Baby Percentile Calculator
See where your baby’s weight and length sit on the WHO Child Growth Standards for their age and sex — the same charts used in child health records worldwide, birth to 24 months.
Weight-for-age percentile
48th percentile
- Weight z-score (SD from median)
- -0.04 SD
- WHO median weight at this age
- 7.93 kg
- Length-for-age percentile
- 50th percentile
- Length z-score (SD from median)
- -0.01 SD
- WHO median length at this age
- 67.6 cm
Weight is on the 48th percentile and length on the 50th percentile of the WHO Child Growth Standards — meaning roughly 48% of healthy babies of the same age and sex weigh less. Both measurements sit inside the −2 to +2 SD range that covers about 95% of healthy children. Any percentile from the 3rd to the 97th is generally considered typical; what matters clinically is the pattern across visits, not one snapshot. This tool is informational and is not medical advice.
How to use this calculator
Choose your baby’s sex (the WHO publishes separate charts for boys and girls), then enter their age in completed months — a baby who is 6 months and 3 weeks old counts as 6 months — along with weight in kilograms and length in centimetres. Weight should be measured naked or in a dry nappy only, and length lying down, the way it is measured at routine check-ups. The headline number is the weight-for-age percentile; the breakdown adds the weight z-score, the WHO median weight for that age, and the same trio for length-for-age. A percentile of 62 means your baby weighs more than about 62% of healthy babies of the same age and sex — and less than the other 38%. The calculator covers birth to 24 months, the age range where babies are measured lying down.
How the calculation works
The WHO Child Growth Standards (2006) were built from the Multicentre Growth Reference Study, which tracked healthy, breastfed children in six countries under optimal conditions — so they describe how children should grow, not merely how a reference population happened to grow. For each age and sex the WHO publishes three LMS parameters: L (the Box–Cox power that corrects skewness), M (the median) and S (the coefficient of variation). A measurement X converts to a z-score with Cole’s LMS formula: z = ((X/M)^L − 1) / (L×S). The z-score counts how many standard deviations the measurement sits from the median, and the percentile is the share of the standard normal distribution below that z. Weight-for-age uses the WHO’s restricted adjustment beyond ±3 SD (extreme values are scaled against the distance between the ±2 and ±3 SD curves, per the WHO igrowup macro), while length-for-age has L = 1 at every age — the WHO models length as normally distributed, so its z-score is the plain (X − M) / (S×M). This calculator embeds the WHO’s published monthly LMS tables verbatim, machine-checked against all 1,500 published percentile values.
Worked example
A 6-month-old boy weighing 7.9 kg and measuring 67.6 cm: the WHO median weight for 6-month boys is M = 7.934 kg with L = 0.1257 and S = 0.10958, so z = ((7.9/7.934)^0.1257 − 1)/(0.1257 × 0.10958) = −0.04 — just a whisker under the median, the 48th percentile. His length compares with a median of 67.6 cm, giving z = −0.01, the 50th percentile. Both sit comfortably inside the −2 to +2 SD band (roughly the 2nd to 98th percentile) that covers about 95% of healthy children. For contrast, a 6-month-old boy weighing 6.4 kg would be at z = −1.93, the 3rd percentile — still within the typical range, but a value a paediatrician would want to see holding steady across visits rather than drifting downward.
Frequently asked questions
What does a baby growth percentile actually mean?
A percentile ranks your baby against healthy babies of the same age and sex on the WHO growth standards. The 25th percentile for weight means your baby weighs more than 25% of healthy babies and less than 75% — it is a position on a distribution, not a score out of 100. The 50th percentile is the median, not a target: half of all healthy babies are below it by definition. Any percentile from roughly the 3rd to the 97th is considered typical, and a small baby tracking steadily along the 10th percentile is usually far less interesting clinically than a baby who drops from the 75th to the 25th between visits.
Which growth charts does this calculator use — WHO or CDC?
The WHO Child Growth Standards (2006), which cover birth to 24 months here. The WHO standards are prescriptive — built from healthy, predominantly breastfed children raised in optimal conditions across six countries — whereas the older CDC 2000 charts are descriptive references of how US children happened to grow, including formula-fed infants. The CDC itself recommends using the WHO charts for children under 24 months, and the UK’s child health record (the red book) has been based on the WHO standards since 2009. Numbers from a CDC-based tool will differ slightly, especially for breastfed babies after 3 months.
Is a low or high percentile a problem?
Not by itself. Percentiles from the 3rd to the 97th (z-scores between about −2 and +2) span the typical range, and healthy babies are found across all of it — someone has to be on the 5th percentile. What paediatricians watch is the trend: a baby crossing two or more major percentile lines downward (say 75th → 25th), a weight-for-age z-score below −2 (possible underweight) or above +2, or weight and length percentiles that diverge sharply. A single measurement is a snapshot with real measurement error in it — wet nappies, wriggling, and different scales all move the number. If this calculator shows a value outside ±2 SD, discuss it with your paediatrician, health visitor or family doctor rather than drawing conclusions from one data point.
What is a z-score and how does it relate to the percentile?
The z-score counts how many standard deviations a measurement sits above or below the median for age and sex: z = 0 is exactly median, z = +1 is one SD above, z = −2 is two SD below. Percentiles are the same information re-expressed as a rank: z = 0 is the 50th percentile, z = −2 is roughly the 2nd, z = +2 roughly the 98th. Clinicians and the WHO prefer z-scores at the extremes because percentiles saturate — the difference between z = −3 and z = −4 is invisible in percentile terms (both round to “below the 0.1st”) but clinically enormous. This calculator reports both.
Why are the charts different for boys and girls?
Boys and girls grow at measurably different rates from birth: at 6 months the WHO median weight is 7.93 kg for boys and 7.30 kg for girls, and the spread around the median differs too. Plotting a girl on a boys’ chart would understate her percentile by a noticeable margin — typically 10–20 percentile points through the first year. Every national implementation of the WHO standards keeps the sexes separate, which is why this calculator asks for sex first.
How accurately do I need to measure my baby at home?
Weight is the easy one: use the same scale each time, weigh naked or in a dry nappy only, and remember a feed can add 100–200 g. Length is genuinely hard at home — clinics use a length board with two people, and a solo tape-measure attempt on a wriggling baby is often 1–2 cm off, which at 6 months is enough to move the result by 15–20 percentile points. Treat home length percentiles as rough, and rely on the values in your child’s health record for anything that matters. Also enter age in completed months (a 6-month-3-week-old is “6 months”), since the WHO tables are indexed by completed month.