When should you check the next size?

The calculator determines exact age from the birth and measurement dates. For a child born before 37 weeks, corrected age is used through age 2. Measurements before age 2 are compared with the WHO 2006 length- and weight-for-age standards; measurements from age 2 use the CDC 2000 stature- and weight-for-age references. When standing height is entered before age 2, 0.7 cm is added for comparison with the WHO recumbent-length reference.

For a measurement y and the reference values L, M, and S at that age and sex, the growth-chart z-score is:

Here M is the reference median, S describes its spread, and L accounts for skewness.

The central path continues the child’s current height and weight z-scores forward through the age-specific WHO or CDC curves. At each future age a, the engine reverses the LMS transformation:

The projected crossing time for a limit T is the earliest future time t satisfying:

The engine finds t* numerically by bisection. Height and weight are solved separately. When a projection passes age 2, it converts the WHO measurement at the transition into the corresponding CDC z-score so the trajectory remains continuous.

Head clearance is optional and is measured with the child positioned in the selected seat mode. If C is the measured clearance and R is the reserve required by the seat instructions, usable clearance is:

From age 2, the seated-height reference SH(a) comes from 6,012 NHANES III sitting-height measurements in the calculator’s 24–108-month age range. Separate male and female curves were fitted with survey-weighted cubic median regression:

The coefficients were estimated with the NHANES examination sample weights using iteratively reweighted least squares. The fitted monthly curves were constrained to remain nondecreasing and rounded to 0.001 cm for the calculator. Values between monthly points are linearly interpolated, so the calculator evaluates the exported monthly curve rather than the unrounded polynomial directly.

The clearance crossing is the first future time at which the increase in reference seated height consumes the usable clearance:

The displayed clearance range repeats this calculation with U reduced and increased by 0.635 cm (0.25 inch):

These are measurement-sensitivity bounds, not a calibrated probability interval. For children younger than 2, the calculator instead applies broad 35%, 50%, and 65% seated-growth shares to the projected gain in overall length.

The central result is the earliest applicable height, weight, or clearance crossing:

For infants whose crossing falls within a published WHO growth-increment interval, a scenario crossing based on percentile increment Δ_p is:

Here I is the published interval length, T is the limit, and y₀ is the current measurement. The range uses the WHO 5th-, 50th-, and 95th-percentile increments when all relevant crossings fit within one published interval. Other height and weight ranges hold lower or higher WHO/CDC z-score channels and solve the LMS crossing equation again. If clearance is entered, each overall boundary is the earliest applicable boundary from the numerical-limit and clearance scenarios.

The chart places height on the horizontal axis and weight on the vertical axis. The central line follows the current growth channel, the shaded area shows the height-and-weight planning scenarios, the straight limit lines show the selected numerical boundaries, and the teal marker identifies the head-clearance crossing.

For a selected clothing brand, the calculator uses the upper child-height and child-weight values from that brand’s official ordinary-apparel chart. If the chart publishes height but not weight, only height is used. “Typical across common US brands” uses rounded equal-brand medians from Carter’s, Gerber Childrenswear, Old Navy, Cat & Jack, and The Children’s Place. The displayed common-brand range records the lowest and highest published upper limits in that group.

Car-seat height, weight, and fit limits come from the selected model, revision, and mode in the calculator’s car-seat catalog. Choosing “Enter manually” uses the height and weight limits supplied by the user.