BMI Calculator

Quick answer

Body Mass Index (BMI) is your weight divided by your height squared. A result from 18.5 to 24.9 is classed as normal weight under the World Health Organization categories used for adults. Enter your weight and height in metric or imperial units to get your BMI, your category and the weight range that fits that normal band for your height.

BMI is a screening number, not a diagnosis. It is quick to calculate and widely used to group adults into weight categories, but it says nothing about muscle, fat distribution, age or health on its own.

This calculator is for adults aged 20 and over. Children and teenagers are assessed with BMI-for-age percentiles instead, which this page does not provide; see the child BMI calculator for that.

Related tools: BMR Calculator: Estimate basal metabolic rate with the Mifflin–St Jeor and revised Harris–Benedict equations. TDEE Calculator: Total daily energy expenditure from BMR and activity level. Calorie Calculator: Daily calories to maintain, lose or gain weight.

How to use the BMI Calculator

1. Choose units: Metric (kg, cm) and Imperial (lb, in).
2. Enter weight (kg, or lb if imperial) (between 2 and 700).
3. Enter height (cm, or inches if imperial) (between 20 and 260).
4. Press Calculate to see the result, and use Reset to start again.

Formula and method

Adult categories follow the WHO classification: below 18.5 underweight, 18.5–24.9 normal, 25.0–29.9 overweight, 30.0 and above obese. BMI does not distinguish muscle from fat and is not valid for children. Informational estimate, not medical advice. Talk to a qualified health professional about your own situation.

Worked examples

These examples are verified: each one is an automated test case that the live calculator must reproduce exactly.

The first line is the headline result (your BMI). Below it the bMI Calculator lists category and healthy weight range, which describe the same case from other angles. IEEE-754 double precision; shown to 1 decimal place unless stated.

Adult BMI categories

These are the World Health Organization categories the calculator applies to adults.

Worked example by hand

For a person who weighs 70 kg and is 175 cm tall: convert the height to metres (1.75), square it (3.0625) and divide the weight by that number. 70 ÷ 3.0625 = 22.857, which rounds to 22.9, so the category is normal weight.

The matching healthy range for a 1.75 m height comes from the same formula run backwards: 18.5 × 3.0625 = 56.7 kg and 24.9 × 3.0625 = 76.3 kg.

In imperial units the formula is 703 × weight in pounds ÷ height in inches squared. For 154 lb and 69 in: 703 × 154 ÷ 4,761 = 22.7.

What BMI does not tell you

BMI does not separate muscle from fat, so very muscular people can be classed as overweight, and older adults can have a normal BMI with little muscle. It does not describe where fat is carried or give any information about fitness, blood pressure or other health markers. Treat it as one input and discuss your own situation with a qualified health professional.

How the result changes with weight

Holding every other input at the first worked example, this table changes only weight. It shows how sensitive the answer is to that one value; each row was produced by running this calculator.

What happens with invalid input

These messages are produced by the calculator itself, so you know what to fix.

• Height (cm, or inches if imperial): entering "10" shows "Height (cm, or inches if imperial) must be at least 20." beside that field and no result.
• Weight (kg, or lb if imperial): left empty shows "Enter weight (kg, or lb if imperial)." beside that field and no result.

Choosing between the BMI Calculator and similar calculators

Each row shows what a related calculator on this site asks for and what it returns, so you can pick the one that matches your question.

Background: body measurements and screening

Body-measurement ratios are screening tools, not diagnoses. BMI ignores body composition and fat distribution, and reviews find waist-to-hip and waist-to-height ratios are useful additions but not clearly better than BMI or waist circumference alone. Where the tape goes matters, so measure at the same sites each time.

Some sources quote a waist-to-hip ratio above 0.8 for women and 0.95 for men as a marker of increased metabolic risk; treat that as a screening flag, and talk to a clinician about what your own numbers mean.

Assumptions and limitations

• Adults 20+ only
• Athletes and older adults may be misclassified
• Not a diagnosis

This calculator gives general information based on published formulas. It is not medical advice, and it cannot account for your health history. Talk to a qualified clinician about decisions that affect your health.

If a required value is empty, not a number, or outside its allowed range, the form marks that field with a message and shows no result instead of a misleading number. The automated checks deliberately try invalid entries for height (cm, or inches if imperial) and weight (kg, or lb if imperial) to confirm this.

Related calculators

• BMR Calculator — Estimate basal metabolic rate with the Mifflin–St Jeor and revised Harris–Benedict equations.
• TDEE Calculator — Total daily energy expenditure from BMR and activity level.
• Calorie Calculator — Daily calories to maintain, lose or gain weight.
• Macro Calculator — Split daily calories into grams of carbohydrate, protein and fat.
• Height Calculator — Convert height between centimeters and feet-and-inches.
• Body Fat Calculator (U.S. Navy) — Estimate body fat percentage from tape measurements (U.S. Navy method).
• Ideal Weight Calculator — Ideal body weight by four published formulas (Devine, Robinson, Miller, Hamwi).
• Waist-to-Hip Ratio Calculator — Waist-to-hip ratio and WHO risk threshold.

Sources

• World Health Organization, BMI classification for adults
• CDC: About Adult BMI

Content last reviewed 2026-11-04. Formulas and worked examples are checked by automated tests against independently computed values.