Short answer: because no law makes baby clothing sizes match. In the United States there is no mandatory sizing standard for children's apparel. Every brand writes its own chart, picks its own age brackets, and grades its own patterns. Two labels that both say "12–18 months" can be built to different measurements, and both are correct.
This is the single most useful thing to understand before you buy anything for a baby online. It converts a frustrating experience — "why does this brand run small?" — into a solvable one.
Is there any official baby sizing standard in the U.S.?
There is a voluntary one, and voluntary is the operative word. ASTM International publishes standard tables of body measurements for infant and children's sizes. Brands may use them, adapt them, or ignore them entirely. Nothing requires a manufacturer to follow them, and nothing requires a label to disclose which chart it used.
Contrast that with children's sleepwear, where the Consumer Product Safety Commission does write dimensions into federal rules — 16 CFR Parts 1615 and 1616 specify maximum body dimensions for snug-fitting sleepwear in sizes above 9 months through 14. That is the exception, and it exists for fire safety, not for fit convenience.
Where exactly do the charts diverge?
In four places, and you can spot all four on a product page.
1. The brackets themselves. Brands don't even cut the year into the same pieces. Kyte Baby lists a single 6–12 months size; Little Sleepies splits the same span into 6–9 months and 12–18 months on some silhouettes. A baby who is "nine months" has no equivalent size in both catalogs.
2. The numbers inside the bracket. Same name, different content:
| Size | Kyte Baby | Little Sleepies |
|---|---|---|
| 0–3 months | 9–12 lbs · 21–23 in | 8–12 lbs · 21–23 in |
| 12–18 months | 22–25 lbs · 29–31 in | 22–26 lbs · 29–31 in |
| 18–24 months | 25–27 lbs · 31–33 in | 26–30 lbs · 31–33 in |
A 28-pound toddler is off the top of one 18–24 month size and mid-range in the other.
3. One chart per brand, or one chart per garment. Little Sleepies publishes different tables for different silhouettes. Most brands publish a single table for the whole catalog, which means a footed sleeper and a two-piece set are graded from the same numbers even though they fit nothing alike.
4. The units. Most U.S. brands publish pounds and inches only. Kyte Baby is the only one of the five leading bamboo brands that publishes kilograms and centimeters alongside them. If you think in kilos — and a lot of families in Miami do — that difference decides whether the chart is usable at all.
Why does one brand "run small" if everyone uses the same body measurements?
Because body measurements are not garment measurements. Between the two sits ease: the extra room a pattern maker adds beyond the body. Two brands can target the identical 22-pound baby and cut garments two inches apart in chest width, because one is designing a relaxed daywear fit and the other a close sleep fit.
You see this in the brands' own language. The same manufacturer will describe an infant sleeper as "relaxed and roomy" and a toddler sleepwear item as "snug fit" — same body, different intent, different pattern.
Does the fabric change how a size fits?
Yes, and it's a real variable rather than an excuse. A knit with elastane stretches and recovers, so it tolerates a baby sitting between two sizes. A woven with no stretch does not. Two garments cut from the same pattern in different fabrics will not fit the same child the same way.
Fiber also affects what happens after the first wash, which is a separate reason a size can feel like it changed. That's covered in do baby clothes shrink, and how much.
How do I compare two brands before I buy?
Ignore the size names entirely and compare numbers.
- Write down your baby's weight and height today, with the date.
- Open both brands' charts. Find the row your baby's numbers land in — not the row matching their age.
- If the two brands put your baby in differently named sizes, that's normal. Buy the size each chart tells you, not the same name in both.
- Check whether the chart is body measurements or garment measurements. If a brand publishes garment measurements too, that's the more useful number.
- Check whether the brand publishes one chart or one per silhouette.
- Read the return window before ordering.
Should I just buy a whole wardrobe from one brand?
It genuinely reduces the guessing, and it's why so many parents end up loyal to one label. Within a single brand, the grading is consistent: once you know it fits, the next size up will behave the way you expect.
The cost is that you're locked into that brand's brackets, including any bracket they don't make. It's a reasonable trade for sleepwear, which you buy in quantity and want to be predictable, and a less necessary one for daywear.
What about hand-me-downs and older sizes?
Older garments follow whatever chart that brand used at the time, and charts get revised. A 12–18 month piece from five years ago is not necessarily built like today's. Measure the garment flat and compare it to something that currently fits, rather than trusting the label.
If you're on the receiving end of a lot of these, what to do with outgrown baby clothes covers sorting and storing them so they're usable when the size finally arrives.
The short version
Baby sizes are brand-specific brackets, not measurements. Use weight and height, read the chart of the brand you're actually buying from, expect the same baby to be different sizes in different catalogs, and treat the age on the tag as decoration. The full reasoning behind that habit is in why baby sizes go by weight, not age, and the between-sizes decision is in how to choose between two sizes.
Sources
- U.S. Consumer Product Safety Commission, Children's Sleepwear Regulations (16 CFR Parts 1615 and 1616) — cpsc.gov
- ASTM International, standard tables of body measurements for infant and children's sizes (voluntary consensus standards) — astm.org
- Kyte Baby, published baby and toddler size chart — kytebaby.com
- Little Sleepies, published sizing page — littlesleepies.com