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Baby clothing sizes, explained: What you need to know to shop for infants at every size

Newborn or 0-3 months? Experts help break down the confusing world of baby sizes and how they relate to clothes.

Baby clothing sizes, explained: What you need to know to shop for infants at every size

It shouldn’t be complicated, but for some reason, baby sizes on clothing are unsettlingly confusing — particularly if you have no frame of reference. (“Can newborns wear 0-3 size clothes?” “And why isn’t that the same thing as newborn-size?!”) And making things more baffling is the fact that, much like the clothes themselves, there’s no one-size-fits-all rule when it comes to figuring out which clothes from what brands will fit your baby when.

“It’s impossible to give hard and fast rules when it comes to baby clothes sizes, especially since brands all size and label things a little differently,” says Jenn Bonnett, a fashion and wardrobe stylist who primarily works with kids. “Additionally, all kids are different sizes, regardless of their age, so it’s important to keep in mind that the sizes we see are based on the average. If your baby is in the 99th percentile, you’re going to need to size up.”

“It’s impossible to give hard and fast rules when it comes to baby clothes sizes, especially since brands all size and label things a little differently. “


But even without there being a universal guide for baby clothes sizes, there are general rules that will make onesie-buying far more fun than stressful. Here, experts break down the confusing world of baby clothing sizes. 

Is newborn the same size as 0-3 months?

“Newborn-size represents an average baby at birth up to 21.5 inches long and 5-8 pounds, which can be a very wide range,” explains Dr. Amna Husain, a pediatrician at Pure Direct Pediatrics in Marlboro Township, New Jersey. 

In terms of how weight relates to 0-3 month clothing, the range is typically 8-13 pounds, according to Husain, but there are other considerations to take into account — namely, length. 

“Infants come in all shapes and sizes, so the range is helpful if you have an infant on the smaller/petite end versus the larger/longer end,” says Husain. “I’ve found that sorting clothes by length versus what the tag says can be helpful since some brands run small and others large.”

How long do babies fit in newborn clothes?

Whether you’re trying to choose a coming-home outfit for your own little one or pick out something sweet for a loved one’s new baby, newborn-size (NB) clothes, or something a bit bigger with some room to grow, such as 0-3 months, is most likely the way to go. Which one will fit right off the bat? Ultimately, it comes down to baby’s size, which is nearly impossible to predict before birth. 

“I recommend having a variety of sizes on hand for your newborn including newborn and 0-3 months,” says Husain.

While babies are known for their ability to grow —and outgrow clothes — at warp speed, Husain notes that not everyone will be packing the newborn-size clothes up after a week or two. “You may have your little one wearing newborn clothing for 4-6 weeks if they’re closer to the 4-5 pound range,” Husain points out. “Every baby is different, but it’s smart for parents to remove the tags and wash a few — not all — newborn clothes before birth. This way, they can be returned if baby quickly jumps to the next size up.”

“I recommend having a variety of sizes on hand for your newborn including newborn and 0-3 months.”


Use the same line of thinking if you’re buying clothes for a newborn or baby shower gift. NB-size clothes may not fit for long — and some bigger babies may never get a chance to wear them — so it’s a safer bet to buy a size or two up for newborns.  

Baby clothing sizes explained

Just as caring for babies gets easier with time, so does cracking the DaVinci-like code of buying clothes for them. Not only will it become more apparent which end of the height and weight spectrum your child is on, but you’ll also become familiar with how favorite brands run. 

“While a big part of choosing the right size baby clothes depends on a particular brand, my personal rule is considering that the tag means ‘up to,’” says Husain. “So if a label says ‘3 months,’ I think of it being about it being able to fit somewhere between 0-3 months, or if it says ‘6 months,’ it can fit somewhere between 3-6 months.” Husain notes that while this rule won’t necessarily work for every item of clothing and every brand, it will make shopping simpler until parents get an idea of baby’s shape, as well as brand preferences.

“My personal rule is considering that the tag means ‘up to.’ So if a label says ‘3 months,’ I think of it being about it being able to fit somewhere between 0-3 months, or if it says ‘6 months,’ it can fit somewhere between 3-6 months.”


Here’s a concrete example: Baby Gap’s 3-6-month size range fits babies 12-17 pounds and 23-27 inches, while Carter’s 3-month clothing fits babies 9-12.5 pounds and 21.5-24 inches and their 6-month clothing fits babies 12.5-17 pounds and 24-27 inches. The difference between 9 and 12 pounds is a lot in babies.

In addition to having a few NB pieces set aside before baby’s birth, as a general rule of thumb, it’s always smart to size up when you don’t know baby’s exact size. For instance, when buying clothes for a 3-month-old, opt for the 3-6 month (or larger) size range instead of the 0-3 month range. 

A few other good reasons for the size-up rule? Clothes may shrink in the laundry over time, but babies will continue to get bigger.

“My son was outgrowing his clothes, so, in a rush one day, I grabbed a couple of Cat & Jack rompers from Target in what I thought was his size,” notes Mia Perez, a mom of three from Howell, New Jersey. “They wound up being super big on him. After a few weeks — and many washes — they wound up fitting, though. And actually, they shrunk so much that my next baby was able to wear them on the earlier side.”

Cheryl Delfino, a mom of two in Los Angeles, had a similar experience with her first baby. “For the first few months of my daughter’s life, she lived in plain Gerber onesies, so when I got a gift card for Baby Gap, I was excited to use it,” Delfino explains. “I bought a bunch of rompers and onesies online in 3-6 months and they were huge! Eventually, she grew into them, but my daughter was swimming in them when she was between 3-4 months old.” 

“Not only will children’s clothes shrink the more you wash and dry them,” says Bonnett, adding that it’s all the more reason to buy clothing with room to grow, “but kids just look cute in oversized sweatshirts or sweaters with the sleeves rolled up — and they get extra length and more wear out of them that way.”

The moral of the story? If you’re still figuring out what sizes fit your baby best, or if you’re trying to buy a gift for someone else, it’s a good idea to refer to a brand’s size chart since there’s always variation. And when in doubt, size up, knowing baby can grow into a larger size. 

To help you out, we compiled some of the top baby brands and sizes. 

Baby clothes size chart by brand*


Label/SizeWeight (lbs)Height/Length (inches)
P (Preemie)Up to 6 lbsUp to 18″
NB (Newborn)6-9 lbs18-21.5″
3 months9-12.5 lbs21.5-24″
6 months12.5-17 lbs24-27″
9 months17-21 lbs27-28.5″
12 months21-25 lbs28.5-30″
18 months25-28 lbs30-32″
24 months28-30 lbs32-34″

Amazon Essentials

Label/SizeWeight (lbs)Height/Length (inches)
P (Preemie)Up to 6 lbsUp to 17″
NB (Newborn)6-9 lbsUp to 21.5″
3 months9-12.5 lbs21.5-24″
6 months12.5-17 lbs24-26.5″
9 months17-21 lbs26.5-28.5″
12 months21-25 lbs28.5-30.5″
18 months25-28 lbs30.5-32.5″
24 months28-30 lbs32.5-34″


Label/SizeWeight (lbs)Height/Length (inches)
PreemieUp to 5 lbsUp to 17″
Newborn (NB)5-8 Ibs17-21″
0-3 months8-12 Ibs21-24″
0-6 monthsUp to 16 Ibs17-26″
3-6 months12-16 Ibs24-26″
6-9 months16-20 Ibs26-28″
12 months20-24 Ibs28-30″
18 months24-28 Ibs30-32″
24 months/2T28-32 Ibs32-34″

Old Navy & Baby Gap

Label/SizeWeight (lbs)Height/Length (inches)
Up to 7 lbsUp to 7 lbsUp to 18″
0-3 months7-12 lbsOld Navy 19-23″
Baby Gap 18-23″
3-6 months12-17 lbs23-27″
6-9 monthsOld Navy 17-19 lbs
Baby Gap 17-20 lbs
6-12 monthsOld Navy 17-22 lbs
Baby Gap 20-22 lbs
Old Navy 27-29″
Baby Gap 28-29″
12-18 months22-27 lbs29-31″
18-24 months27-30 lbs31-33″

Target brands (including Cat & Jack, Cloud Island)

Label/SizeWeight (lbs)Height/Length (inches)
Preemieup to 5.5 lbsup to 17.5″
Newborn6-8 lbs18-20″
0-3 months9-12 lbs20.5-23.5″
3-6 months12.5-16 lbs24-25.5″
6-9 months16.5-19.5 lbs26-27.5″
9-12 months20-23 lbs28-30.5″
18 months23.5-26 lbs31-32.5″
24 months26.5-28 lbs33-35.5″
* According to size charts on the brand sites at the time of this posting.

What’s the difference between 24 months and 2T?

If you’re wondering about one of the biggest baby clothing mysteries — what’s the difference is between 24 months and 2T — the answer is fairly concrete: It’s mainly about the fashion and whether or not a child is potty-trained

“The biggest difference between size 24 months and 2T is the style,” says Jessica Herman, director of customer experience at the children’s clothing subscription service, KIDBOX. “Size 2T typically is more mature toddler styles, where size 24 months has more babyish clothes, such as onesies, particularly geared toward kids who are still in diapers.” 

In other words, it’s much easier for a child or parent to pull up and down a pair of pants when it’s potty time, as opposed to unsnapping a onesie. Also, some brands, such as Baby Gap, even note that, in addition to accommodating extra room in their clothing for a diaper, their sizes are representative of clothing with a diaper on.

While it certainly would be convenient if baby sizes were as simple as following a label, alas, few things parenting-related are so simple. That said, once you figure what works best for your little one, it’s easy. Until then, though, better to be safe than sorry. ”Whenever you’re unsure about sizing, always size up,” says Herman. “Kids are constantly growing, and it’s better to be too big than too small.”