Calculator
What the Child BMI Calculator does
Child 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) 8 years, plus months 0 months and sex Boy gives BMI 15.9.
Enter Age (years), Plus months, Sex and Weight and the Child BMI Calculator reports BMI, BMI-for-age percentile and CDC category. You can nudge a single number and watch the effect straight away.
Most people who open the Child BMI Calculator are people tracking a goal. Context matters more than the number itself. Age, build and activity all shift it.
Your measurements stay on your device. Health numbers are nobody else’s business, including ours.
The meaning of each field, the formula in use, and a worked example that starts from the default inputs are all further down.
What do the Child BMI Calculator fields mean?
The Child BMI Calculator uses 5 inputs. You do not have to fill everything in before seeing an answer; the starting values already produce one.
| Field | What to enter | Default |
|---|---|---|
| Age (years) | measured in years; accepts 2 to 12 | 8 |
| 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 | 26 |
| Height | measured in cm; accepts 60 to 230 | 128 |
How does the Child 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.
Range checks come first, so the formula only ever sees values it can handle.
Worked example
These are the values the Child BMI Calculator loads by default, and the result it produces from them. This is the worked example the page starts from, reproduced here so you can check the arithmetic.
| Age (years) | 8 years |
|---|---|
| Plus months | 0 months |
| Sex | Boy |
| Weight | 26 kg |
| Height | 128 cm |
| BMI | 15.9 |
|---|---|
| BMI-for-age percentile | 52nd percentile |
| CDC category | Healthy weight (5th to 85th percentile) |
| Healthy BMI range at this age | 13.8 to 18.0 |
| Overweight and obesity cut-offs | 85th: 18.0 · 95th: 20.1 |
| What this means | A boy aged 8 with a BMI of 15.9 is at the 52nd percentile for boys of that age: inside the 5th to 85th percentile… |
Why children need a percentile, not a threshold
A healthy 5-year-old has a BMI around 15.5. On the adult chart that is severely underweight. BMI falls through early childhood, bottoms out at about age 6 (the "adiposity rebound"), then climbs steadily to adult values by 20. A fixed threshold cannot follow that curve, so the CDC expresses a child's BMI as a percentile among children of the same age in months and sex.
| Age | 5th percentile | 85th percentile | 95th percentile |
|---|---|---|---|
| 2 years | 14.7 / 14.4 | 18.1 / 18.0 | 19.3 / 19.1 |
| 4 years | 14.0 / 13.7 | 16.9 / 16.8 | 17.8 / 18.0 |
| 6 years | 13.7 / 13.4 | 17.0 / 17.1 | 18.4 / 18.8 |
| 8 years | 13.8 / 13.5 | 18.0 / 18.3 | 20.1 / 20.7 |
| 10 years | 14.2 / 14.0 | 19.4 / 20.0 | 22.2 / 23.0 |
| 12 years | 15.0 / 14.8 | 21.0 / 21.7 | 24.2 / 25.3 |
What to keep in mind
- The percentile compares the child with the CDC reference population of the same age and sex, so a boy and a girl with identical height and weight can land in different categories.
- Age is used to the month. A child of 8 years 11 months is compared against 8-year-11-month-olds, not against 8-year-olds, and the cut-offs move by roughly 0.1 BMI per month at this age.
- The categories are screening bands, not diagnoses. A child in the 85th to 95th band who is muscular or mid growth-spurt may be entirely healthy; a paediatrician looks at the growth trajectory, not one point.
- Above the 97th percentile the CDC now uses an "extended BMI-for-age" method for precise values; this calculator flags that region rather than reporting a spurious decimal.
- Measure height without shoes, standing straight against a wall, and weight in light clothing. A 1 cm height error moves BMI by about 0.25 at this size.
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
- Enter Age (years) in years (2 to 12).
- Set Plus months in months (0 to 11).
- Pick a value for Sex: the options are Boy or Girl.
- Key in Weight in kg (5 to 250).
- Fill in Height in cm (60 to 230).
- Each keystroke triggers a fresh calculation, which is cheap because it happens locally.
- You get BMI, BMI-for-age percentile, CDC category, Healthy BMI range at this age, Overweight and obesity cut-offs and What this means back.
- Copy Link puts the current inputs in the URL, so bookmarking it brings the same numbers back.
What this tool does not do
- Body composition, ethnicity and age all shift what a "normal" result means.
- These formulas come from population averages and cannot account for your medical history.
- Results are informational: check anything consequential against a second source.
- It validates ranges, not intent. A value can be accepted and still be the wrong one to use.
Frequently asked questions
Enter Age (years) in years, Plus months in months, Weight in kg and Height in cm. Look for the unit beside each label; the result is reported in a stated unit as well.
Age (years) accepts 2 to 12 years. Out-of-range entries are refused up front, which is safer than silently clamping them.
With the values loaded when the page opens, BMI comes out as 15.9. The figure is derived from the inputs above, so it updates whenever they do.
Not at all: the whole thing runs offline once the page has loaded, which tells you nothing is being sent.
No. There is no account, no usage cap and no trial period. The site is funded by advertising rather than by charging for the tools.
The number is exact; its meaning is approximate. Two people with identical results can be in genuinely different situations, which is why this is a screening tool rather than a diagnostic one.