WHO Child Growth Standards
Baby Weight Percentile Calculator
Use one dated baby weight to calculate a WHO weight-for-age percentile and z-score from birth through the day before the second birthday.
This focused page answers weight relative to age. It is a reference comparison for one measurement, not a diagnosis or growth trend. For several measurements together, use the Baby Growth Percentile Calculator.
What baby weight-for-age means
Weight-for-age answers: where does this dated weight fall compared with WHO references for babies of the same exact age and sex? It does not use length as the comparison axis.
A percentile is a reference position, not a target, normality label, or treatment goal.
What weight-for-age does not tell you
One weight-for-age point does not establish weight status, nutrition adequacy, feeding adequacy, illness, or a diagnosis. Weight-for-length uses a different comparison basis and can be calculated in the comprehensive Baby Growth tool when both measurements are available.
BMI-for-age is not the supported infant output here; age-2-and-older BMI belongs in the Kids BMI Calculator.
How to weigh a baby consistently
- Use an infant scale on a stable surface when available.
- Use a consistent scale, clothing approach, and time context; keep clothing and diaper handling conservative and consistent.
- Record the date with the weight and repeat an uncertain reading.
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 clinical measurements are needed to assess a pattern. It does not calculate a WHO velocity percentile or predict future growth.
Weight-for-age, weight-for-length, and BMI are different
Weight-for-age compares weight at an exact age. Weight-for-length compares weight at the same recumbent length and does not use age as its chart axis. The Kids BMI Calculator is an age-2-and-older standing-height screening tool.
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. For age 2 and older weight-for-age, use the CDC Weight Percentile Calculator. That page uses the child CDC reference and does not continue the WHO infant page.
Worked example
A girl born January 1, 2026 and weighed February 20, 2026 is 50 days old. A weight of 5.3 kg produces about the 76th percentile with a z-score of about 0.72 on the WHO girls weight-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 weight-for-age LMS table. It uses the existing WHO adjusted-z method for weight-based results; 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.
- Chronological age is used; corrected age and preterm chart selection are not automatic.
- Accurate dates, units, scale technique, and repeatability matter.
- One weight-for-age result is not a diagnosis, weight-status label, feeding plan, 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 weight-for-age compare?
It compares one submitted weight with the WHO weight-for-age distribution for babies of the same exact age and selected sex. It does not account for recumbent length.
Does this result diagnose underweight or poor growth?
No. A single weight-for-age percentile is a reference position, not a diagnosis, weight-status label, or conclusion about feeding or health.
Why do the birth and measurement dates matter?
WHO infant references change across individual age days. The calculator uses the actual dates rather than a rounded month estimate.
How should I weigh a baby?
Use an infant scale when available, a consistent method and equipment, minimal clothing, and a dated reading. Repeat an uncertain measurement rather than averaging guessed values.
What if I also have length?
Use the Baby Growth Percentile Calculator for weight-for-age together with recumbent length, 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 Weight Percentile Calculator for age-based child weight.
Does the site save my baby’s weight?
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 weight-for-age, length, weight-for-length, and head-circumference percentiles before age 2 and what one result can and cannot show.