# Baby Percentile Calculator _Weight, length or head circumference against WHO's growth standards, for your baby's exact age in days._ - Source: https://proba.baby/en/tools/baby-percentile-calculator - Language: English - Last verified: v1.0.4 · 7 Sep 2026 - Last updated: 2026-10-10 ## How it works Choose what you measured — weight, length or head circumference — and whether your baby is a boy or a girl. WHO publishes separate tables for boys and girls and no combined one, so the calculator does not guess. Then give the age, either as a birth date and the day of the measurement or typed in days, weeks or months, and enter the number in kg or lb and oz, cm or inches. The result appears as you type: the percentile, the z-score behind it, WHO's median and 3rd–97th range for that exact age, and where the measurement sits on WHO's chart. A percentile is a position, not a grade. On the 30th percentile, about 30 in 100 healthy babies of the same age and sex measure less and 70 measure more. The standards behind it are WHO's: the Multicentre Growth Reference Study followed about 8,500 children in Brazil, Ghana, India, Norway, Oman and the USA, raised in conditions that favour healthy growth, with the breastfed baby as the model. In the US, the CDC recommends these WHO charts from birth to 24 months. The result also says whether the measurement is inside WHO's typical range. WHO's growth-assessment course says children who are growing normally are generally on or between the −2 and +2 z-score lines, roughly the 2nd to the 98th percentile. Only outside those lines does the calculator suggest talking to your pediatrician, and it names the line. One measurement is a snapshot: home scales differ, a wriggling baby measures short, and what your pediatrician reads is whether your baby keeps following their own curve over several visits. For a baby born before 37 weeks, choose the week of birth and the calculator scores corrected age, as the ProbaBaby app does. The calculation runs in your browser, and your inputs live in the address bar, so you can bookmark a result or send it to a partner. Below the calculator are WHO's month-by-month tables for all three measurements. ## The math WHO describes each measurement with three numbers for every day of age from birth to 730 days, separately for boys and girls: L (the skew), M (the median) and S (the spread). The z-score is z = ((x ÷ M)^L − 1) ÷ (L × S), or ln(x ÷ M) ÷ S when L is 0. The percentile is the share of a normal distribution below that z: Φ(z) × 100. Length and head circumference have L = 1, which makes them plain normal distributions. A worked example. A girl measured at 4 months is 4 × 30.4375 = 121.75 days old, which WHO rounds to the day-122 row: L = −0.0053, M = 6.428 kg, S = 0.12401. At 5.2 kg, z = ((5.2 ÷ 6.428)^−0.0053 − 1) ÷ (−0.0053 × 0.12401) = −1.71, and Φ(−1.71) = 4.4%, so she is on the 4th percentile: in the 3rd–15th band, and inside WHO's typical range because −1.71 is above the −2 line. Weight gets one extra rule. Beyond ±3 SD the LMS curve would produce weights no baby has, so WHO continues in steps the width of the gap between its 2 SD and 3 SD lines — the restricted method in WHO's anthro software. Length and head circumference never need it. A value so extreme that WHO's software flags it as a likely error (weight z below −6 or above 5, length beyond ±6, head beyond ±5) gets a check-the-measurement note instead of advice. Age is counted in whole days, the way WHO's anthro software counts it: an age typed in months is multiplied by 30.4375 and rounded half up, so a measurement on day 23 is scored against day 23 rather than rounded to a month. For a baby born before 37 weeks, (40 − weeks at birth) × 7 days come off the age first. Before that due-date equivalent WHO has no row, so there is no percentile, and the 0–2 year standards end at day 730. The percentile is shown as a whole number, with under the 1st and over the 99th in the tails, where more precision would be false. The flag uses WHO's own cut-offs on the z-score shown: below −3, −3 to −2, −2 to +2, +2 to +3 and above +3, and a value exactly on a line counts in the less severe group, as WHO's training course instructs. The chart band (3rd, 15th, 85th, 97th) is for display only: the 3rd percentile is z = −1.88, still inside the typical range. In this site's test suite every score matches WHO's published tables to ±0.01 z, and the tables below match WHO's printed monthly tables cell by cell. ## FAQ **What does my baby's percentile mean?** It is where your baby sits among healthy babies of the same age and sex. On the 40th percentile, about 40 in 100 measure less and 60 measure more. It describes size, not health, and half of all healthy babies are below the 50th by definition. What your pediatrician watches is whether your baby keeps roughly to the same curve over time. **What percentile is normal for a baby?** WHO's growth-assessment course says children who are growing normally are generally on or between the −2 and +2 z-score lines, roughly the 2nd to the 98th percentile. A baby steadily on the 5th or the 95th can be perfectly well. Outside those lines the calculator suggests talking to your pediatrician, because WHO uses them as cut-offs that deserve a closer look — not because they are a diagnosis. **Why do I have to choose boy or girl?** Boys and girls grow on different curves, and WHO publishes separate tables for each with no combined one. Picking one at random would move the answer by several percentiles, so the calculator, like the ProbaBaby app, asks rather than guesses. **Is this the same as the CDC growth chart?** For babies under 2 in the US, yes: the CDC recommends the WHO growth standards from birth to 24 months and its own CDC charts from age 2. On the WHO charts the CDC prints, the −2 and +2 lines are labelled the 2nd and 98th percentiles. This calculator stops at 24 months for the same reason. **How do I use it for a premature baby?** Enter the birth date and choose the week your baby was born. Before 37 weeks the calculator subtracts (40 − weeks at birth) × 7 days and scores that corrected age, the method the AAP describes. If you type the age instead, type the corrected age. Before the due-date equivalent there is no WHO percentile, because WHO's curves start at term. **Why is this different from the percentile at the clinic?** Usually it is the age or the measurement. Clinics often round age to a week or a month, while this calculator uses the exact day, and in the first weeks a few days move the answer noticeably. Scales, clothing and a baby who will not lie straight for length shift the number too. Some countries use their own charts, such as the UK-WHO charts, which are based on the WHO standards. **Does a high weight percentile mean my baby is overweight?** No. Weight-for-age cannot tell you that, because a long baby weighs more. WHO's growth-assessment course says a high weight-for-age is better judged with weight-for-length or BMI-for-age, which compare weight with length. Check length here too, and ask your pediatrician if both numbers worry you. **Can I track my baby's growth over time?** This calculator scores one measurement at a time. In the ProbaBaby app, growth percentiles come from WHO's own per-day LMS tables, generated from the WHO anthro package — a measurement on day 23 is scored against day 23 — and each measurement is kept, so you can see the curve. A free account is required to use the app. ## Sources - WHO — Child Growth Standards: weight-for-age, length-for-age, head circumference-for-age — LMS tables by day and by month, boys and girls; © World Health Organization: https://www.who.int/tools/child-growth-standards/standards - WHO — Training Course on Child Growth Assessment, Module C: Interpreting growth indicators (2008) — Children growing normally are generally on or between −2 and +2 z-scores; cut-offs at −2 and −3; a point on a line is in the less severe category: https://www.who.int/publications/i/item/9789241595070 - WHO — Multicentre Growth Reference Study — About 8,500 children in six countries; the breastfed infant as the normative model: https://www.who.int/tools/child-growth-standards/who-multicentre-growth-reference-study - WHO anthro — WHO's reference software (R package) — Per-day LMS tables, age in days (months × 30.4375), restricted LMS beyond ±3 SD for weight, implausible-value flags: https://github.com/WorldHealthOrganization/anthro - CDC — Using the WHO growth charts — WHO standards from birth to 24 months in the US; −2 and +2 SD labelled the 2nd and 98th percentiles: https://www.cdc.gov/growth-chart-training/hcp/using-growth-charts/who-using.html - RCPCH — UK-WHO growth charts, 0–4 years — The UK charts are based on the WHO Child Growth Standards: https://www.rcpch.ac.uk/resources/uk-who-growth-charts-0-4-years - AAP HealthyChildren.org — Corrected Age for Preemies — Actual age minus the weeks born early (40 − gestational age at birth): https://www.healthychildren.org/English/ages-stages/baby/preemie/Pages/Corrected-Age-For-Preemies.aspx --- ProbaBaby (listed on the App Store as “Pregnancy & Baby Tracker - PB”) is a free iPhone, iPad and Apple Watch app for pregnancy, newborn and postpartum tracking, built for twins, in 32 in-app languages, with an optional AI baby-face prediction that computes Mendelian genetics (Punnett squares) from the traits you enter before any image is generated. Tracking, tools, PDF reports and family sharing are free; only AI predictions use credits — 5 starter credits cover your first Scientific prediction — and no subscription is required. Requires iOS 26 or later. Not available on Android.