Diaper Size Chart
Diaper sizing runs on weight, not age, and the ranges overlap between sizes on purpose because babies of the same weight have different builds. This chart lists the weight range for each size along with the signs that tell you to move up, plus realistic buying quantities so you do not end up with an unopened case your baby outgrew.
Sizes by weight
Preemie: under 6 lbs / 2.7kg.
Newborn (N): up to 10 lbs / 4.5kg. Many babies use this for only 2-4 weeks.
Size 1: 8-14 lbs / 3.6-6.4kg.
Size 2: 12-18 lbs / 5.4-8.2kg.
Size 3: 16-28 lbs / 7-12.7kg. The longest-worn size for most babies, often 6-12 months of use.
Size 4: 22-37 lbs / 10-16.8kg.
Size 5: 27 lbs / 12.2kg and up.
Size 6: 35 lbs / 16kg and up.
Note the deliberate overlaps: a 17 lb baby could reasonably be in size 2, 3, or 4 depending on build and brand.
Signs the size is wrong
Move up a size if: red marks around the thighs or waist after a change, the tabs reach the very edge of the landing strip, frequent leaks despite correct fitting, or the diaper no longer covers the belly button area fully.
Move down a size if: gaps around the legs, the diaper sags or slides down, or the tabs overlap significantly in the middle. A too-large diaper leaks just as reliably as a too-small one, and is a common overlooked cause of blowouts.
Nighttime leaks specifically often mean it is time to size up for overnight even while the current size still works during the day, which is why many parents run one size larger at night.
How many to buy of each size
Newborns go through 8-12 diapers a day, dropping to about 6-8 by a few months and 4-6 after the first year. That is roughly 250-300 diapers in the first month alone.
Buy sparingly in newborn size (one pack, maybe two) since babies outgrow it fast and some skip it entirely. Size 1 and 2 also move quickly. Size 3 is where stocking up actually pays off, since most babies stay in it for many months.
Keep receipts and buy no more than one unopened box ahead. Most retailers exchange unopened diaper packs for a different size, which is the practical safety net against guessing wrong.
Brands fit differently
Weight ranges are broadly consistent across brands, but cut is not: some brands run narrow and long, others wide and short. A size 3 that leaks constantly in one brand can fit perfectly in another at the same weight.
If leaks persist after checking fit and sizing, trying a different brand before sizing up is usually the faster fix, particularly for babies with very chunky or very slim thighs.