Baby Weight Percentile Calculator

A baby weight percentile compares your baby's weight with healthy babies of the same sex and age. At the 40th percentile, your baby weighs more than about 40 out of 100 of those babies. This calculator uses the WHO Child Growth Standards, which give reference values for each day of age from birth¹.

What a Baby Weight Percentile Measures

It shows where one weigh-in sits compared with babies who grew well. It does not measure health, body fat or how much your baby eats. Babies on the 10th and 90th percentiles can both be growing well.

The Formula

WHO describes each day of age with three numbers: L (skew), M (median weight) and S (spread)². Your baby's weight becomes a z-score, then a percentile.

z = ((weight ÷ M)^L - 1) ÷ (L × S)

Percentile = share of a normal bell curve below z, × 100

This is the LMS method WHO used to build its standards, with a small fix for weights beyond 3 standard deviations². For a boy aged 73 days (10 weeks 3 days), L = 0.1870, M = 5.9174 kg and S = 0.12088¹. At 5.8 kg (12 lb 12.6 oz) his z-score is -0.17, the 43rd percentile.

How This Calculator Computes Your Result

Enter sex, the birth and weigh-in dates (or age in weeks and days) and weight in pounds and ounces, kilograms or grams (1 lb = 0.45359237 kg). The tool looks up that exact day in the WHO daily table, from day 0 to 730, without rounding to months.

You get the percentile, z-score, the median weight for that age and the WHO screening range. Add a lying-down length for weight-for-length, or up to 10 weigh-ins for a growth line.

What Your Result Means

CDC recommends the WHO charts for babies under 2. It uses 2 standard deviations (the 2.3rd and 97.7th percentiles) as screening lines, not the 5th and 95th used on older CDC charts³.

A result outside this range is a reason for a closer look, not a diagnosis. Median (50th percentile) weights by age¹:

How to Measure Correctly

Use the same scale each time, with your baby undressed or in a clean diaper, at a similar time of day. For length, lay your baby flat with the head against a firm board and the legs gently straightened. Babies under 2 are measured lying down, and lying-down length runs about 0.7 to 0.8 cm longer than standing height³.

Weight-for-Length: Is Weight in Proportion?

Weight-for-length compares weight with babies of the same length, whatever their age, using WHO tables from 45 to 110 cm⁶. A girl of 15 lb 6 oz at 7 months is on the 23rd percentile for age, but at 26 in long she is on the 30th for her length, so her weight is in proportion. CDC uses the 97.7th percentile as the line for high weight-for-length on WHO charts³.

Tracking Weight Over Time

Weight often does not follow one line in the first year, and UK chart guidance says it usually tracks within one centile space⁵. A centile space is the gap between neighboring lines on UK-WHO charts⁴. The tool shows how many lines and spaces your baby has crossed since the first weigh-in.

UK NICE guidance suggests a closer look after a fall of 1 or more spaces if birth weight was below the 9th centile, 2 or more if it was between the 9th and 91st, 3 or more if it was above the 91st, or if current weight is below the 2nd centile⁴. A girl who goes from the 64th percentile at birth (3.4 kg) to the 11th at 6 months (6.3 kg) has fallen 2.4 spaces, so the tool suggests sharing her weigh-ins with her doctor.

Early drops are common. The weight loss of the first days usually stops after 3 or 4 days, and most babies are back to birth weight by 3 weeks⁴, so the tool checks these rules only from 3 weeks. Moving up is common too: formula-fed babies tend to gain faster than the WHO charts after about 3 months³.

Babies Born Early

The WHO standards left out babies born before 37 weeks³. Premature babies are plotted by corrected age, usually until age 2⁷. Enter weeks at birth and the tool uses adjusted age. A girl born at 32 weeks who is 12 weeks old and weighs 4.2 kg is on the 57th percentile at her adjusted age of 4 weeks; by actual age she would be on the 1st percentile (z-score -2.37).