Onesies 101- The Ultimate Guide to All Things Onesie (2024)

As a mom, I am all about onesies! From everyday wear to emergency diaper bag blow-out outfits, they are the epitome of versatility. Read on to learn everything there is to know about baby onesies!

Why do Babies Wear Onesies?

Onesies have many uses. First and foremost, they make an excellent base layer for keeping baby warm and comfortable. Onesies keep body heat in by covering the stomach and back at all times and don’t ride up like a t-shirt, thanks to those handy snaps. Add a shirt or a sweater over top or just put a onesie under every outfit and you’re in business. In hot weather, onesies make a cool and convenient shirt and they’re also great for keeping scratchy fabrics and other uncomfortable materials from bothering baby’s sensitive skin.

Where onesies are the real MVP, however, is keeping little hands out of diapers, and keeping diapers on little bums. Babies are naturally curious, so it makes sense that if there is access to the inside of their diaper, they might explore. This can lead get too nasty messes on their hands, maybe even sharing that mess with walls or crib slats. A onesie blocks access to the top of the diaper thereby cutting down on your chances of having to clean up something truly unfortunate.

Keeping diapers on babies is equally important. Because onesies typically have three strong snaps in a hard-to-reach (for the wearer) place, they act as a guard against diaper removal. Whether you’re dealing with a curious baby, a birthday suit inclined toddler, or simply the will of gravity, a good snug onesie really helps keep the diaper on, which keeps messes where they belong until changing time.

If you’re interested, check out our blog post on the history of onesies over here.

How Long Do Babies Wear Onesies?

Every parent has their own preference for when their baby should stop wearing onesies. Some use them until their child starts walking (which can, of course, range from 8 - 18 mos), while others use them only for added warmth at night. I always keep my babies in onesies as long as they are wearing diapers. A common time to stop using onesies is when your child potty trains. This can be anytime between 18 months and 3 years old and involves a series of criteria. Some children may potty train earlier than 18 months, and some later than 3 years old, so look for readiness rather than relying on dates. Signs of readiness include staying dry, an awareness of peeing or pooping in their diaper, and a desire for independence. Onesies can be replaced with short-sleeved shirts as your child learns to use the bathroom. If your child isn’t ready to potty train by the time she outgrows 24M onesies, which are the largest most manufacturers make, consider buying or making some onesie extenders.

How Many Onesies Do I Need?

How many onesies you need depends on how often you’re willing and able to do laundry. Most newborns go through 8-12 diapers a day, and there’s no telling how many of those will be blowouts requiring a change of clothing. It’s not unrealistic to assume you may go through 4 onesies in one day with a newborn, although that won’t be true every day. At 4 onesies a day, you’d need 28 to get through one week. If you have a baby shower, you’ll probably get the majority of what you need as gifts. As your baby grows, you’ll start to get an idea of how many onesies you go through in a week, and that can help you decide how many of the larger-sized onesies to buy.

Personally, I usually have at least 20 onesies in each size, even if they go through fewer of them in the larger sizes. Newborns will have more blowouts and spit-up, but older kids will get into mud and markers and spill their food. I also like to have extras on hand to pack into diaper bags, overnight bags, and for use as an emergency change of clothes to keep in the car. I’d err on the side of too many onesies rather than too few because there’s nothing worse than reaching into the drawer for a clean onesie and finding nothing there. (Having more onesies also allows me to do less laundry, which is always a win in my book.)

What Size Onesies Should I Buy?

Knowing that newborns need a lot of onesies doesn’t help you to know which size they will wear. You’d like to think they will wear size “newborn,” but that isn’t always the case. Some babies are born large, and will never wear newborn onesies. Others are born premature and need to wear preemie-sized onesies. Another factor is that different brands have slightly different size charts.

Here are the size charts of two national brands, Carter’s and Gerber, which are the brands of onesies that we use for our custom onesies here at Kate & Meri

Carter’s Brand Onesies:

Gerber Brand Onesies:

There are several things to note here. First, notice that Gerber onesies have sizes like 0-3 months, which, as you might expect, are designed to fit the average baby for the first three months of life. Carter’s onesies, however, are simply sized 3M. Size 3M onesies are expected to fit a baby up until it is 3 months old. The same applies to the larger sizes of both brands. A Gerber onesie in size 18 months is designed to fit a baby up until they turn 18 months old, just as Carter’s onesies in size 24M are designed to fit until baby turns 24 months old.

The second difference between these brands is that Carter’s onesies run slightly larger. Their newborn size fits a baby one pound heavier and half an inch longer, while the Gerber newborn size will accommodate a baby one pound smaller and one inch shorter. It may not sound like much, but if your baby is particularly large or small you may want to stick to the size that more closely lines up with their measurements.

Remember that these sizes are generalizations based on averages and that your baby may be wearing sizes smaller or larger depending on his birth weight and growth rate. Babies born smaller than 5 or 6 pounds will need to wear preemie onesies, and will probably be in smaller sizes that don’t line up with his actual age from then on. Babies born over 8 or 9 pounds will skip size newborn entirely and may or may not line up with the approximate ages after that. Go by baby’s size, not his age. And of course, the ultimate test is putting the garment on and seeing how it fits.

Where Can I Buy Onesies?

Onesies are available just about anywhere! We’re of course biased and think that a personalizedonesie from Kate & Meri is exactly what you need but there are lots of options for buying onesies.

If you're building up a stockpile, consider a multipack of generic, solid-colored onesies, which come in packs of up to 15. You can also find onesies at Target and Walmart, as well as specialized baby clothing stores such as Carters and Baby Gap. Onesies are available in pretty much every department and/or clothing store, so you can still find them even if none of the well-known retailers are close to your home.

Where Are Onesies in the Store?

If you need some onesies today or you’re unable to order online, head to a store near you and ask for help finding the baby section. Onesies are usually organized by size and displayed with other items of the same size to make purchasing outfits easier. You’ll see plain or lightly-patterned onesies packaged together, similar to packages of underwear for older kids. Some onesies will be on small hangers. These will usually be more expensive and have messages or characters on them. You may also find onesies packaged as part of a layette set. A layette set is a group of coordinating clothes for babies, such as this one, which includes 3 onesies, 2 pairs of pants, a dress, a sweater, a headband, and 3 bibs.

How Much Do Onesies Cost?

The cheapest onesies will be a multipack of plain white, short-sleeved onesies, which can cost as little as $2 per onesie. The next cheapest will be a multipack of solid colored, short-sleeved onesies in colors such as grey, blue or pink, which can be as little as $2.40 per onesie. It is possible to find cheaper onesies at dollar stores, but it is my experience that there is such a thing as too cheap. Anything cheaper than a $2 onesie is likely to have thin fabric and may wear out quickly, which is not great, especially if you plan to use the onesies for more than one baby.

If you’re interested in getting some cute or clever onesies, you’ll have to pay a little more. You can find cute but generic patterns for as little as $2.50-$3.75 each at stores like Walmart or Target and they range from $14-$17 apiece from places like Carter’s and Baby Gap. Our personalized onesies start at 12.99. And if you want character or licensed material, you could shell out at much as $15 for a single onesie, such as this pack of 3 MLB-themed onesies for $45 from Carter’s. (That’s pretty steep. Even Disney onesies are only $5 each at Target.)

*All prices are for short-sleeved onesies and prives are subject to change

If you’re interested in buying a large number of generic onesies, here’s a list of comparable packs of different brands and where to buy them. As you can see, the cheapest come from Walmart, then Amazon, Target, and then Carter’s and Gerber. I personally avoid buying baby items at dollar stores, as I’ve found you get what you pay for.

*All prices are for short-sleeved onesies

How Do I Fold Onesies?

You can check out our entire post here:how to fold baby onesies.

Onesies: the Universal Baby Outfit

Whatever the occasion, a onesie is a great choice for dressing your infant or toddler. A plain onesie makes a perfect layering piece for any outfit. Patterned onesies pair well with pants, shorts, or a skirt to make an outfit on its own, and any sort of onesie goes well under pajamas to keep your little one snug all night. Whatever the weather, consider using a onesie to complete your baby’s ensemble.