BMI Calculator | Check Your Body Mass Index

Quick Start Guide

  1. Choose your units: Select Metric (kg, cm) or US units (lbs, ft/in) before entering your details.
  2. Enter height, weight, age, and gender: The core inputs used to calculate your BMI and weight category.
  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: Use reverse mode to calculate the weight needed to reach a specific BMI.
  7. Track your progress: Save entries to build a BMI history chart over time.

Understanding BMI

The BMI formula

BMI = weight (kg) ÷ height (m)². It is a quick screening ratio, not a direct measure of body fat — two people with the same BMI can have very different body compositions.

Who should not rely on BMI alone

BMI does not distinguish muscle from fat, so athletes and very muscular individuals can score as "overweight" despite low body fat. It is also less reliable for older adults, pregnant women, and children/teens, and does not account for waist circumference or fat distribution — a body fat or waist-to-height measurement gives more context.

Reverse mode: target weight

Switch to Target Weight to work backwards from a goal BMI: enter your height and target BMI, and the calculator solves for the weight needed to reach it.

Waist-to-height ratio

Enabling waist context adds a waist-to-height ratio alongside your BMI. A ratio under 0.5 is commonly considered lower risk — this can flag health risk in cases where BMI alone looks normal, particularly for abdominal fat.

Features

Reverse Mode: Switch to "Target Weight" mode to calculate what weight you need to reach a specific BMI.

Goal Trajectory: Set target BMI and weekly change assumption to estimate timeline.

Progress Tracking: View your BMI history over time with an interactive chart.

Clinical Guardrails: Under-18 input gets pediatric guidance messaging rather than adult cutoffs.

Waist Context: Optional waist-to-height ratio improves risk interpretation quality.

Global Settings: Height can be saved as a global variable for use across all health calculators.

Common Use Cases

Weight Monitoring: check your BMI and see which category you're in (underweight, healthy, overweight, obese), switching between metric and US units as needed.

Goal Setting: switch to Target Weight mode to find the weight needed for a desired BMI, or add a goal BMI and weekly change rate to see an estimated timeline.

Health Check: quickly confirm whether you're in the healthy weight range (18.5–24.9), or use reverse mode to find your target weight from a goal BMI.

Fitness Progress: track your BMI over time with the progress chart, and copy or export results for health or fitness logs.

Frequently Asked Questions

For adults, BMI 18.5-24.9 is generally considered healthy. Under 18, BMI should be interpreted with pediatric percentile guidance rather than adult cutoffs.
BMI can overestimate body fat in muscular individuals since muscle weighs more than fat. Athletes may have a high BMI but low body fat. Consider using body fat percentage or waist circumference as additional measures.
Use Reverse mode to compute target weight from goal BMI. In normal mode, you can also set a goal BMI and weekly change assumption to estimate a timeline to target.
Waist-to-height ratio can add risk context that BMI alone may miss. A commonly used heuristic is to keep waist below half of height.

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