Best Sleep Sacks: How to Pick the Right TOG

The sleep sack is probably the most useful, and most misunderstood, item on a newborn's shopping list. Useful because it safely replaces loose blankets in the crib, removing the suffocation risk that comes with loose bedding. Misunderstood because most parents pick one based on looks alone, ignoring the number that actually matters: the TOG rating, which measures how much warmth the fabric provides.

In this guide we'll cover how to pick the right TOG for the season, how to get the sizing right, and how to avoid the most common mistakes that leave a baby either sweaty or cold overnight.

What TOG means, and why it matters more than fabric

TOG is a unit of thermal insulation: the higher the number, the heavier and warmer the sack. You'll typically find sacks rated at 0.5 TOG (summer), 1.0 TOG (mid-season), and 2.5-3.5 TOG (winter, in unheated rooms). The fabric (cotton, muslin, bamboo) affects breathability, but TOG is what determines whether your baby will be too warm or too cold — which is why it's the first thing to check on the label, before color or pattern.

A wrong TOG rating is the single most common, and most easily fixed, cause of temperature-related night wakings.

A general temperature/TOG reference chart

As a general reference (always adjust based on how your own baby runs warm or cool): above 24°C/75°F in the room, a 0.5 TOG sack with a light bodysuit is usually enough. Between 20-24°C/68-75°F, a 1.0 TOG sack with light pajamas works well for most babies. Between 16-20°C/61-68°F, you'll typically move to a 2.5 TOG sack with heavier pajamas. Below 16°C/61°F, a 3.5 TOG sack is often the right call, with an extra layer if the room isn't heated.

How to choose the right size

Sleep sack sizing is almost always based on the baby's height, not age like regular clothing — a detail plenty of first-time parents miss, buying based only on the months printed on the packaging. A sack that's too big reduces the swaddled, contained feeling that helps many newborns feel secure, and in footed styles can create a risk of sliding down. A sack that's too small restricts movement and can become uncomfortable within just a few weeks.

A note on methodology: sizing and TOG guidance in this article is based on major manufacturers' size charts and a direct comparison across several models. Warmth perception varies from baby to baby — always check for a sweaty neck or cold hands/feet to fine-tune your choice.

The most common sizing mistakes

Buying by the age printed on the package. Always check your baby's actual height against the manufacturer's size chart.
Layering too much under an already-warm sack. Overheating is just as real a risk as being cold, and often underestimated.
Ignoring fabric in summer. Even with the right TOG, a low-breathability fabric can feel uncomfortable on hot nights.
Not checking the zipper. Pinch-guard zippers and an easy-access opening for night diaper changes make a real practical difference.

When it's time to size up

A clear sign is when the baby's shoulders start reaching the edge of the opening, or the sack no longer allows free leg movement. On average, given typical first-year growth, most babies move up a size roughly every 3-4 months, but it's worth checking their height against the chart monthly so you're not caught off guard at a seasonal change.

The bottom line

Picking the right sleep sack isn't about looks — it's about matching TOG to your room temperature, sizing based on actual height rather than age, and paying attention to practical details like zippers and access. Get those three things right and most common mistakes take care of themselves.

This article is for informational purposes only. For specific questions about your baby's sleep safety, consult your pediatrician.