Calculator
What the Teen BMI Calculator does
Teen BMI Calculator: calculates BMI, BMI-for-age percentile and CDC category from Age (years), Plus months and Sex. Formula: z = ((BMI / M)^L - 1) / (L x S), then percentile = Φ(z). Example: age (years) 15 years, plus months 0 months and sex Boy gives BMI 20.2.
BMI, BMI-for-age percentile and CDC category, derived from Age (years), Plus months, Sex and Weight. Each change is reflected immediately, which suits quick what-if comparisons.
Context matters more than the number itself. Age, build and activity all shift it. The Teen BMI Calculator exists to take that particular chore off your hands.
Nothing leaves the tab. You can watch your browser network panel while you use it and see for yourself.
If you want to check the arithmetic, the field table, the method and a worked example are further down.
What do the Teen BMI Calculator fields mean?
The Teen BMI Calculator uses 5 inputs. Every box starts with a working value, so there is a real result on screen before you touch anything.
| Field | What to enter | Default |
|---|---|---|
| Age (years) | measured in years; accepts 13 to 19 | 15 |
| Plus months | measured in months; accepts 0 to 11 | 0 |
| Sex | choose from Boy or Girl | Boy |
| Weight | measured in kg; accepts 5 to 250 | 55 |
| Height | measured in cm; accepts 60 to 230 | 165 |
How does the Teen BMI Calculator work?
BMI is weight (kg) / height (m)^2 as usual, but L, M and S come from the CDC BMI-for-age table for the exact age in months and sex, so the same BMI maps to a different percentile at 8 and at 15. Under the 5th percentile is underweight, 85th to 95th overweight, 95th and above obesity.
Nothing is computed until the inputs pass their checks, so you will not get a confident answer built on a typo.
Worked example
These are the values the Teen BMI Calculator loads by default, and the result it produces from them. The figures are the tool’s live defaults and its live output, so you can reproduce them exactly.
| Age (years) | 15 years |
|---|---|
| Plus months | 0 months |
| Sex | Boy |
| Weight | 55 kg |
| Height | 165 cm |
| BMI | 20.2 |
|---|---|
| BMI-for-age percentile | 54th percentile |
| CDC category | Healthy weight (5th to 85th percentile) |
| Healthy BMI range at this age | 16.6 to 23.5 |
| Overweight and obesity cut-offs | 85th: 23.5 · 95th: 26.8 |
| What this means | A boy aged 15 with a BMI of 20.2 is at the 54th percentile for boys of that age: inside the 5th to 85th percentile… |
Why the adult thresholds do not apply until 20
The healthy BMI range keeps rising through adolescence. At 13 the 85th percentile for boys is about 22; by 19 it is 26.4, which the adult chart would already call overweight. Puberty adds muscle, bone and, in girls, essential body fat on a schedule that varies by two or three years between individuals, so a teenager's BMI is only meaningful as a percentile for their exact age and sex.
| Age | 5th percentile | 85th percentile | 95th percentile |
|---|---|---|---|
| 13 years | 15.5 / 15.3 | 21.9 / 22.6 | 25.2 / 26.3 |
| 14 years | 16.0 / 15.8 | 22.7 / 23.3 | 26.0 / 27.3 |
| 15 years | 16.6 / 16.3 | 23.5 / 24.0 | 26.8 / 28.1 |
| 16 years | 17.1 / 16.8 | 24.2 / 24.7 | 27.6 / 28.9 |
| 17 years | 17.7 / 17.2 | 24.9 / 25.2 | 28.3 / 29.6 |
| 19 years | 18.7 / 17.8 | 26.4 / 26.1 | 29.7 / 31.0 |
What to keep in mind
- The reference stops at 20 years. From 20 the adult categories (18.5, 25, 30) apply, and the CDC 95th percentile has been designed to meet the adult 30 threshold at that point.
- Early and late developers of the same age can differ by two years of growth. A late-developing 14-year-old is compared with 14-year-olds who have mostly finished their growth spurt, which can push a healthy reading toward the low end.
- Athletic teenagers, especially in strength sports, often read high because BMI cannot separate muscle from fat. A waist measurement adds the information BMI lacks.
- Eating-disorder screening uses the low end of this chart: a BMI-for-age under the 5th percentile, or a rapid fall through percentiles, is a reason to see a doctor regardless of the absolute number.
- The percentile is the figure to track over time. A teenager whose BMI rises from 20 to 22 between 14 and 17 has usually stayed on the same percentile line.
Reference data: CDC 2000 Growth Charts, BMI-for-age, 2 to 20 years (Kuczmarski et al., 2002). Category definitions: CDC, Defining Childhood Weight Status.
Read next: Understanding BMI: What the Number Measures, and What It Misses
How to use it
- Fill in Age (years) in years (13 to 19).
- Type Plus months in months (0 to 11).
- Pick a value for Sex: the options are Boy or Girl.
- Set Weight in kg (5 to 250).
- Put Height in cm (60 to 230).
- Live output means you can search for a target value by nudging an input up and down.
- The panel reports BMI, BMI-for-age percentile, CDC category, Healthy BMI range at this age, Overweight and obesity cut-offs and What this means.
- The link encodes your inputs, so saving it is enough to reproduce the result later.
What this tool does not do
- Population-level formulas cannot account for medical conditions, medication or pregnancy.
- This is a screening estimate, not a diagnosis.
- The output is a number, not a judgement. Deciding what it means is still your job.
- The tool cannot tell whether a number was entered in the unit you intended, so double-check the labels.
Frequently asked questions
Enter Age (years) in years, Plus months in months, Weight in kg and Height in cm. The unit is shown beside each box, and the result carries its own label, so there is nothing to convert by hand.
Age (years) accepts 13 to 19 years. If a value falls outside, the tool says which field is at fault instead of guessing.
With the values loaded when the page opens, BMI comes out as 20.2. That number tracks the inputs, so a small edit shows its sensitivity immediately.
Nothing is logged. The tool is client-side code; your data stays where you entered it.
Free, and there is no upsell. The whole tool is the free version.
Reliable as a rule of thumb, unreliable as a verdict. Equations of this kind are fitted to large groups, and the spread around the fitted line is wide enough that any individual can sit well off it.