Why does the right diaper size matter?

A diaper that’s too big or too small could actually be the cause of leaks and blowouts, especially overnight.

If your baby is in a diaper that’s too big: gaps around their legs and waist can create areas that allow both liquid and solid waste out.

If your baby is in a diaper that’s too small: it won’t have room for proper containment, forcing liquid out the legs cuffs or up the back

This leads us to the next reason you want to ensure your baby is in the right size—their output increases through the first months and years of life. Bladder capacity grows along with your baby, nearly doubling within the first 2 years:

Table 1

Bladder capacity in children under 24 monthsOpens in a new tab.

Different diaper sizes will have variances in liquid capacity to account for these changes. Smaller diaper sizes are purposefully designed with smaller liquid capacities, due to the smaller bladder capacities of the babies in those sizes.

This also means less absorbent material is required for the core of the smaller sizes, allowing the diapers to be constructed with a smaller surface area—fitting newborns and younger infants without getting too bulky.

For instance, Coterie’s Size 4 diapers are tested and proven to hold over 16 fl oz, while smaller sizes like Newborn and Size 1 won’t have quite as large of a liquid capacity. So making sure your baby is in the right size diaper ensures that its liquid capacity is equipped to handle their output.

How are diaper size charts determined?

We now know diapers work best when they fit properly, but how can you be sure you have the right size?

There isn’t one standardized system for diaper sizing or fit, so don’t assume the same size will work across every brand. You’ll want to reference the size chart for the specific diaper brand you’re buying.

According to Coterie’s director of product development, “to develop diaper size charts we look at and combine the following:

1. CDC Growth ChartsOpens in a new tab. and US Anthropometric Reference DataOpens in a new tab. by age and sex to determine how select body measurements correspond to weight
2. Product measurements to ensure minimum and maximum waist/hip/leg dimensions can cover body sizes of children in corresponding weight ranges for each size—even for those on the low and high ends of the ranges
3. Consumer feedback on how the product fit their babies and children overall—around the waist and hips, and around the legs—from user studies conducted during product development
4. Baby’s clothing size charts are also considered, as well as underwear sizes for training pants”

The most accurate diaper size charts go by weight. Age isn’t a reliable metric for diaper sizes because there’s such a wide variation of weights, lengths, and body shapes between babies at every age—for example, a 99th percentile 6 month old boy weighs 23 lbs while a 3rd percentile 6 month old girl weighs 13 lbs.Opens in a new tab.

Coterie Diaper Size Chart

Coterie Training Pants Size Chart

Even more so for the bigger sizes, as toddlers and big kids can have similar waist and hip circumferences (and other relevant measurements) at a larger range of weights due to maturing body composition and increasing height.

How should a diaper fit?

• Waist snug right below the belly button (The Pant has a higher-rise waist that should sit right above the belly button)
• No gaps around the legs or waist
• Room to comfortably run a finger around the legs and waist
• Don’t forget to pull out the leg ruffles to create a seal as an extra barrier to stop leaks and blowouts

How should a newborn diaper fit?

Parents should take special care to fit their newborn’s diaper around the umbilical cord stump.

Always adhere to the specific instructions provided by your healthcare provider for caring for the umbilical cord stump. But as it heals, you generally want to keep it clean, dry, and exposed to fresh air, avoiding tight clothing around your baby’s stomach.Opens in a new tab.

Do not pull your newborn’s diaper up over the umbilical cord stump. Either fold their diaper down under it or use a diaper with an umbilical cord cutout.

Coterie diapers in Newborn and Size 1 are designed with overlapping tabs and a contoured umbilical cord cutout so the diaper comfortably and snugly fits around the umbilical cord stump without having to fold the diaper down.

Signs your diaper is too small (size up)

• Waist sits more than 2 fingers below the belly button
• Tabs barely close at the edges of the landing zone
• Leaves lasting red marks or indents, especially on the waist and thighs
• You experience a sudden increase in leaks and blowouts

Signs your diaper is too big (size down)

• Sags around the legs, leading to leaks
• Gaps at the waist and/or bunched up leg holes
• Tabs close together or overlap (with a couple exceptions especially for Coterie diapers in the smallest sizes)

Should the diaper tabs overlap?

Generally, diaper tabs should meet near the center of the diaper landing zone without overlapping. In most cases, that’s a sign your diaper is too big and overlapping the tabs will create gaps that bunch up around the waist.

However, Coterie diapers in Newborn and Size 1 come with Overlapping Tabs as a special feature (you can see how they work hereOpens in a new tab.). This is because we heard from a lot of parents that they needed a more customizable, flexible fit for smaller babies.

According to the World Health Organization (WHO), The average full-term baby is around 7.5 lbs at birth, with normal weights ranging from 5.5 to 8.8 lbs—that range can be quite significant when it comes to how a small diaper fits. The Overlapping Tabs on Coterie’s Newborn and Size 1 diapers help parents secure the diaper snugly around the waist in those first days and weeks, especially on babies that fall on the smaller end of the spectrum.

How many diapers do I need in each size?

Results from Coterie’s 2025 Diapering Study showed that 88% of parents reported that they’ve already purchased diapers ahead of their baby’s arrival, making it the most common baby preparation activity, even above choosing a name.

However, since every baby grows at their own pace—and you’ll likely outgrow some sizes much faster than others—it’s best to stock up gradually instead of buying a lot of diapers in every size far in advance.

Babies grow most rapidly from 0-4 monthsOpens in a new tab.—gaining approximately 1.5 to 2 lbs and 1 inch per month. This means they’ll go through the smaller sizes of diapers more quickly. Babies will generally stay in larger sizes longer because their growth rate slows down after the first 6 months, and even more so after the first year.

So, if you’re buying diapers ahead of baby’s arrival, you’ll only want to get about a 1-month supply—approximately 200 diapers—in sizes Newborn and 1. If friends and family are giving diapers as a new baby gift or for your baby shower, you may want to request they get more diapers in sizes 3, 4, and 5, as your baby will likely be in those diaper sizes for the longest amount of time.

Since babies grow out of the Newborn size the quickest, Coterie offers the option of a box that’s split between Newborn and Size 1 diapers—you get 3 packs of Newborn (93 diapers) and 3 packs of Size 1 (99 diapers) to get you through that first month.

Coterie’s Size Assist, is another helpful feature that guides Coterie subscribers through diaper size changes, helping keep babies in the right size as they grow, and preventing parents from ending up with shipments of diapers that are too small. Here’s how it works:

• You’ll get a free trial pack of the next size up with your first shipment of diapers
• You get personalized nudges that recommend the next size or product based on Coterie’s extensive sizing data and how long you’ve been in each size
• You can add on single packs of diapers or training pants. This can be especially helpful if you’re right on the edge of sizing up and only need a pack or two more in your current size, but not a whole additional shipment.

How many diapers do I need per day?

As with most things, how many diapers a baby will go through in a day can depend on their individual eating, drinking, and defecating habits. However, you will certainly need to change diapers more often in the first few months.

This mostly comes down to the fact that newborns poop and pee more frequently, with that frequency decreasing as they grow.

At one month of age, babies urinate an average of 20.1 times per day, whereas by 3 years of age that number halves to an average of 10.8 times per day.Opens in a new tab.

Defecation frequency was highest at 3 weeks of age with a median of 4 times per day, 3 times per day at 10 weeks, and 2 times per day by 17 weeks.Opens in a new tab. During the first 5 months, stool frequency was higher in infants who were exclusively breastfedOpens in a new tab., so parents should be aware that exclusively breastfed babies may need more diaper changes per day.

Modern disposable diapers are highly absorbent and don’t necessarily need to be changed after every wetting, but it is especially important to change dirty diapers as soon as possible to help prevent diaper rash. Diaper rash, formally known as diaper dermatitis, occurs in approximately 50% of infantsOpens in a new tab.—this aligns with data collected from Coterie’s 2025 Diapering Study, in which 52% of parents said they have experienced a skin issue like diaper rash with their child. Prolonged exposure to urine and feces—in combination with diapers trapping heat and moisture against the skin—can alter skin’s pH, contribute to a breakdown of the skin barrier, and result in irritation.

With this in mind, studies on preventing diaper rash have found that newborns should ideally be changed every 2 hours during the day, while older infants may only need a diaper change every 3 to 4 hours.Opens in a new tab.

Coterie diapers are delivered on a monthly basis, in boxes optimized according to the average number of changes babies need as they grow:

Do I need to size up for overnight diapers?

In baby’s first few weeks, you’ll likely be doing multiple overnight diaper changes so having a diaper that’s specifically designed to work through long stretches of sleep isn’t as much of a necessity. This is because newborns usually need to feed every 2 to 3 hrs, or more, especially before they regain their birthweight. Since they can be sleepy feeders, it’s often helpful to do a diaper change before a middle of the night feed to wake them up enough for a full feed.

Your newborn is also learning how to poop as their digestive system “comes online” after birth, and they’ll often do it throughout the night. Since dirty diapers should be changed immediately, this requires overnight diaper changes.

However, as your baby grows, you’ll likely be able to phase out those middle of the night changes—but when? It’s an incredibly common parent question on forums like Reddit, “when will my baby stop pooping overnight?” And the answers are commonly between 6-10 weeks, give or take a few.

Once you reach that stage when your baby isn’t having dirty diapers at night, you actually don’t need to change their diaper overnight. And as long as they don’t have a serious diaper rash or other skin issue you’re managing, you especially don’t need to wake them to change their diaper.

This is when overnight diapers—with enough absorbency to last for up to 12 hrs—become useful. Most major brands of overnight diapers start at Size 3, when babies are of a size and age where their digestive systems are a bit more mature and they’re more likely to sleep longer stretches overnight. However, brands like Coterie don’t make separate overnight diapers; they just have one highly absorbent diaper that works for both day and night.

Coterie diapers are designed to work all night without additional hacks like sizing up, adding extra inserts, or doubling up on diapers. However, if you’re between sizes, you will likely want to go for the larger size overnight.