How Long Are Babies in Size 1 Diapers?

Many parents who love to plan ahead want to know just how long their baby will wear size 1 diapers. 

Though not an exact science, research has developed a fairly accurate growth trajectory that helps parents predict how long their little ones will wear each size of diaper. 

Diaper Size by Weight

For starters, diaper sizes are determined by weight.  So, since no two babies grow the same, the length of time in size 1 diapers can vary significantly from family to family. 

A myriad of factors can affect a newborn’s growth rate.  Some of these factors include feeding practices (formula vs. breastfeeding), family health history, whether or not there is an illness at birth, length of gestation, and the list goes on. 

All of that to say, there is not a guaranteed system for determining the exact growth pattern of your baby. 

However, with the help of science, we can make some pretty accurate guesses just how quickly your little one will grow and hence, how long you can plan to have them in size 1 diapers.

Recommended Diaper Sizes

After researching all of the major brands of diapers on the market, I compiled a short list of the average recommended weight ranges for each of the first few diaper sizes. 

How Long Will Baby Be in Size 1 Diapers?

So, how long can one expect their little one to be in the size 1 category? The average baby will be ready for size 1 diapers as early as birth, or during the first month or two, and size 1 will fit them comfortably for approximately 3-4 months. 

According to studies, the average birth weight of a newborn ranges between 5.8 and 10 pounds.  That means a newborn can fit into size 1 diapers shortly after birth. Anything less than 5.8 pounds would generally fit into the preemie diaper category. 

Taking into consideration all the various factors that affect growth, researchers and medical professionals have concluded that the average newborn will grow 1.5-2 lbs per month for the first 6 months of his or her life. Hence, baby should outgrow size 1 diapers by 4 to 5 months old.

Let’s look at an example.  If a baby weighs in at 7 pounds at birth, it will reach the recommended weight range for size 1 diapers within a few weeks.  If growth continues as usual, it will take the infant approximately 3-4 months to outgrow the recommended weight limit for size 1 diapers. 

As you can see in the chart, there is an overlap in the weight amounts assigned to each diaper size.  So this time frame can be shortened or extended depending on the parents preference. 

Planning Ahead for Diapers

If you are looking to buy diapers ahead of time, this is a great diaper cost calculator tool. This calculator will help you determine how many diapers to buy based on the average amount of diaper changes a day for each diaper size. 

Using the research listed above, this calculator uses the gender, weight, and your choice of budget category to determine how many diapers and wipes you will use at each stage of development.  Once you select your options, this calculator also breaks down the payments by quarters so you can budget accordingly. 

This is a great tool in helping you plan ahead! It also helps if you buy the most affordable diaper brands. 

Planning for Newborn Diapers

Let’s look at some examples. Using the diaper cost calculator, let’s go back to our original example of a baby boy born weighing in at 7 lbs at birth. 

This brings us to the following table that tells approximately how many diapers we will need in each diaper size category.

So let’s break it down even further. 

Let’s say you want a rough estimate of how many newborn diapers to have on hand.  Research shows that the average newborn will need to be changed approximately 8-12 times a day.  If your child is born into and continues in the trajectory of normal growth, your baby will need approximately 3-4 weeks worth of newborn diapers. 

According to the calculator, this will range between 210-300 newborn diapers.  These can be purchased in smaller packs/bags (around 40-50 diapers) or can be purchased in bulk boxes. 

Depending on which of the top diaper brands best suits your family, a box of diapers can range from 120-134 diapers per box.  Some moms enjoy using the newborn phase to purchase different brands of diapers in smaller packs until they find the best fit for their little one.  Once they’ve determined the best fitting diaper brand, then they can transition to ordering size 1 diapers in bulk boxes.

Planning for Size 1 Diapers

Going back to our example, after the first 3-4 weeks, this little 7 pound nugget has probably grown to nearly 9 pounds. The number of diaper changes per day has likely decreased slightly. 

Using the same diaper cost calculator, we can determine that our baby will use approximately 230-270 size 1 diapers per month.  If your infant continues to grow at the same rate, you will likely need somewhere between 460-550 size 1 diapers before your little one is ready to move on. 

Depending once again on which brand your family has chosen, you can get between 96 and 196 diapers in each size 1 box.  Size 1 diapers can also be purchased in smaller bags that usually contain about 30-35 diapers in each bag (lasting the average family about 1 week). 

So, unless you have a giant closet devoted to storing diapers, most moms with experience recommend only buying 1-2 boxes of size 1 diapers at a time.  

FAQ’s About Diaper Sizing

How will I know when my baby needs to move up to the next diaper size?

There are a few common issues that are tell-tale signs its time to change diaper sizes. 

• Frequent diaper blowouts is the most common sign that it is time to move up diaper sizes.  If you are suddenly experiencing a lot of diaper leaks but your little one is still having regular bowl movements, this could mean its time to try the next diaper size up. 
• If you are having to tug harder than usual to secure the diaper tags, or if you notice consistent red marks on your baby’s legs or waistline, it probably means your little one is ready to try the next size up.  Most diaper companies say that a sign of a good fitting diaper is if you can easily slide two fingers under the waistband of the diaper.  If you the diaper tabs keep popping open or you have to pull the tabs too tight to secure them, it is probably time to move up to the next diaper size. 
• Check your diaper brand’s diaper sizing chart, usually available on the side of the diaper box or on their website. if you notice that your baby’s weight is on the higher end of their recommended sizing guidelines, it’s probably a good idea to start buying the next size up. 
• Frequent diaper rashes is another sign that your infant’s diaper might be too snug on his/her bum.  A good fitting diaper allows for a certain amount of airflow that avoids rashes between diaper changes.  If the diaper is too small, you will probably not only experience leaks, but also redness and soreness.  So, if this is the case and you’ve ruled out food-related issues, you may want to try either another brand or the next size up.     

Do I need to buy Newborn diapers or just start with size 1 diapers?

Many moms with experience agree that you should not focus on the Newborn diaper size. 

The recommended weight limit for newborn diapers is between 8-10 lbs, depending on the brand.  Some babies weigh this much right out of the gate.  So, many moms recommend having a couple of weeks worth of newborn diapers on hand but also having a large quantity of size 1 diapers in stock. 

The average growth chart predicts that babies will be ready for size 1 diapers within 3-4 weeks.  So, while it’s really not possible to predict how quickly your little one will start putting on the pounds, it’s much better to be prepared in advance than surprised and unprepared. 

How many size 1 diapers should I order?

This depends a lot on how often the baby gets their diaper changed.

As in infant grows, the number of diaper changes per day usually decreases.  In the first few weeks, it’s not uncommon to need to change your baby’s diaper every couple of hours.  As they approach the weight limit for size 1 diapers (8-14 lbs.), most diaper companies and researchers agree that your baby will probably need 6-9 diaper changes per day.  This comes to approximately 225 size 1 diapers per month. 

Many moms of experience recommend buying only 1-2 large boxes of diapers at a time for a couple of reasons. 

First, you need ample storage space to house that many diapers. 

Second, if your child hits a sudden growth spurt, you probably don’t want to be stranded with a bunch of diapers your little one can no longer use.  However, in that event, you may want to consider finding a local center or shelter that would be glad to put those diapers to good use.  

Will formula affect my baby’s growth rate out of size 1 diapers?

The short answer is yes, formula has been known to help babies “chunk up” faster than breastfed babies.  This does not mean formula-fed babies are healthier than breastfed babies.  However, studies have shown that breastfed babies tend to gain weight at a slower pace than formula-fed babies. 

Pediatricians and researchers did note that these effects do not typically manifest themselves until after the baby is 4 months old. 

So, for the question at hand, formula should not affect how long your baby is in size 1 diapers.  As noted earlier, the average baby will reach the recommended size 2 diaper weight around 3-4 months.