Transparent method
The formula
Worked example
Example inputs
How to interpret the result
BMI is a population screening tool built from just two numbers, height and weight, and it was never designed to diagnose any individual. A bodybuilder and a sedentary person of the same height and weight can land on an identical BMI while carrying completely different amounts of muscle versus fat, because the formula has no way to tell the two apart.
Before you rely on it
What to check
Look at BMI as one data point among several, not a standalone verdict. Body composition, waist measurement and overall health history tell a fuller story than this number alone.
The common error
Where people go wrong with BMI calculator
Reading a single BMI figure as a personal diagnosis. It was built and validated as a population-level screening tool, and applying it to judge one specific person's health status is a use it was never designed for.
Sensitivity evidence
How weight changes the body mass index
| Weight | Body mass index | Weight at BMI 24.9 (upper healthy range) |
|---|---|---|
| 128 | 20 | 159 |
| 144 | 22.6 | 159 |
| 160worked example | 25.1 | 159 |
| 176 | 27.6 | 159 |
| 192 | 30.1 | 159 |
Every input, tested
Which input moves the body mass index most
| Input | Tested from | To | Body mass index at each end | Swing |
|---|---|---|---|---|
| Height | 60 | 74 | 31.2 to 20.5 | 10.7 (43%) |
| Weight | 144 | 176 | 22.6 to 27.6 | 5 (20%) |
Two variables at once
Body mass index by weight and height
| Weight \ Height | 54 | 67 | 80 |
|---|---|---|---|
| 128 | 30.9 | 20 | 14.1 |
| 144 | 34.7 | 22.6 | 15.8 |
| 160 | 38.6 | 25.1 | 17.6 |
| 176 | 42.4 | 27.6 | 19.3 |
| 192 | 46.3 | 30.1 | 21.1 |
Step by step
The worked example, input by input
| Input | Value used | What it means |
|---|---|---|
| Weight | 160 | Body weight in pounds. |
| Height | 67 | Height in inches. |
| Body mass index | 25.1 | |
| Weight at BMI 24.9 (upper healthy range) | 159 | |
Inputs, definitions and assumptions
Weight
Body weight in pounds.
Height
Height in inches.
Where the inputs come from
Weight and height should be measured, not remembered: weigh yourself in the morning without shoes and heavy clothing, and measure height against a wall. This version uses pounds and inches, and the formula is 703 times weight divided by height squared. Enter your current values for an adult; children and teenagers are assessed with growth charts by age and sex, so this tool does not apply to them.
Using the body mass index in a real decision
A change over time can matter more than a single reading.
When the result stops applying
BMI does not distinguish muscle from fat, so athletes and people with a lot of muscle can register as overweight while being lean. It can underestimate risk in older adults who have lost muscle, and health risks at a given BMI differ across ethnic groups. It does not show where fat is stored. It is a population screening tool, not a diagnosis.
What to compare it with
See fat distribution with the waist-to-hip ratio and body fat percentage calculators, and the weight that matches a BMI range on the healthy weight range page. The weight to target BMI calculator shows how far you are from a chosen value, and the calorie and TDEE pages show energy needs. A clinician can interpret results alongside other measures.
People also ask
Frequently asked questions
What is a healthy BMI range?
For adults, 18.5 to 24.9 is considered the healthy weight range, though health risk also depends on waist size, fitness, age and medical history.
Why can BMI be wrong for athletes?
Muscle weighs more than fat, so a muscular person can have a high BMI with low body fat. BMI cannot tell the difference.
Does BMI work for children?
No. Children and teens are evaluated with age- and sex-specific growth percentiles, not the adult categories.