WHO Child Growth Standards
Baby Length Percentile Calculator
Use one dated lying-down recumbent length to calculate a WHO length-for-age percentile and z-score from birth through the day before the second birthday.
Under 2: recumbent length. The age-2-and-older Height Percentile Calculator uses standing stature. For several infant measurements together, use the Baby Growth Percentile Calculator.
What length-for-age answers
Length-for-age answers where this dated recumbent length falls compared with WHO references for babies of the same exact age and sex. It uses length and age, not standing-height measurement.
A percentile is a reference position, not a target, normality label, or treatment goal.
Recumbent length versus standing height
Recumbent length is measured with the baby lying down. Standing stature is measured upright and is used by Child Growth Lab's age-2-and-older CDC height calculator. These measurements are related but are not interchangeable inputs.
At the age-2 handoff, both the reference chart and the measurement method can change. A change in percentile should not automatically be treated as a growth problem.
How to measure recumbent length
- Use a flat, firm surface or an infant length board when available; two adults can help position the baby safely.
- Keep the head in a consistent position, straighten the body gently, extend the legs without forcing them, and place the heels together against the footboard.
- Record the date and repeat an uncertain measurement using the same method rather than averaging guessed values.
Exact age and why dates matter
The WHO reference changes across infancy. The actual birth date and measurement date select the integer WHO age-day row; a rounded month can select a different reference.
One measurement versus a growth trend
This page answers a one-date question. Repeated, accurately dated measurements are needed to assess a pattern. It does not calculate a WHO velocity percentile or predict future height.
Length-for-age, weight-for-length, and growth velocity are different
Length-for-age compares recumbent length at an exact age. Weight-for-length compares weight at the same recumbent length and does not use age as its chart axis. Growth Velocity compares two dated measurements arithmetically.
Preterm babies and corrected age
This calculator uses chronological age and does not automatically apply corrected age or select a preterm chart. The Corrected Age Calculator explains a timeline only; an appropriate healthcare professional should confirm the chart and age basis.
The age-2 handoff
This focused WHO page ends on the day before the second birthday. From age 2 onward, use the CDC Height Percentile Calculator with a standing-height measurement. That page uses the child CDC reference and does not continue the WHO infant length page.
Worked example
A girl born January 1, 2026 and measured February 20, 2026 is 50 days old. A recumbent length of 57.1 cm produces about the 72nd percentile with a z-score of about 0.57 on the WHO girls length-for-age reference.
This is a reference position, not a target or clinical conclusion.
How the calculation works
The calculator uses the actual dates, exact WHO age day, selected WHO sex chart, and the reviewed WHO length-for-age LMS table. No separate focused algorithm is introduced.
Provenance, source identity, checksums, and permission details remain available in the collapsed technical section and Methodology.
Limitations
- Birth through the day before the second birthday only.
- Primary input is recumbent length, not standing height.
- Chronological age is used; corrected age and preterm chart selection are not automatic.
- Accurate dates, units, technique, and repeatability matter.
- One length-for-age result is not a diagnosis, future-height prediction, or trend.
- The focused page does not save measurements or create a longitudinal record.
Sources
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- 6WHO Child Growth Standards data used under WHO Permission Request 202609680, granted 2026-07-16.
No WHO logo is used. Child Growth Lab is independent and is not affiliated with or endorsed by WHO.
Helpful context
Frequently asked questions
What does baby length-for-age compare?
It compares one lying-down recumbent length with the WHO length-for-age distribution for babies of the same exact age and selected sex. It is not a standing-height result.
Is recumbent length the same as standing height?
No. Under age 2, the WHO infant reference uses recumbent length measured lying down. The age-2-and-older Height Percentile Calculator uses standing stature.
Does one length percentile diagnose short stature or growth failure?
No. One length-for-age percentile is a reference position, not a diagnosis, an endocrine assessment, or a conclusion about future height.
How should I measure a baby’s length?
Use a flat firm surface or infant length board, position the head consistently, extend the legs gently without forcing them, align the heels, and repeat an uncertain measurement.
What if I also have weight?
Use the Baby Growth Percentile Calculator for length-for-age together with weight-for-age, weight-for-length, or head circumference.
Does this calculator use corrected age?
No. It uses chronological age from the actual birth and measurement dates. A healthcare professional should confirm the chart and age basis for a baby born preterm.
What happens at age 2?
This focused WHO page ends the day before the second birthday. From age 2 onward, use the CDC Height Percentile Calculator with a standing-height measurement.
Does the site save my baby’s length?
No. The calculation runs in the browser and is not saved as a child health record or sent to an external calculation service.
Understand this result
What Do Baby Growth Percentiles Mean?
Understand WHO recumbent-length, weight, weight-for-length, and head-circumference percentiles before age 2 and what one result can and cannot show.