Sleep Sack Size Chart (XS–L) + Fit Tips (No More Guessing)

If you’ve ever bought a sleep sack and then paused at bedtime thinking, “Wait… is this too big?” you’re not alone. Sleep sack sizing can feel weirdly confusing; especially because babies don’t grow in neat little “age = size” boxes.

Here’s the good news, mama-to-mama: choosing the right sleep sack size comes down to two simple things—a secure fit at the top (neckline + arm openings) and room to move at the bottom (hips + legs).

This guide walks you through our sleep sack size chart (XS–L), how to choose between sizes, what a safe fit should look like, and when it’s time to size up so you can stop guessing and feel confident at bedtime.

If you’re also wondering what baby should wear under a sleep sack by room temperature, you’ll love our Sleep Sack TOG Chart: What to Wear Under a Sleep Sack.

If you want the simplest “do this, then that” plan:

Choose your sleep sack size based on your baby’s current weight and length (not age).
Prioritize a secure neckline and arm openings so the sack can’t shift upward.
If baby is between sizes, don’t automatically size up; top fit matters more than extra length.

A sleep sack can be a little long and still fit beautifully. A sleep sack that’s too loose at the top is the one you want to avoid.

Use this chart as your starting point, then use the fit tips below if your baby is between sizes.

XS: 7–13 lb (3–6 kg) | 19–25 in (50–64 cm)
S: 14–20 lb (6–9 kg) | 22–27 in (56–69 cm)
M: 18–30 lb (8–13 kg) | 26–33 in (66–84 cm)
L: 28–42 lb (12–19 kg) | 33–40 in (84–100 cm)

If you notice overlap (like 18–20 lb or 28–30 lb), that’s normal. In those cases, use the tie‑breaker below.

If your baby falls into an overlap range, here’s what I’d do:

If sizing up makes the neckline or arm openings feel loose or gappy, choose the smaller size.
If baby is near the top of the smaller size range and looks snug through the torso or length, go up.

And if you’re ever unsure, the safest anchor is always the top fit: a sleep sack should sit securely at the shoulders and stay clear of baby’s face when they wiggle.

You don’t need anything fancy, just two quick numbers.

1) Weight

Use your most recent weight (or a quick home check). Weight is the main anchor for most sleep sack size charts.

2) Length

Measure baby from the top of the shoulder down to the heel (or crown to heel either is fine, just be consistent). Length helps confirm baby will have comfortable space through the body of the sack.

If your baby is in between sizes, check both weight and length, then use the fit checklist below as the deciding factor.

Here’s what you’re aiming for at bedtime:

1. The neckline should feel secure and sit at the shoulders, staying clear of baby’s mouth and nose.

2. The arm openings should not be overly wide; arms should move freely, but you don’t want big gaps.

3. The bottom should be roomy; baby should be able to kick and bend their legs naturally, without tightness through the hips or legs.

One little mama tip: don’t check hands and feet as your “temperature test.” They can feel cool even when baby is comfortable. Instead, check baby’s chest or the back of the neck for warmth and dryness.

A sleep sack is usually too big if:

The neckline gapes or sits lower than it should.
The arm openings look overly wide or roomy at the sides.
The top looks loose enough that it could shift upward when baby wiggles.

If you’re seeing those signs, sizing down is often the safer choice even if the sleep sack looks “shorter” than you expected.

You’re usually ready for the next size if:

The sleep sack feels snug through the chest, hips, or legs.
Baby’s feet are pressing firmly into the bottom (not just touching—pushing).
The zipper looks strained or the fabric doesn’t lay smoothly once zipped.
After a growth spurt, baby seems less comfortable and the fit looks tighter than it used to.

Newborns are tiny, wiggly, and unpredictable which is exactly why top fit matters most in the early stage.

Many families start with swaddling and then transition to a sleep sack when baby starts showing signs of rolling, or when baby simply fights the swaddle. If you’re in that transition season, it can help to keep both options on hand: baby swaddles for earlier nights, and baby sleep sacks for the next stage.

A quick thing most parents don’t realize: layers underneath can change how a sleep sack fits. A thick pajama layer can make a sleep sack feel tighter through the torso, so once you choose the size, it’s worth checking fit again once baby is dressed for sleep.

If you want the full temperature guide, our Sleep Sack TOG Chart makes this part feel so much easier.

For the simplest bedtime rotation, these basics keep it stress-free: baby pajamas for most nights, convertible footies when you want quick changes, and baby bodysuits as an easy base layer when the room is warm.

If there’s one takeaway from this whole guide, it’s this: choose a sleep sack size that fits securely at the top and gives baby room to move at the bottom.

If you’re ready to simplify bedtime, start with a breathable baby sleep sack, then build a small rotation of baby pajamas, convertible footies, and baby bodysuits so you can adjust for comfort without guessing every night.