Diaper sizes by weight: the complete chart
All US brands, every size, in one chart. Plus the three signals that beat the weight chart every time.
Diapering · Reference
TL;DR
Diaper size is by weight, not age. NB fits 6–10 lbs, Size 1 fits 8–14 lbs, Size 2 fits 12–18 lbs, Size 3 fits 16–28 lbs, Size 4 fits 22–37 lbs, Size 5 is 27+ lbs. The ranges overlap on purpose. When you size up depends on leaks and red marks, not pure pounds. Most US babies skip Newborn entirely or use it for under two weeks.
The age guidelines printed on diaper boxes are wrong for about 30% of babies. They're written for the average. Your baby's weight curve isn't average. Size by weight, with leak frequency as the tiebreaker.
The complete size chart
These ranges are nearly identical across Pampers, Huggies, Honest, Hello Bello, and Costco Kirkland. Cloth diapers use different sizing (newborn / one-size / toddler) and aren't on this chart.
| Size | Weight range (lbs) | Typical age range | Use duration |
|---|---|---|---|
| Preemie | Under 6 lbs | NICU / under 37 weeks | Days to weeks |
| Newborn (NB) | 6–10 lbs | Birth to 1 month | 0–4 weeks (often skipped) |
| Size 1 | 8–14 lbs | 1–4 months | 2–3 months |
| Size 2 | 12–18 lbs | 2–7 months | 3–4 months |
| Size 3 | 16–28 lbs | 5–24 months | 5–8 months |
| Size 4 | 22–37 lbs | 1.5–3 years | 6–12 months |
| Size 5 | 27+ lbs | 2.5–3.5 years | Through potty training |
| Size 6 | 35+ lbs | 3–4 years (overnight) | Mostly nighttime |
| Size 7 | 41+ lbs | 4+ years (overnight) | Nighttime / special needs |
The three signals that override the chart
1. Two leaks in a row
One leak is normal. Diapers fail. But two leaks in a row, at the leg gathers or up the back, means the absorbent core can't keep up or the fit is wrong. Time to size up.
Overnight leaks are the earliest signal. Two nights in a row, size up before you start seeing daytime leaks too.
2. Red marks at thighs or waist
Pull off the diaper and look. Red indents at the leg gathers or waistband, lasting more than a few minutes? The diaper is too tight. Size up.
This usually happens before the weight chart says to. Babies who carry weight in their thighs (perfectly normal) hit the size-up trigger by waistband fit, not raw pounds.
3. The two-finger test
Slide two fingers between the waistband and your baby's belly. If two fingers don't fit easily, it's too tight. If you can fit four fingers, it's too loose. Aim for two-finger snug.
Calculate your baby's exact size
Enter age and weight. Get size, daily count, monthly cost, and time-until-size-up.
Try the calculatorWhy the size ranges overlap
Notice that Size 1 (8–14 lbs) and Size 2 (12–18 lbs) both cover 12–14 lbs. That's intentional. A baby in the upper end of Size 1 may stay there for weeks before they need to size up, depending on body shape. A leaner baby may sit in the lower end of Size 2 for a while.
The overlap is your buffer. Don't bulk-buy when your baby is in the overlap zone. They could go either way for a couple of weeks.
Brand-by-brand differences
Pampers Swaddlers
Run slightly small. If your baby is at the upper edge of a size, Pampers will leak first. Their newborn-line fit is the best for tiny babies though.
Huggies Little Snugglers
Wider waistband, more generous leg gathers. Better for chunky-thighed babies. Slightly worse on absorbency than Pampers in our testing.
Honest Co.
The opposite of Pampers. Runs slightly large. A baby in mid-Size 1 may swim in Honest Size 1. Absorbency is solid. Price-per-diaper is the highest of the mainstream brands.
Hello Bello
Slim cut, runs true to chart. Best for slim babies. Worst for chunky thighs (red marks happen sooner).
Kirkland (Costco)
Made by the same factory as Huggies. Sizing is identical. Absorbency is comparable. Price is half. The objective best price-per-diaper option.
The newborn-size question
About 60% of US babies skip newborn entirely or use it for under two weeks. The exceptions: babies under 7 lbs at birth, preemies, and slow-weight-gain babies.
If you're stocking up before delivery, get 1 box of newborn and 2 boxes of size 1. If your baby comes in at 8+ lbs, donate the unused NB to a NICU. If your baby is 6 lbs, you'll burn through that NB box in a week and need more — Amazon Prime ships in a day.
When the chart is wrong
The chart fails for these specific babies:
- Tall and lean. They hit the upper weight of a size by length, not bulk. Size up by leak count, not pounds.
- Chunky-thighed. They hit the size-up trigger by red marks before they hit the upper weight.
- Slow weight gain. They may stay in size 1 for 4+ months instead of 2–3.
- Heavy overnight wetters. They may need to size up just for overnight and stay in current size during the day.
If any of these match your baby, ignore the box guideline. Use leak frequency as your real signal.
Overnight diapers — different rules
Brands sell overnight versions: Pampers Swaddlers Overnights, Huggies OverNites. They're built for 12-hour wear without leaks. Most parents move to overnights around 3–6 months, when sleep starts consolidating.
Overnight leaks before 3 months usually mean fit (size up the regular diaper) or feeding volume, not a need for the overnight version.
The right way to stock up
- Pre-baby. 1 NB box and 2 Size 1 boxes max. Don't go bigger.
- 0–6 months. Buy 2 boxes at a time. Sizes change fast.
- 6–18 months. Bulk-buy window. Sizes last 4–6 months. Costco runs are worth it.
- 18+ months. Don't over-stock past size 5. Potty training is coming.
Most parents over-stock NB and Size 1, then under-stock Size 3 and up (which actually lasts a while). Aim for the inverse.
Questions parents ask
Pulled from Google's "People Also Ask" box for this topic, answered by our editors with the research and our test-family notes.
What size diaper should my baby be wearing?
Diaper size goes by weight, not age. Newborn covers 8 lbs and under, Size 1 covers 8 to 14 lbs, Size 2 covers 12 to 18 lbs, Size 3 covers 16 to 28 lbs. Brands overlap, so use the weight range as the guide, not the size number on the box.
When should I move up a diaper size?
When you see overnight leaks, red marks on baby's thighs, or trouble snapping the tabs without pulling. Many babies size up before the weight chart says so — those are the body-level signals that the diaper is undersized.
Do diapers run small or big?
It varies. Pampers Swaddlers run small in the legs. Huggies Little Snugglers run slightly larger. Kirkland and Honest run a bit more generous. Use the weight ranges as a starting point, then watch the leak/red-mark/tab signals to decide whether your specific baby fits the size.
How long do babies stay in each diaper size?
Newborn: 2 to 4 weeks. Size 1: 2 to 3 months. Size 2: 3 to 4 months. Size 3: 4 to 8 months. Size 4 onward: longer, often 8 to 18 months as growth slows. Buy in pack sizes, not bulk warehouse boxes, until you know your baby's body type.