BMI Calculator
Calculate BMI for adults aged 20 or older, or estimate a BMI-for-age percentile for children and teens aged 2–19 using sex-specific CDC growth reference data. BMI is a screening measure, not a diagnosis.
Educational estimate. Calculator results are for planning and information only, not financial, tax, medical, legal, or engineering advice. Verify important decisions with official sources or a qualified professional.
BMI Calculator
Calculate Your Body Mass Index
📐 Formula & Method
BMI Formula (Metric)
Divide your weight in kilograms by the square of your height in meters.
BMI Formula (Imperial)
Divide your weight in pounds by the square of your height in inches, then multiply by 703.
Child BMI-for-Age Percentile
For ages 2–19, sex- and age-specific L, M, and S parameters come from the CDC 2000 BMI-for-age growth reference. Severe obesity is flagged at 120% of the 95th percentile or BMI ≥ 35.
📋 How to Use
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Choose Adult or Child/Teen. The child mode is for ages 2–19; adult mode is for ages 20 and older.
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Select sex. Adult thresholds are unchanged by sex, while child percentiles use sex-specific growth data.
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Select your preferred unit system (Metric or Imperial).
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Enter weight in kilograms or pounds.
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Enter height in centimeters or total inches.
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Calculate BMI. Adults also receive an obesity class and healthy-weight interval; children receive a BMI-for-age percentile and category.
What is BMI and Why Does It Matter?
Body Mass Index (BMI) is a numerical value calculated from a person's weight and height. It is a widely used screening tool to identify potential weight-related health issues. BMI was developed by Belgian mathematician Adolphe Quetelet in the 1830s and has since become one of the most commonly used health indicators worldwide.
While BMI is a useful population-level screening tool, it is not a diagnostic measure. It does not directly measure body fat, muscle mass, or fat distribution. However, research has shown that BMI correlates with direct measures of body fat, such as underwater weighing and dual-energy X-ray absorptiometry (DXA).
For adults aged 20 or older, BMI below 18.5 indicates underweight, 18.5–24.9 healthy weight, 25.0–29.9 overweight, 30.0–34.9 obesity class 1, 35.0–39.9 class 2, and 40 or above class 3.
For children and teens aged 2–19, BMI must be interpreted relative to age and sex. This calculator uses CDC BMI-for-age LMS parameters and reports underweight below the 5th percentile, healthy weight from the 5th to below the 85th, overweight from the 85th to below the 95th, and obesity at or above the 95th percentile.
If your BMI falls outside the normal range, it is recommended to consult a healthcare professional for a more comprehensive health assessment. Lifestyle changes including balanced nutrition and regular physical activity can help achieve and maintain a healthy BMI.
🔬 Methodology & Accuracy
Formula: Uses the standard mathematical formula shown in the Formula & Method section above. All computations run client-side in your browser — no data is sent to our servers.
Data sources: Tax bands, contribution limits and regulatory rates are taken from official US (IRS, SSA) and UK (HMRC, gov.uk) publications for the current tax year, and updated when bands change.
Last reviewed: June 2026 · General formula used: BMI Formula (Metric) · Accuracy: Results are precise to two decimal places using IEEE-754 double-precision arithmetic. Intended for educational and planning use only.
For informational purposes only. Results are estimates based on the inputs and formulas provided. For financial, tax, medical, or legal decisions, consult a qualified professional. Rates and regulations change — always verify current figures with official sources.