whether adult categories apply.
  3. View your BMI and category: See your BMI value, weight category, and health information update instantly.
  4. Add waist context (optional): Enable waist-to-height ratio for extra risk screening beyond BMI alone.
  5. Set a goal: Add a goal BMI and weekly change assumption to see an estimated timeline.
  6. Switch to Target Weight mode: Enter your height and a target BMI to see the weight needed to reach it.
  7. Track your progress: Save entries to build a BMI history chart over time.

Frequently Asked Questions

For adults 20 and older, the CDC's healthy weight range is a BMI of 18.5 to 24.9. From ages 2 to 19, BMI is read on age- and sex-specific percentile charts instead. Many charts call this the normal BMI range.

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