How to Read Baby Sleep Sack Sizing Charts When Ordering Online in Canada

Age Ranges on Sizing Charts Are a Starting Point, Not the Answer

Sizing charts for baby sleep sacks look deceptively simple — a column of age ranges, a column of weights, maybe a height range if you’re lucky. But the number that actually matters most when ordering online is weight, not age. Babies at the same age can differ by four or five pounds, and a sleep sack sized for a 6-month-old on the chart may fit a 4-month-old who’s tracking heavy or a 7-month-old on the leaner side.

The reason this matters isn’t just comfort. A sleep sack that is too tight may restrict movement, leading to discomfort and potential developmental concerns, while one that is too loose poses risks of suffocation, especially for younger infants. That’s a meaningful safety gap — and it’s exactly the kind of thing a sizing chart is supposed to help you avoid, provided you know how to read it.

The reason so many parents get sleep sack sizing wrong is they rely solely on age or body weight/height — the advice is to consider both when shopping. Most quality sizing charts give you three columns to work with: age range, weight range, and height (or body length). When those three line up cleanly for your baby, the decision is straightforward. When they don’t — say, your 9-month-old is 28 inches tall but only 17 lbs — you’ll need to prioritize. In most cases, weight drives fit at the neck and armholes, while height determines whether the sack is long enough to stay on properly overnight.

What Loulou Lollipop’s Sizing Chart Actually Tells You

Loulou Lollipop’s sleep bag sizing chart covers three infant size bands:

A few things stand out here. First, the age ranges are wide — the 6–18 M band covers a full year of development. That’s intentional. Some brands that once based sleep bag sizes on narrow age ranges have updated their charts to provide a more accurate fit prediction, because babies and children are all different sizes regardless of their age. Second, notice that the weight ranges in adjacent bands overlap slightly. A baby at 16 lbs sits at the top of the 0–6 M range and the bottom of the 6–18 M range. In that overlap zone, height becomes the deciding factor — if your baby is already 27 inches or taller, the larger size is the better call.

The chart is labeled as a reference, and Loulou Lollipop’s customer service team is available for questions on sizing — which is worth knowing if your baby is sitting right between bands. What the chart doesn’t tell you directly is how the sack should feel once it’s on, which brings us to the fit check.

For Canadian parents shopping the sleep bag collection, the sizing applies consistently across TOG weights — whether you’re ordering the 0.5 TOG muslin for a warm summer bedroom or the 2.5 TOG TENCEL™ version for a cold Prairie winter night.

The Fit Check You Should Do Before the First Night

Once the sleep sack arrives, run through this before putting your baby down in it for the first time.

Neck opening: Slide two fingers horizontally under the neckline. It should feel snug around those fingers without pulling. A sleep sack should be well-fitted around the neck and the armholes to avoid your baby slipping through. If the neckline is so loose that your baby could tuck their chin into the sack, it’s too big.

Armholes: The armholes should be the right size to prevent the baby from slipping inside the sack, yet allow freedom of movement. Tight armholes that leave red marks after a nap are a sign the sack is too small; armholes where the baby’s arm can slide fully inside are a safety risk.

Bottom length: The sack should be bell-shaped at the bottom to allow for easy leg movement. There should be room for the classic froggy leg position — knees bent, hips open. The sleep bag should fit snug but not tight, and there should be plenty of room at the bottom for your baby’s legs to assume the froggy position. This matters for hip development, not just comfort.

Overall length: An excessively long or wide sleep sack can present hazards — the additional fabric might bunch up around your baby’s face, posing a suffocation risk. If the sack extends well past the feet when the baby is lying flat, it’s worth checking whether the smaller size would still accommodate the weight range.

And importantly: do not size up to the next size until your child meets the weight requirement, for safety reasons.

Where Canadian Parents Often Get Tripped Up

Ordering online adds a layer of uncertainty that in-store shopping removes — you can’t hold the sack up against your baby before checkout. A few patterns come up often.

Sizing by clothing size, not sleep sack size. Sleep sacks and clothing do not share the same sizing logic. A baby in 9-month clothing may fit a 6–18 M sleep sack, but that’s a coincidence, not a rule. Always go back to the weight and height columns on the specific sleep sack chart, not the size printed on their onesie.

Assuming all brands use the same ranges. They don’t. Other brands may opt for small, medium, and large sizing or different age ranges, and it is important to keep in mind that each brand may have different specs when it comes to height and weight ratios for correlating sizes. A 6–18 M from one brand may be cut significantly longer or shorter than the same label from another. When switching brands — or ordering from a new one for the first time — treat the chart as if you’ve never sized a sleep sack before.

Sizing up for longevity. It’s tempting to buy the next size up so the sack lasts longer. With clothing, that’s usually fine. With sleep sacks, it’s a different calculation. Although you may prefer to size up in your baby’s clothes so they have room to grow, a sleep bag should always be a perfect fit for your baby’s weight. The neck and armhole fit is what determines safety — extra length at the bottom is less of a concern than extra room at the top.

Forgetting about TOG alongside size. Sizing and TOG are separate decisions, but they’re related. In Canada, bedroom temperatures swing considerably between seasons. It can be helpful to have different TOG-rated sleep sacks for seasonal changes — a lightweight sleep sack for summer and a thicker one for winter will help keep your child comfortable. A correctly sized 0.5 TOG sack for July in Vancouver is a different product than a correctly sized 2.5 TOG for January in Edmonton, even if the size label is the same. Getting the size right matters; getting the TOG right matters equally.

For parents navigating both decisions at once, Loulou Lollipop’s sleepsuits collection includes options across TOG weights with the same consistent sizing framework, which makes it easier to compare across styles without recalibrating from scratch.

A Quick Reference for Common Sizing Scenarios

These are the situations that come up most often when ordering a sleep sack online in Canada:

Baby is at the top of one weight range and bottom of the next: Go up. The neck and armhole fit loosens as babies grow, so a sack that fits snugly at 16 lbs will be noticeably looser at 20 lbs. Sizing up earlier rather than later gives a better fit window.

Baby is tall for their weight: Prioritize height. A sack that’s too short will ride up and restrict leg movement. The bottom of the sack should always extend past the feet.

Baby is heavy for their height: Prioritize weight. The neck and armhole dimensions are set by weight in most sizing charts. A sack that’s snug at the neckline is a safety issue regardless of how the length fits.

Baby is right in the middle of a range: Check the fit at the neckline specifically. If two fingers fit comfortably but not loosely, you’re in good shape. If your baby is between sizes, a size that leaves a little room to grow is okay — as long as it passes the fit test.

Ordering as a gift: When in doubt, size up. A sleep sack that’s slightly large is easier to exchange than one that’s too small, and the recipient can always return for the right size. If you know the baby’s current weight, use that as the primary filter on the chart rather than guessing by age.

The Loulou Lollipop sleep bag sizing chart is available on the site and applies across the full sleep bag range — a consistent reference point whether you’re ordering a muslin summer sack or a TENCEL™ waffle style for colder months.