Diaper Size Table
| Diaper size | Baby weight (lb) | Baby weight (kg) | Approx child age |
|---|---|---|---|
| Newborn (N) | under 10 | under 4 | first few weeks only |
| Size 1 | 8–14 | 3–6 | 2–4 months |
| Size 2 | 12–18 | 5–8 | 3–6 months |
| Size 3 | 16–28 | 7–13 | 6–10 months |
| Size 4 | 22–37 | 9–17 | 7–17 months |
| Size 5 | 27 and up | 12 and up | older than 2 years |
| Size 6 | 35 and up | 16 and up | older than 3 years |
| Size 7 | 41 and up | 19 and up | older than 4 years |
| Size 8 | 46+ | 21+ | older than 5 years |
Diaper sizing runs on weight, not age - the single rule that prevents most nursery sizing mistakes. The chart's weight ranges (Newborn under 10 lb, Size 1 at 8-14, Size 2 at 12-18, Size 3 at 16-28) overlap on purpose: a baby at the top of one size is usually ready for the next, and brands print overlapping ranges so you can pick by build, not by birthday. Age columns exist only as a rough preview of what to expect; the scale is the authority.
The two failure modes are opposite: too small leaves red pressure marks at the waist and thighs and blows out up the back, while too big gaps at the legs and leaks sideways at night. The practical rhythm most parents land on: buy the current size in small packs, watch for the top-of-range signals (red marks, frequent blowouts, tabs that barely meet), and size up the moment night diapers start failing - which is why the chart's ranges, printed as overlaps, matter more than any single number.
How to use
- Weigh first, buy second: match the baby's current weight to the chart row and start there - a 17 lb baby is Size 3 by weight even if the age column says 6-10 months and yours is 5.
- Fit-check with two fingers: a properly sized diaper lets one or two fingers slide under the waistband without digging; ruts or red lines mean size up, and gaps you can fit a whole hand into at the leg mean size down or fasten tighter.
- Watch the overnight signal: daytime dryness with night leaks is the classic top-of-size symptom - the diaper's capacity is being outrun by a bigger bladder, not by anything you did wrong; move up a size (or to the brand's overnight line in the same size) rather than doubling up.
Frequently asked questions
What size diaper does a 20 lb baby wear?
By weight, 20 lb lands in Size 3 (16-28 lb) for most brands - possibly Size 4 (22-37 lb) if the baby is long-waisted or the 3s leave red marks. The age column is a red herring here: 20 lb is typical around 6-9 months, but a chunky 4-month-old and a slim 14-month-old can wear the same size. What confirms the choice is the fit, not the chart: two fingers under the waist, no red lines, no leg gaps.
How do diaper sizes compare between brands?
The weight ranges are close across major brands - the chart's table (Newborn under 10 lb up to Size 8 at 46 lb+) is the industry's common pattern - but cut and stretch differ, so the same claimed size can fit differently. Brands also break the pattern at the ends: newborn lines run small and some offer a preemie size below Newborn, while Sizes 7 and 8 exist mainly in one or two brands for toddlers past 35 lb. Treat the chart as the shared skeleton and the first pack of any new brand as the fit test.
When should I move up a diaper size?
Three signals, in order of reliability: red marks at the waist or thighs that persist after refastening, leaks or blowouts that return despite a fresh fit, and tabs that no longer reach the landing strip comfortably. Weight crossing the chart's range top is the early warning; the fit signals are the go decision. Parents who size up on the early side report fewer blowouts - the cost difference is small and the laundry difference is not.
How many diapers does a baby use by size?
Newborns burn through 8-12 changes a day, and the daily count slides as sizes climb - roughly 8-10 a day through Size 2, then 6-8 a day for the bigger sizes as bladders grow and solids begin. Stretched over the weight ranges, the middle sizes (3 and 4) are where babies spend the most months, which is why bulk-buying Size 3 feels efficient and bulk-buying Newborn feels wasteful: many babies exit Newborn within weeks. Buy the small sizes small, and stock the size your baby currently wears plus one.