Calculator form
Example data table
| Case | Sex | Age | Length input | Z score | Percentile | Interpretation |
|---|---|---|---|---|---|---|
| A | Boy | 6 months | 67.6 cm | -0.01 | 49.6% | Within expected range |
| B | Girl | 9 months | 67.0 cm | -1.30 | 9.7% | Within expected range |
| C | Boy | 18 months | 79.0 cm | -1.21 | 11.3% | Within expected range |
| D | Girl | 15 months | 73.0 cm | -1.65 | 5.0% | Within expected range |
Formula used
This calculator applies the WHO LMS growth method for length-for-age from birth through 24 months. It linearly interpolates the monthly L, M, and S values between whole-month ages when needed.
Z score formula
z = (((X / M)^L) - 1) / (L × S)
Where X is the adjusted child length, M is the age-specific median, S is the generalized coefficient of variation, and L is the Box-Cox power. Percentile is then estimated from the standard normal cumulative distribution.
Measurement adjustment
When a child under 24 months is measured standing, the tool adds 0.7 cm to estimate recumbent length before calculating the z score.
How to use this calculator
- Select the child’s sex.
- Choose whether you want to enter age directly or derive it from dates.
- Choose recumbent length or standing height.
- Enter the length or height in centimeters.
- Press the calculate button to display the result above the form.
- Review the z score, percentile, median difference, and interpretation.
- Use the export buttons to save the result as CSV or PDF.
- Discuss out-of-range results with a pediatric professional when needed.
Frequently asked questions
1. What does length for age measure?
It compares a child’s measured body length with the expected distribution for children of the same sex and age. It helps identify whether linear growth appears typical, low, or unusually high.
2. What is a z score in this calculator?
The z score shows how far the child’s adjusted length sits above or below the WHO median, measured in standard deviation units. A value near zero is close to the median.
3. What percentile should worry parents?
Percentiles alone do not confirm a problem. A low percentile, a z score below minus two, poor growth trend, feeding issues, illness, or developmental concerns deserve pediatric review.
4. Why does standing height get adjusted?
For children under two, WHO standards are based on recumbent length. Standing height is usually shorter, so the tool adds 0.7 cm before comparing the child with the reference standard.
5. Can I use this for children older than 24 months?
No. This page is designed for birth through 24 months. Older children should use a height-for-age calculator based on the correct WHO or national reference tables.
6. Is one low reading enough to diagnose stunting?
No. A single reading can be affected by measurement error, posture, equipment, and health context. Diagnosis should consider repeated measurements, medical history, diet, and professional assessment.
7. How accurate is date-based age calculation?
It converts the difference between birth date and measurement date into months using the average month length. This is usually practical for screening and routine growth monitoring.
8. Should parents track trends instead of one result?
Yes. Growth trend over time is often more informative than one measurement. Repeated values help reveal steady progress, slowing growth, recovery, or the need for closer evaluation.