The child growth planner answers one question. Given a child's current height and weight, when are they likely to reach the upper limit of a clothing size or a car-seat mode? It projects the child's growth along a reference curve and finds the first date on which the projection reaches a limit taken from a published chart or manual.

The limits come from two catalogs maintained on the site. The growth projection comes from the WHO and CDC reference curves.

Clothing limits

Children's clothing sizes in the United States are labeled by age band, but the sizes are defined by height and weight, and brands define them differently. A 12-month size at one brand may have an upper height limit an inch above another's.

The planner's clothing catalog records the upper height and weight limits from each included brand's published size chart for ordinary apparel. When a brand publishes height but not weight, only height is used. The "typical across common U.S. brands" option uses rounded medians of the upper limits across Carter's, Gerber Childrenswear, Old Navy, Cat & Jack, and The Children's Place. The result also shows the range of limits observed among those brands. For the newborn size, the typical upper limits are 21.5 inches and 8 pounds, with brand limits ranging from 20 to 21.5 inches and 7 to 9 pounds.

The catalog follows ISO 8559-1 in treating a size as defined by body measurements rather than by age. Age bands are shown as labels only.

Car-seat limits

Car-seat limits are more consequential and are handled more strictly. The catalog records each seat by family, by revision and by use mode. Rear-facing, forward-facing and booster modes on the same seat have different limits. A seat's limits can also change between manufacturing revisions. Each entry links to the manufacturer's documentation it was taken from.

Limits are of two kinds. Numeric maximums for height and weight can be projected from growth. Fit requirements cannot be projected. The one-inch head clearance rule for many rear-facing modes, the harness position relative to the shoulders, and belt fit in booster mode all depend on the child's proportions and on the seat. The planner shows these rules alongside the date rather than forecasting them. Some manuals combine criteria with "and" or "or" logic, and the catalog records that logic rather than flattening it to independent limits.

A numeric maximum in the catalog is inclusive. The planner treats reaching it as the date to re-check the seat. This follows NHTSA guidance to keep a child in each mode until the manufacturer's limit is reached. A null limit in the catalog means the manufacturer publishes no numeric threshold for that measure in that mode, which is common for standing height in rear-facing modes.

Combining limits with growth

For each applicable limit, the planner projects the child's current z-score forward. It uses the WHO 2006 standard before age 2 and the CDC 2000 reference after. It then solves for the first age at which the projected height or weight reaches the limit. Height and weight are solved separately, and the earlier crossing is the main re-check date. A child who is tall for their weight reaches height limits first. The planner reports which limit governs.

For head clearance, the planner uses a seated-height reference derived from NHANES III sitting-height data for ages 2 through 8. It finds the date on which projected seated growth uses up the measured clearance, less the reserve the seat's manual requires.

Reading the result

The central date is a projection under the assumption that the child stays on the current growth channel. The range shows the date under slower and faster scenarios. For clothing, being a few weeks early or late has no consequence. For car seats, the date is a prompt to check the seat against the manufacturer's instructions. The fit requirements listed with it may be reached before the numeric limit is.

The planner does not replace the manual. Manufacturers update limits between revisions, and the catalog's revision selector exists because the same model name can carry different limits. Where the catalog has not yet recorded a seat's model numbers, the planner asks the user to confirm the seat label and offers manual entry of the limits.