BMI With Age Calculator
Calculate BMI from height and weight, then use age to select the correct interpretation: adult categories for age 20+ or CDC BMI-for-age references for ages 2–19.
Calculate BMI with age
Choose units and enter age, growth-chart reference sex, height and weight.
How BMI With Age Is Calculated
The BMI formula itself does not include age: BMI = weight in kilograms ÷ height in meters squared. Age determines how the number is interpreted. For a person age 20 or older, this page uses the standard adult cutoffs. For a child or teen age 2 through 19, it looks up the CDC BMI-for-age reference for the entered age in whole months and the selected growth-chart sex.
The child and teen result uses the CDC 2000 growth-chart L, M and S values bundled with this calculator. Values below the age-specific 95th percentile are converted to a z-score and percentile using the published LMS method. At or above the 95th percentile, the calculator reports “95th percentile or higher” and shows the percentage of the age-specific 95th-percentile BMI. That avoids pretending an ordinary adult cutoff applies to a growing child.
Adult BMI Categories at Age 20 or Older
| BMI | Adult category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 to 24.9 | Healthy weight |
| 25.0 to 29.9 | Overweight |
| 30.0 or higher | Obesity, divided into classes 1–3 |
Age and sex are not variables in the standard adult BMI formula or its usual category cutoffs. Age can still matter clinically because body composition, muscle mass, health conditions and functional goals vary. The number is a screening reference, not a personalized target.
Child and Teen BMI-for-Age Categories
| CDC percentile range | Category |
|---|---|
| Below the 5th percentile | Underweight |
| 5th to below the 85th | Healthy weight |
| 85th to below the 95th | Overweight |
| 95th percentile or higher | Obesity |
| At least 120% of the 95th percentile, or BMI at least 35 | Severe obesity |
A percentile is a reference comparison, not a score or diagnosis. For example, a BMI-for-age at the 75th percentile means the BMI is equal to or higher than the BMI of about 75% of children of the same age and sex in the reference population. It does not mean that 75% of the body is fat or that a child is “75% healthy.”
Worked Examples
Adult example: 70 kg and 1.75 m gives 70 ÷ 1.75² = 22.86, displayed as BMI 22.9. At age 20 or older, that falls in the standard adult healthy-weight category.
Child or teen example: the same BMI can have different percentiles at different ages because expected growth changes over time. The calculator therefore uses age in months and the appropriate CDC reference curve before showing the category. Accurate height, weight and age entries are important because small measurement differences can move a result near a percentile boundary.
Important Limits
BMI compares weight with height but does not directly measure body fat, muscle, bone, fat distribution or metabolic health. It can be less informative for athletes with high muscle mass, during pregnancy and in other individual circumstances. For children and teens, one measurement is also less informative than a growth pattern observed over time.
This calculator is intended for general education. It is not a medical device, diagnosis or treatment recommendation. If a result raises a concern—especially for a child or teen—discuss accurate measurements and the growth trend with a qualified healthcare professional.
Sources and Calculation Reference
- CDC Child and Teen BMI Calculator — interpretation for ages 2–19.
- CDC Growth Charts Data Files — BMI-for-age LMS values and formula.
- CDC Child and Teen BMI Categories — percentile boundaries and severe-obesity definition.
Frequently Asked Questions
Does age change the adult BMI formula?
No. For adults age 20 and older, BMI uses height and weight, and the standard category cutoffs do not change with age or sex.
Why does a child’s calculator ask for sex?
The CDC BMI-for-age growth references are sex-specific. The selection chooses the reference curve used for the percentile comparison; it does not change the BMI formula.
Can I use adult BMI for a 17-year-old?
Use an age- and sex-specific BMI-for-age reference for ages 2 through 19. Adult cutoffs begin at age 20.
Is BMI a diagnosis?
No. It is one screening measure and should be considered with other health and growth information.