Rex says
Same equations the textbooks use — just way less awkward. Pop in your stats and I'll tell you what the number actually means for you.
Try a scenario
Click to load — tweak from there.Inputs
Result
Estimated percentile
43.8
z-score
-0.16
Psst — share this and help Rex grow
One click, a permanent link with your numbers baked in.
How to use this
- 1Enter sex.
- 2Enter age (months).
- 3Enter weight (lb).
- 4Enter reference average weight for age (lb) (lb).
- 5Enter reference standard deviation (lb) (lb).
- 6Read your estimated percentile on the right — it updates as you type.
- 7Hit Share to keep the scenario or send it to someone.
About this calculator
Pediatricians plot weight and height (and weight-for-length under age 2) against CDC or WHO growth charts to see where a child falls relative to peers of the same age and sex, since percentile tracking over time matters more than any single measurement. This calculator uses simplified normative mean and standard deviation approximations by age band to estimate a z-score and convert it to a percentile, giving you a ballpark read between well-child visits — it is an estimate, not a diagnostic substitute for the actual CDC/WHO chart your pediatrician uses.
Worked example
Using the values the calculator loads with:
Inputs
- Sex: Female
- Age: 24 months
- Weight: 27 lb
- Reference average weight for age (lb): 27.5 lb
- Reference standard deviation (lb): 3.2 lb
Results
- Estimated percentile: 43.8
- z-score: -0.16
What each field means
Inputs
- Sex
- Pick the option that matches your situation — the maths changes per option. Choices: Female, Male.
- Age (months)
- The age used in the calculation, measured in months. Starts at 24 months so you have a working example on load. Accepted range: 0–240 months.
- Weight (lb)
- The weight used in the calculation, measured in lb. Starts at 27 lb so you have a working example on load. Accepted range: 2–250 lb.
- Reference average weight for age (lb) (lb)
- The reference average weight for age (lb) used in the calculation, measured in lb. Starts at 27.5 lb so you have a working example on load.
- Reference standard deviation (lb) (lb)
- The reference standard deviation (lb) used in the calculation, measured in lb. Starts at 3.2 lb so you have a working example on load.
Results
- Estimated percentile
- Returned as a decimal number and shown as the headline result. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- z-score
- Returned as a decimal number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
FAQ
Where do I find the reference mean and standard deviation for my child's age?
The CDC publishes LMS parameter tables by exact age in months and sex for weight, height, head circumference, and BMI, available free on cdc.gov/growthcharts. This calculator uses those inputs directly so results track the same statistical method pediatricians use, without hardcoding a single lookup table.
Is a low or high percentile a problem?
Not by itself — a consistently tracked 10th percentile child growing steadily along that line is generally healthy, since percentile reflects where a child falls in the population distribution, not a pass/fail grade. What concerns pediatricians is a percentile that crosses two or more major percentile lines in either direction, which can signal a growth or nutrition issue worth investigating.
Why do doctors use weight-for-length instead of BMI under age 2?
BMI as a ratio behaves oddly in infants and toddlers because body proportions change rapidly with age, so the CDC/WHO charts use weight-for-length up to 24 months and switch to BMI-for-age afterward. Make sure you're using the right chart type for your child's age.
Accuracy and limitations
- Population formulas describe averages; individual bodies vary, especially at the extremes of height, age, or muscle mass.
- Results do not account for medical conditions, medication, or pregnancy.
- Use this as a directional reference, not a diagnosis — talk to a clinician before acting on it.
Related tools
Cite this calculator
Writing about this topic? Grab a citation — every link helps keep these tools free.
RevenueLab. (2026). Child Growth Percentile Calculator. Retrieved from https://www.revenuelab.fyi/toolbox/growth-percentile-calculator
<p>Source: <a href="https://www.revenuelab.fyi/toolbox/growth-percentile-calculator" target="_blank" rel="noopener">Child Growth Percentile Calculator — RevenueLab</a> (2026).</p>
Source: [Child Growth Percentile Calculator — RevenueLab](https://www.revenuelab.fyi/toolbox/growth-percentile-calculator) (2026).