How Do Baby Sizes Work?
Why Baby Clothes Never Seem to Fit … and What to Do About It
As a parent, there’s one universal truth we can all agree on: kids grow fast. One day those tiny onesies fit just right, and the next? Suddenly nothing in the drawer works and your laundry pile is full of clothes they wore once.
But here’s the thing: it’s not just growth spurts causing chaos in your child’s closet. It’s also the wildly inconsistent sizing across baby and toddler brands.
Let’s break down why baby clothes rarely fit the way you expect, and what you can do to make shopping (and dressing your kiddo) easier.
There’s No Standard in Baby Clothing Sizes
Unlike adult clothing, there’s no official sizing standard across brands when it comes to babies and toddlers. That means a 12–18 month onesie from one store might fit like a 9-month size somewhere else.
And if your little one is tall, short, chunky, slim, or has that amazing toddler-belly (we see you, 9-month-old squish!), it gets even trickier.
Buying by Age Doesn’t Work for Every Kid
Just because your child is 18 months old doesn’t mean they’ll fit into “18M” clothing.
Some kids wear 2T by their first birthday. Others are still rocking 12-month pants at age 2. Age-based sizing doesn’t account for things like:
Height vs. weight
Torso vs. leg length
Diaper or pull-up bulk
So when you’re trying to plan ahead for seasons, growth, or school/daycare outfits… guesswork just isn’t going to cut it.
That’s Why We Created the Perfect Fit Calculator
At Next Size Up, we take sizing seriously — and personally.
We built our Perfect Fit Calculator to help you get a better match for your kiddo’s real size, not just their age. By entering their DOB, height, and weight, we recommend a bundle size that’s more likely to actually fit them right now — and give you a little wiggle room for growing, too.
Because nothing’s worse than finally ordering that new wardrobe… only to realize half of it is already too snug.
Secondhand Should Still Feel Thoughtful
Our bundles are made from high-quality, gently used baby and toddler clothes that are sorted, cleaned, and curated like they’re going to our kids. No random grab-bags here — just a better way to shop secondhand, sustainably and affordably.
When you use our size calculator, you help us make that bundle even better for your little one — and avoid another “ugh, this doesn’t fit” moment.
Ready to Find Their Real Size?
Try our Perfect Fit Calculator: it takes 30 seconds, and could save you hours of frustration (and returns).
Because when your child’s clothes actually fit, your whole day runs a little smoother. 🌱