Methodology

Every answer on this site comes from two things: a projection of how big a baby will be on a given date, and the size chart a brand publishes. This page explains both, including where the method is weak. This is a sizing estimate, not medical advice.

1. Growth data: the WHO Child Growth Standards

Growth projections use the World Health Organization Child Growth Standards, weight-for-age and length/height-for-age, for boys and girls from birth to 60 months. The tables were downloaded from the WHO website on October 4, 2026 and are stored unmodified with the project.

• WHO weight-for-age standards
• WHO length/height-for-age standards

Length versus height. WHO measures babies lying down (recumbent length) until 24 months and standing up (height) from 24 months. Standing height is about 0.7 cm less than length for the same child. The estimate switches tables at 24 months, exactly as the standards do. A measurement you enter is read as length if the child was under 24 months that day, and as height otherwise.

Boy, girl, or not sure. The growth chart setting only selects the growth curve. "Not sure" averages the boy and girl curves. It has no connection to the color theme.

2. The LMS method, in plain language

The WHO tables describe, for each age, the middle of the range (M), how spread out babies are (S), and how lopsided the spread is (L). With those three numbers, any measurement can be turned into a z-score: a number that says how far from the middle a baby sits. Zero is the middle of the chart.

1. If you enter a current weight or length, the z-score is calculated at the baby's age on the measurement date: z = ((X / M)^L - 1) / (L * S). Without measurements, the estimate assumes the middle of the chart (z = 0).
2. The z-score is limited to the range from -3 to 3.
3. The estimate assumes the baby stays on the same z-score, and reads the size at the target age: X = M * (1 + L * S * z)^(1 / L).

Weight and length are projected separately. Ages between whole months are interpolated between the monthly WHO values.

3. The uncertainty band

Babies do not follow one curve exactly. Each projection is repeated half a z-score lower and half a z-score higher (z - 0.5 and z + 0.5). That produces the "most likely" size and the "could be" sizes on every result. The further away the date, the more the range matters.

If the measurements you enter fall below the 1st or above the 99th percentile, the result includes a note that size predictions are less reliable at the edges of the growth charts, and that current measurements are the best guide. The site does not comment on whether a baby is big or small.

4. Babies born early: corrected age

For a baby born early, the growth projection uses corrected age: age counted from the due date instead of the birth date (chronological age minus the weeks born early). Corrected age is used until the child reaches 24 months corrected. After that, chronological age is used. Before the original due date, the estimate uses the newborn values, which is a rough guide only.

5. Babies who are not born yet

With a due date, the estimate assumes birth on the due date and growth along the middle of the chart. Update it with real measurements after the birth.

6. Brand size charts

The database covers 28 brands. Every number comes from the brand's own published chart: the brand's size chart page, its official size guide, or its official storefront at a major retailer. Blogs, forums, and other size tools are never used. No value is estimated or filled in. If a brand does not publish a number, it is left empty.

Each brand page lists the source link, the retrieval date, and the chart in the brand's original wording. Charts were retrievedon October 4, 2026. Brands that could not be verified from an official source are left out.

7. Picking a size

• A size fits when the projected weight and length both fall inside the ranges the brand publishes for it.
• If weight and length point to different sizes, the larger one is used, because a garment has to fit both ways.
• If a brand publishes only weight or only height, that one measurement is used.
• If a brand publishes only ages, age is used and the result is marked "lower confidence".
• When the baby lands between two sizes, "room to grow" picks the larger size and "snug" picks the smaller. The same rule covers two sizes whose ranges overlap, and a value that sits exactly where one size ends and the next begins: it counts as inside both, so "snug" gives the smaller size and "room to grow" the larger.
• A few published charts contain a range that starts below the size before it. The published numbers are kept, but a later size is never treated as starting below an earlier one.

8. Brand vs brand

Two sizes are compared by how much their published ranges overlap, measured as the shared part divided by the combined part, averaged over weight and height when both brands publish both. An overlap of 60% or more is called a close match. Between 25% and 60% is a partial match. Below that, the page says there is no clean match.

9. Runs small, true to size, runs large

Based on published size charts compared with other brands. For every size label that at least three other brands also use, the brand's published ranges are compared with the middle (median) of the other brands' ranges for that label. If a brand's ranges sit more than 25% of a typical size step above the median, it is labeled "runs large" (the same label fits a bigger baby). The opposite is "runs small". This reflects published charts only. It is not based on fit reviews, fabric, shrinkage, or cut.

10. Seasons

Seasons are for the United States (northern hemisphere): winter is December to February, spring is March to May, summer is June to August, fall is September to November.

Limitations

• Growth is not perfectly steady. Babies move between curves, especially in the first months.
• The WHO standards describe healthy growth under good conditions worldwide. Individual babies vary.
• Size charts describe the baby a size is made for, not the garment. Cut, fabric, stretch, and shrinkage are not captured.
• Brands change their charts. Retrieval dates are shown so you can judge freshness.
• Some brands were only partly verifiable. Those charts start or stop where the verified data does, and the brand page says so.

This is a sizing estimate, not medical advice. For questions about a child's growth, talk to a pediatrician.