For educational purposes only. Not personalised health advice. This calculator has not been reviewed by a registered dietitian, physician or exercise physiologist. Results are population estimates and can be wrong for you individually. Speak to a qualified professional before acting on them, particularly if you are pregnant, under 18, managing a medical condition, or have a history of disordered eating.
Baby Weight Percentile Calculator
The Baby Weight Percentile Calculator computes baby weight percentile from the relation z = ((weight / M) raised to L minus 1) / (L x S); percentile = standard normal CDF of z x 100. It takes 4 inputs (baby weight in kg, lms reference l value for age and sex, lms reference m median weight for age and sex in kg, lms reference s coefficient of variation for age and sex) and returns the baby weight percentile. These calculators are for anyone tracking a cycle, a pregnancy or a new baby, and for the midwives, nurses and doctors supporting them, turning recorded dates and measurements into the timing and growth figures used to anticipate what comes next, follow progress against expected patterns, and decide what to raise at the next appointment. Enter your values below and the result updates instantly, and you can share a permalink that pre-fills the exact calculation. These outputs follow directly from the dates and measurements you enter, so use dates exactly as recorded rather than rounded recollections, keep weight and length figures in one unit system where a tool asks for them, and read any single result as less telling than the pattern that builds up over repeated cycles or measurements. For example, with baby weight = 11 kg, lms reference l value for age and sex = 1, lms reference m median weight for age and sex = 10 kg, lms reference s coefficient of variation for age and sex = 0.1, the baby weight percentile works out to 84.134474, and the worked example further down the page shows every step so you can follow the arithmetic and reproduce it by hand. The method is the standard form documented by WHO Child Growth Standards, and the figure above each result carries the date it was last verified. Cycles, pregnancies and babies all vary widely, and these figures describe typical ranges rather than an assessment of any individual, so discuss anything that worries you, or any result that falls outside what you expected, with your midwife, doctor or pediatrician.
With Baby weight = 11 kg, LMS reference L value for age and sex = 1, LMS reference M median weight for age and sex = 10 kg, LMS reference S coefficient of variation for age and sex = 0.1, the result is 84.134474.
Applies to: any numeric inputs. Method source: WHO Child Growth Standards, checked 2026-06-23.
The formula
z = ((weight / M) raised to L minus 1) / (L x S); percentile = standard normal CDF of z x 100
Worked example
With Baby weight = 11 kg, LMS reference L value for age and sex = 1, LMS reference M median weight for age and sex = 10 kg, LMS reference S coefficient of variation for age and sex = 0.1:
- z score = 1.0000
- percentile = normal CDF of z x 100 = 84.13
- Baby Weight Percentile = 84.134474
This worked example is one of the automated golden-value tests this calculator must pass before it can publish.
What this assumes
- Inputs are real numbers in the units shown.
- The result is the exact value of z = ((weight / M) raised to L minus 1) / (L x S); percentile = standard normal CDF of z x 100; general information, not professional advice.
Frequently asked questions
What formula does this use?
z = ((weight / M) raised to L minus 1) / (L x S); percentile = standard normal CDF of z x 100, the standard form documented by WHO Child Growth Standards.
Does the result ever change over time?
No. This is a pure formula with no external rate, so the same inputs always give the same result.
Official sources and verification
- Method: WHO Child Growth Standards, checked 2026-06-23.
This content is provided for educational purposes only and has not been reviewed by a qualified specialist in this field. See our methodology. General information, not professional advice.