BMI Calculator
Free BMI Calculator with instant results. Calculate your Body Mass Index, get your ideal weight range, and receive AI-powered health recommendations.
Adult BMI quick reference
What your BMI result can—and cannot—tell you
Body Mass Index is calculated as weight in kilograms divided by height in metres squared. It is an adult screening measure, not a diagnosis and not a direct measurement of body fat. Muscle mass, body composition, pregnancy, medical history, symptoms, and other health measures can change how a result should be interpreted. CDC guidance on BMI describes these uses and limitations.
Adult screening categories
- Underweight
- Below 18.5
- Normal weight
- 18.5 to below 25
- Overweight
- 25 to below 30
- Obesity
- 30 or above
Fixed adult categories are not appropriate for people ages 2–19, who require age- and sex-specific BMI-for-age guidance. See the CDC child and teen calculator.
Published calculation and QA details
Formula version BMI-2026.1 · Last QA
BMI formula with a worked example
For metric measurements, divide weight in kilograms by height in metres squared. For US measurements, multiply pounds by 703, then divide by height in inches squared.
An unrounded BMI of about 29.07 falls in the adult overweight screening category. The category is descriptive; it is not an individual diagnosis.
How the reference weight range is calculated
The calculator reverses the standard formula at BMI 18.5 and just below 25. At 1.70 metres, that corresponds to approximately 53.5–72.0 kg (118.0–158.7 lb). This is a BMI-derived reference range, not a prescribed target or an “ideal” weight.
BMI Prime is BMI divided by 25. A value of 1.00 is the upper boundary of the standard adult normal-weight category; it does not add a separate health assessment.
- Standard formula
- BMI = weight (kg) ÷ height (m)². US inputs are converted using 1 inch = 0.0254 metres and 1 pound = 0.453592 kilograms.
- Rounding
- Category thresholds use the unrounded result. BMI is displayed to 1 decimal place, BMI Prime to 2, and reference weights to 1 decimal place. BMI Prime divides the unrounded BMI by 25.
| Category | Unrounded BMI |
|---|---|
| Underweight | < 18.5 |
| Normal weight | 18.5 to < 25.0 |
| Overweight | 25.0 to < 30.0 |
| Obesity Class I | 30.0 to < 35.0 |
| Obesity Class II | 35.0 to < 40.0 |
| Obesity Class III | 40.0 and above |
| Case | Inputs | BMI | Category |
|---|---|---|---|
| Metric reference | 175 cm, 70 kg | 22.9 | Normal weight |
| Metric upper-normal example | 180 cm, 75 kg | 23.1 | Normal weight |
| US-unit conversion | 5 ft 10 in, 180 lb | 25.8 | Overweight |
Adult BMI questions
How is BMI calculated?
BMI is calculated by dividing your weight in kilograms by your height in metres squared. Formula: BMI = weight (kg) ÷ height² (m²). In imperial units: BMI = (weight (lbs) × 703) ÷ height² (in²). For example, a person weighing 70 kg at 1.75 m has a BMI of 70 ÷ (1.75 × 1.75) = 22.9.
What do the BMI categories mean?
Adult BMI screening categories are underweight (below 18.5), normal weight (18.5 to below 25), overweight (25 to below 30), Class I obesity (30 to below 35), Class II obesity (35 to below 40), and Class III obesity (40 or above). These categories do not diagnose a condition or establish an individual’s health risk, treatment, or urgency.
How does BMI differ for men vs. women?
Standard adult BMI thresholds do not change by sex, but BMI does not measure sex-related differences in body composition. That limitation is one reason BMI should not be interpreted as a direct body-fat measurement.
Can BMI be used for children?
People ages 2–19 require BMI-for-age percentiles based on age and sex. This fixed-category adult calculator is intended for people aged 20 and older; use an authoritative child and teen BMI resource for younger users.
What is BMI Prime?
BMI Prime divides BMI by 25, the exclusive upper boundary of the standard adult normal-weight category used by this calculator. It is another expression of the same screening result and does not add an individualized health assessment.
Can BMI be used during pregnancy?
BMI calculated from weight during pregnancy does not account for pregnancy-related changes in body composition and fluid. Use maternity-care guidance and discuss questions with a qualified healthcare professional.
Methodology, sources & limitations
This calculator uses a published formula and the values entered in the calculator to generate the result.
- Formula
- BMI = weight(kg) / height(m)²
- Jurisdiction & units
- Intended for US, IN, EU, GB, AU, CA, Global. Supports both units where applicable.
- Assumptions & limitations
- Results are screening or educational estimates based on the inputs you provide. They do not diagnose, treat, or replace advice from a qualified clinician.
- Review status
- Formula and automated QA completed June 21, 2026; independent professional review is pending. Next scheduled review: September 21, 2026.
Validation sources
- CDC Growth Charts
- WHO Global Database on BMI
BMI Calculator — Answer & Method
Calculate adult Body Mass Index with the standard weight-to-height formula. Supports metric and US units and shows adult screening categories, BMI Prime, and a reference weight range.
Formula: Body Mass Index
BMI = weight(kg) / height(m)²
Example Calculation
A person weighing 70 kg at 1.75 m tall has a BMI of 22.86, classified as Normal weight by WHO standards.
Important limitation
This is an educational estimate, not a diagnosis or a substitute for professional medical advice.
Key Facts
- BMI is calculated as weight in kilograms divided by height in meters squared.
- WHO classifies BMI below 18.5 as underweight, 18.5-24.9 as normal, 25-29.9 as overweight, and 30+ as obese.
- BMI does not directly measure body fat percentage.
- CDC recommends BMI screening for adults aged 20 and older.
Sources & Validation
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Understand BMI methodology, limitations, and alternative measures.