MasterTool

Baby Growth Percentile Calculator

Calculate a baby's weight-for-age and length-for-age percentiles (0–60 months) using the WHO Child Growth Standards LMS tables.

WHO Child Growth Standards LMS last updated · reference source

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Dated static reference; no live data is fetched. Verify current source values and assumptions before relying on results.

What this tool does

Calculate a baby's weight-for-age and length-for-age percentiles (0–60 months) using the WHO Child Growth Standards LMS tables. WHO expanded LMS tables embedded at 25 sampled ages (monthly in year 1, then every 3–6 months) with linear interpolation — see src/data/health-tables.ts. A percentile is a position in the reference population, not a grade; trends over time matter more than any single point.

How to use the Baby Growth Percentile Calculator

  1. Enter or select age (months).
  2. Enter or select sex.
  3. Enter or select weight in lb.
  4. Enter or select length / height in in.
  5. Read the calculated result; change any measurement to compare alternatives.

Formula

z = ((x/M)^L − 1)/(L·S) with WHO weight-for-age and length/height-for-age LMS parameters for the baby's age and sex; percentile = Φ(z)
months
Age (months)
sex
Sex
weight
Weight (lb, before metric conversion)
length
Length / height (in, before metric conversion)

WHO expanded LMS tables embedded at 25 sampled ages (monthly in year 1, then every 3–6 months) with linear interpolation — see src/data/health-tables.ts. A percentile is a position in the reference population, not a grade; trends over time matter more than any single point.

Worked example

For baby growth percentile calculator, the following measurements illustrate the exact method: Age (months): 12; Sex: male; Weight: 20; Length / height: 30.

Inputs

  • Age (months)12
  • SexMale
  • Weight20 lb
  • Length / height30 in

Result

  • Weight-for-age percentile28.75
  • Length/height-for-age percentile57.69
  • Weight z-score-0.56
  • Length/height z-score0.19

Results explained

Weight-for-age percentile
Weight-for-age percentile from the formula above. WHO expanded LMS tables embedded at 25 sampled ages (monthly in year 1, then every 3–6 months) with linear interpolation — see src/data/health-tables.ts. A percentile is a position in the reference population, not a grade; trends over time matter more than any single point.
Length/height-for-age percentile
Length/height-for-age percentile from the formula above. WHO expanded LMS tables embedded at 25 sampled ages (monthly in year 1, then every 3–6 months) with linear interpolation — see src/data/health-tables.ts. A percentile is a position in the reference population, not a grade; trends over time matter more than any single point.
Weight z-score
Weight z-score from the formula above. WHO expanded LMS tables embedded at 25 sampled ages (monthly in year 1, then every 3–6 months) with linear interpolation — see src/data/health-tables.ts. A percentile is a position in the reference population, not a grade; trends over time matter more than any single point.
Length/height z-score
Length/height z-score from the formula above. WHO expanded LMS tables embedded at 25 sampled ages (monthly in year 1, then every 3–6 months) with linear interpolation — see src/data/health-tables.ts. A percentile is a position in the reference population, not a grade; trends over time matter more than any single point.

Frequently asked questions

With the WHO LMS method: published L, M and S parameters for each age and sex convert a measurement to a z-score, and the z-score to a percentile — the share of the WHO reference population below that measurement.

Exactly average for the reference population — half of healthy babies measure below it and half above. Any percentile along a consistent curve can be perfectly healthy; there is no 'best' percentile.

WHO Child Growth Standards (0–60 months), embedded from WHO's official expanded LMS tables. The CDC recommends WHO charts for under-2s in the US; this tool extends the same WHO series to age 5.

A single point rarely should. Crossing two major percentile lines over time, or weight and length diverging sharply, are the patterns clinicians watch — raise them at your baby's check-ups rather than interpreting them alone.

The WHO day-by-day LMS table is embedded at 25 ages and interpolated, and small length-measurement errors (babies wriggle) move percentiles noticeably at this age. Treat the result as a good approximation, and rely on your clinician's plotted curve for decisions.