Frequently Asked Questions

Andrea Okerholm Huttlin

February 13, 2018


Why your SizeCast might not be what you expected

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How does SizeCast predict future clothing sizes?

Your child's height, weight, and birthday are used to find the percentile in the WHO or CDC growth chart -- the very same chart that your pediatrician uses. That percentile is used to project future heights and weights, which are compared to the size charts of each selected clothing brand.

Can I forecast my baby's sizes before birth?

I know of no data that compares ultrasound measurements to post-birth measurements. Ultrasound weight estimates are known to be up to 30% off. If your baby has not yet been born, I would use the dropdown menu to select the size you expect your baby to be based on the height and weight of your immediate family members.

How accurate can I expect my SizeCast to be?

There are several things at play here: the underlying WHO/CDC growth model, and how consistently the garments are produced.

SizeCast's forecast is based on the published manufacturer size chart. If a manufacturer doesn't stick to their own size chart, there's not a lot I can do about that. Some brands are better than others about producing consistently-sized garments.

SizeCast assumes that your child will remain at the same height and weight percentile for the forecast period. This isn't really true: kids have growth spurts and they often cross the growth curves. Over time, children as a rule regress to the mean (tall babies grow more slowly and short babies catch up).

The underlying growth data was taken from the NHANES study of American children. It may or may not represent children from other parts of the world well.

The accuracy of your measurement is also very important. Baby sizes are only 2 inches apart, so if your measurement is off by a half inch, that's going to affect the results.

Clothing manufacturers size their clothes based on the average child's proportions. If your child has a longer or shorter torso than average, you'll probably find that you need a different size on top and bottom. At the moment, SizeCast doesn't support this. Extremely tall or short children may also find this to be true, because their proportions are not the same as the ones being used to manufacture the clothing. (Ex: a 12 month old with a 2T waist may find that the pants are too long because toddlers are leggier than infants.)

All that considered, a SizeCast is still much better than guessing based on calendar age! I would recommend not forecasting more than 12 months out and be sure to update your measurements frequently - at least once a quarter for infants.

My extremely tall or short baby's forecast is totally inaccurate!

The WHO/CDC growth models assume a certain distribution of heights and weights across the population of all children. This models most children well, but it does break down at the extremes. Very tall and very short children are just not represented well by this model, and you'll see some strange-looking forecasts.

What about diaper sizes?

Anecdotally, my understanding is that the weights listed on diaper packages don't mean much. My own daughter was always in a diaper size much larger than might be predicted by her weight. So without better data, I can't add diaper sizing.

Can you add my favorite brand?

Yes! Send an email to with a link to the manufacturer's size chart.


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