Sizing baby clothes can feel like a guessing game. Babies grow in fits and starts, and no two are built quite the same — some are long and lean, some are round and cozy, and the number on the tag only tells part of the story.
So here’s everything you need to choose the right Magnetic Onesie the first time: our full size chart, a simple way to measure at home, and a note on why our bigger sizes fit close to the body.
Start with weight, then check length and chest
The fastest way to size your baby is by weight — it’s the single best predictor of fit. If your baby falls between two sizes, or you’re shopping a little ahead of a growth spurt, size up. A onesie that’s a touch roomy today will be just right in a few weeks.
If you want to be precise, measure length and chest and match them to the chart below.
How to measure:
- Length — lay your baby down and measure from the shoulder to the ankle.
- Chest — measure across the chest, just under the arms.
A soft tape measure is ideal, but a piece of string measured against a ruler works just as well.
Why 6–12M and up are a snug fit
You’ll notice our chart marks 6–12M and larger as a snug fit — and that’s by design, not by accident.
Once a baby is on the move, a closer fit is both safer and more comfortable. Snug-fitting cotton sleepwear moves with your baby instead of bunching up, and it’s the standard for one-piece layers at these ages. So if a 6–12M feels closer to the body than your baby’s other clothes, that’s exactly right — it’s meant to.
Our onesies are 100% GOTS-certified organic cotton, which softens and gives a little with every wash, so a snug fit settles in beautifully over the first few wears.
A few quick tips
- Between sizes? Size up. Babies grow faster than they shrink out of a onesie.
- Shopping for a gift? 3–6M is the safest all-around choice for a newborn gift.
- Heading into a growth spurt? Buy the next size up now so you’re never caught short at 3 a.m.
Size chart (text version)
| Size | Weight | Length | Chest |
|---|---|---|---|
| 0–3M | 8–12 lb · 3.6–5.4 kg | 18 in · 46 cm | 9.5 in · 24 cm |
| 3–6M | 12–17 lb · 5.4–7.7 kg | 21 in · 53 cm | 10 in · 25.5 cm |
| 6–12M | 17–22 lb · 7.7–10 kg | 26 in · 66 cm | 11 in · 28 cm |
| 12–18M | 22–27 lb · 10–12.2 kg | 27.5 in · 70 cm | 11.5 in · 29 cm |
| 18–24M | 27–30 lb · 12.2–13.6 kg | 30 in · 76 cm | 12 in · 30.5 cm |
Still not sure?
We’re happy to help you choose. Reply to any of our emails or write us at support@magneticonesie.com with your baby’s weight and age, and we’ll point you to the right size.