Weight-for-Length Percentile Calculator

Weight-for-length answers how a submitted weight compares with WHO references for babies of the same sex at the same recumbent length. It uses length as the x-axis, not age.

The primary result is a percentile and z-score. Neither output is a diagnosis, health label, treatment goal, or automatic conclusion about feeding.

Because WHO weight-for-length is indexed by recumbent length, a birth date is not needed for this numerical reference calculation. The page therefore does not pretend to verify a baby's age eligibility from an input it does not request. Use the documented under-2 workflow and ask an appropriate healthcare professional to confirm chart and age context when needed.

Weight-for-age compares weight with babies of the same exact age. Length-for-age compares recumbent length at an exact age. Weight-for-length compares weight at a matching recumbent length and cannot be substituted for either age-based indicator.

It is also not infant BMI. For age 2 and older, the site's weight-relative-to-standing-height screening tool is the Kids BMI Calculator.

1. Use a consistent infant scale when available, with minimal clothing and conservative, repeatable diaper handling.
2. Measure recumbent length lying flat on a firm surface or length board, keep the head positioned consistently, extend the legs gently, and align the heels.
3. Record both measurements and repeat an uncertain reading rather than averaging guessed values.

The documented WHO weight-for-length table covers 45.0–110.0 cm. The focused calculator allows both endpoints and rejects values outside the range; it does not extrapolate.

At an exact 0.1 cm source key, the stored WHO L/M/S row is used. At a non-key length such as 57.15 cm, the calculator linearly interpolates L, M, and S between the 57.1 and 57.2 cm rows, then applies the same WHO adjusted-z method. It does not interpolate a final percentile from two separate results.

This page answers a single weight-and-length reference question. Repeated, accurately dated measurements are needed to assess a pattern. It does not calculate a WHO velocity percentile or predict future growth.

This focused calculation does not take a birth date and does not automatically apply corrected age or select a preterm chart. A healthcare professional should confirm the appropriate chart, measurement method, and age basis for a baby born preterm.

This page is intended for the WHO infant workflow through the day before the second birthday. Child Growth Lab does not extend it into a WHO weight-for-height or CDC weight-for-stature calculator. For age 2 and older weight relative to standing height, use the Kids BMI Calculator; the combined Child Growth Chart Calculator can review height, weight, and BMI together.

A girl with a recumbent length of 57.1 cm and weight of 5.3 kg produces about the 65th percentile with a z-score of about 0.40. This exact-row example uses the 57.1 cm WHO source key.

At 57.15 cm with the same weight, the lower and upper keys are 57.1 and 57.2 cm and the interpolation fraction is 0.5; the current validated result is about the 64th percentile.

The calculator converts metric or US inputs using the existing unit helpers, selects the WHO sex-specific weight-for-length table, preserves the 45.0–110.0 cm source range, interpolates L/M/S only when the submitted length is not a stored 0.1 cm key, and applies the reviewed WHO adjusted-z method.

Provenance, source identity, checksums, permission details, length keys, and interpolation fraction remain available in the collapsed technical section and Methodology.

• 1
  WHO — Weight-for-length/height↗
• 2
  WHO — Child Growth Standards↗
• 3
  WHO — Child Growth Standards software and data resources↗
• 4
  CDC — Using Growth Charts↗
• 5
  AAP — Anthropometric Measurements↗
• 6
  WHO 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.