Sleep sack sizing looks like clothing sizing and doesn't behave like it. Buying a size up, which is sensible for almost everything else you'll buy a baby, is the one thing not to do here.
Why too big is a problem
A sleep sack has to fit at the neck and the armholes. If it's too large, a baby can slip down inside it, which puts loose fabric over the face — the exact thing the sack exists to prevent.
So the rule inverts. For clothes, buy ahead. For sleep sacks, buy the size they are now.
How they're actually sized
Two systems, and they aren't interchangeable.
By age band. S at 0–6 months, M at 6–12, L at 12–18 — roughly 68, 75 and 82cm of length. Age bands are a shortcut for length, and babies vary, so treat them as a starting point.
By length in centimetres. An 85cm or 95cm sack tells you what it actually is. If you know your baby's height, this is the more reliable number.
Newborn cocoons and quilted wraps are sized differently again — 3M, 6M, 9M, with real dimensions attached — because they're shaped to contain rather than to hang.
Check the fit, not the label
Two checks, both quick.
- The neck. It shouldn't be possible to pull the opening up over the chin.
- The armholes. Snug enough that a shoulder can't come through, loose enough not to mark.
Length below the feet is fine and expected. Room at the top is not.
A note on buying as a gift
This is the one baby item where the usual gift instinct — buy bigger, they'll grow into it — works against the product. If you're buying ahead, buy for the season rather than the size: a lighter sack for a summer baby heading into autumn, a warmer one for the reverse.
Or sidestep it. A one-size sack covering 0–6 months takes the decision away entirely, and the parents can size up when they need to.
Temperature, briefly
Warmth is about the room, not the sack. A bamboo cotton bag suits roughly 10–20°C; below that you layer underneath rather than buy thicker. Check the back of the neck — hands and feet run cool on babies and tell you very little.