The child growth planner reports a central date on which a child is projected to reach a height or weight limit, and a range around it. The central date assumes the child continues along the current growth channel. The range describes how the date changes under slower or faster growth scenarios.
Neither is a measurement of how fast the child is growing now.
The planner uses one measurement, taken today. It does not use a series of measurements, and it cannot calculate the child's own velocity.
Growth-channel projection
The central projection holds the child's current z-score constant and reads the reference curve forward. The planner uses the WHO 2006 standard before age 2 and the CDC 2000 reference after. A child at the 60th percentile today is assumed to be at the 60th percentile at every future age. The projected height or weight at each age is the reference value for that percentile. The crossing date is the first age at which the projected value reaches the limit.
This is the standard assumption for growth projection when only a single measurement is available. It is also the assumption most likely to be wrong in infancy. Mei and colleagues (2004) found that crossing two major percentile lines was common before age 2. Between age 2 and puberty, children tend to track their channel and the assumption holds better.
Where the range comes from in infancy
For children under 2 whose crossing falls within a published interval, the planner uses the WHO growth-velocity standards published in 2009. Those standards describe the distribution of length and weight increments among the WHO study children over intervals of one to six months. The planner takes the 5th, 50th, and 95th percentile increments for the applicable interval and calculates the crossing date under each, giving slow, typical and fast scenarios.
The increments describe how much the WHO children grew over an interval, not how much a particular child will grow. A child whose recent growth has been at the 95th percentile of increments is not identified by the planner, because the planner has no earlier measurement to compare.
Where the range comes from later
Beyond the velocity standards' age range, or when the crossing falls outside a published interval, the planner uses a different method. It repeats the projection with the child's z-score moved to a lower and a higher growth channel and reports the resulting dates. These are scenario bounds. They show how sensitive the crossing date is to the assumed channel, and they are not calibrated to contain the true date with any stated probability.
The distinction matters for reading the range. A range of March to May does not mean a 90% or 95% chance that the limit is reached between March and May. It means that under the planner's slower and faster scenarios, the crossing would fall in March or May.
Head clearance
For car-seat head clearance, the planner uses a separate seated-height reference built from 6,012 NHANES III sitting-height measurements for ages 2 through 8. The range around the clearance date is produced by adding and subtracting 0.635 cm, a quarter of an inch, from the measured clearance. That range represents measurement sensitivity, not growth variation, and the planner labels it as such.
What would change the estimate
A second measurement a few months later, entered as a new calculation, gives a new central date from the child's updated position. Comparing the two dates is the closest the planner comes to velocity. If the child has moved up a channel, the new date will be earlier than the old one predicted.
A clinician with a series of measurements can calculate the child's actual velocity and compare it with the WHO velocity standards directly. That comparison, not the planner's projection, is the appropriate tool for a question about whether a child is growing normally. The planner's question is narrower: given today's measurement, when is a specific size limit likely to be reached, and how much does that date move under plausible alternatives.