Baby clothing sizes can be confusing. A “6-12 months” label doesn’t always fit a 9-month-old, and sizes vary from brand to brand. This US baby clothes size guide explains what each size label means, why weight and length matter more than age, and how to shop so clothes fit now and a little later. For exact Little Styles measurements, see our size guide page, then browse the shop by size.
How US Baby Sizes Work
Most US brands label baby clothes by age range in months: Newborn, 0-3M, 3-6M, 6-12M, 12-18M and 18-24M. Some brands use single numbers like 3M, 6M, 9M and 12M, where the number usually means “up to” that age. After 24 months, sizing typically switches to toddler sizes like 2T, 3T and 4T.
Why age is only a starting point
Babies grow at very different rates. Two babies of the same age can be several sizes apart. That’s why most size charts also list weight and height. When your baby’s weight and length point to different sizes, go by length for one-piece items like sleepers and rompers, because a too-short torso is uncomfortable.
Typical US Baby Size Chart
The ranges below are typical for US brands and are meant as a general guide. Always check a brand’s own chart, including ours.
| Size | Approx. Weight | Approx. Height |
|---|---|---|
| Newborn | 5-8 lb | up to 21.5 in |
| 0-3M | 8-12 lb | 21.5-24 in |
| 3-6M | 12-16 lb | 24-26.5 in |
| 6-12M | 16-21 lb | 26.5-29 in |
| 12-18M | 21-25 lb | 29-31.5 in |
| 18-24M | 25-28 lb | 31.5-34 in |
Size-by-Size Breakdown
Newborn (NB)
Newborn sizes fit smaller babies in their first weeks. Many babies outgrow them quickly, and larger babies may never wear them. Buy only a few pieces in this size.
0-3 Months
This is the size most babies wear right after the newborn stage. It’s worth stocking well with bodysuits, sleepers and a few pants. Our New Arrivals include plenty of 0-3M basics.
3-6 Months
Babies become more active, start rolling and may begin reaching for toys. Look for stretchy fabrics and outfits that don’t ride up when they move.
6-12 Months
This size covers a long stretch of time, so it’s often the best value. Many babies start crawling and sitting, so pants with soft knees and rompers with snaps are practical. It’s also the safest size to choose when you’re buying a gift and aren’t sure of the baby’s size.
12-18 Months
Many children begin walking in this range. Two-piece outfits become more common, and you’ll want clothes that allow easy movement.
18-24 Months
Toddlers are on the move. Durable fabrics and easy pull-on waistbands matter. After this size, many brands switch to 2T.
How to Measure Your Baby
Measuring takes a minute and saves a lot of guessing.
- Weight: Use the latest number from your pediatrician, or weigh yourself holding the baby and subtract your own weight.
- Height (length): Lay your baby flat on their back, gently straighten their legs, and measure from the top of the head to the bottom of the heel.
- Chest: Measure around the fullest part of the chest, under the arms.
Smart Sizing Tips When Buying Baby Clothes
Size up when in doubt
A slightly big outfit can be rolled at the cuffs and worn longer. One that’s too small can’t be worn at all.
Account for shrinkage
Cotton can shrink slightly, especially in a hot dryer. If you plan to tumble dry, consider going up a size.
Think about the season
If you’re buying ahead, match the size to the season your baby will be wearing it. A 6-12M winter coat for a baby born in spring may arrive in the middle of summer.
Remember the diaper
Cloth diapers add bulk. Families using cloth often size up in bottoms and one-piece outfits.
Sleepwear fit
In the US, children’s sleepwear above 9 months must be flame resistant or snug-fitting. Follow the label on snug-fit pajamas and don’t size up too much, as they’re designed to fit closely.
Buying Baby Clothes as a Gift
Gift givers often buy newborn sizes, which means many babies receive more newborn clothes than they can wear. Choosing 6-12M or 12-18M is a kind, practical alternative. Pair a larger size with something the baby can use right away, like booties or a hat. Our gift sets are a convenient place to start.
Signs Your Baby Has Outgrown a Size
- Snaps strain or won’t close.
- Sleepers pull at the shoulders or feet when legs are straight.
- Sleeves or pant legs end well above the wrists and ankles.
- Red marks appear at the waistband or cuffs.
When you see these signs, it’s time to move up. Pass outgrown items to family or donate them.
Shopping Little Styles by Size
Every Little Styles product lists available sizes from 0-3M through 18-24M. Choose a size on the product page, and if it doesn’t fit, you can return it within 30 days. US orders ship for a flat $5.99, and orders over $50 ship free.
Why Sizes Differ Between Brands
You may notice that a 6-12M bodysuit from one brand fits like a 3-6M from another. Brands design around their own fit models and choose how much room to leave for growth, diapers and movement. Some cut their clothes slim for a tailored look, while others cut generously so outfits last longer. Fabric matters too. Stretchy rib knits and jersey give more flexibility, while woven fabrics like poplin or denim have very little stretch.
How to handle mixed sizes in one wardrobe
It’s normal for your baby to wear different sizes from different brands at the same time. Instead of sorting by the label, sort by fit. Keep a small bin for “almost outgrown” pieces and another for “too big for now.” When you shop again, note which brands run small or large for your baby, and adjust your size choice for that brand.
Separates vs. one-piece outfits
Separates, like a top and pants, are more forgiving because each piece only has to fit one part of the body. One-piece outfits have to fit the full torso length, so babies with long bodies often need to size up in rompers and sleepers before they need a bigger size in tops.
Frequently Asked Questions
What size should a 6-month-old wear?
Many 6-month-olds wear 6-12M, but some still fit 3-6M. Check your baby’s weight and length against our size guide.
Is 6M the same as 3-6M?
Usually, yes. Many brands label “6M” to mean “up to 6 months,” which matches 3-6M. It’s best to check the brand’s chart.
What comes after 18-24 months?
Most US brands move to toddler sizes, starting with 2T. Toddler sizes are cut with more room for walking and less room for diapers.
Should I go by weight or height?
Use both, but if they disagree, length usually matters more for one-piece outfits.
Do Little Styles clothes run true to size?
Our sizing follows typical US ranges. Each product page notes any fit details, and you can return or exchange within 30 days.