The right TOG for your baby’s sleep sack depends on how warm the nursery is. TOG is a measure of thermal resistance, essentially how much warmth the fabric traps. A lower TOG means a thinner, cooler sack; a higher TOG means thicker and warmer. Most families need a 1.0 TOG for typical indoor temperatures (68–73°F) and a second option for seasonal extremes.
TOG Ratings by Room Temperature
TOG stands for Thermal Overall Grade, a British standard for measuring how well a textile insulates. The scale for baby sleep sacks ranges from 0.2 (barely more than a sheet) to 3.5 (heavy winter warmth). Here’s the general guide:
- Above 80°F (27°C): 0.2 TOG, essentially a single layer of muslin
- 73–79°F (23–26°C): 0.5 TOG, lightweight for warm rooms or summer
- 68–73°F (21–23°C): 1.0 TOG, the most commonly used year-round weight
- 61–68°F (16–20°C): 2.5 TOG, suited for cooler bedrooms or winter
- Below 60°F (16°C): 3.5 TOG, for cold environments without central heating
Most homes with climate control sit between 68°F and 73°F, which puts you squarely in 1.0 TOG territory. If your home runs cool in winter or you keep the thermostat low at night, a 2.5 TOG is the better pick for those months. Many parents end up owning two: a lighter one for warmer weather and a heavier one for cold nights.
What to Dress Baby in Underneath
The sleep sack isn’t the whole picture. What your baby wears under it adds warmth too, and getting the combination right matters more than the TOG alone. Lightweight, breathable fabrics like cotton onesies or footed pajamas work best as a base layer.
In a warm room (above 75°F) with a 0.2 or 0.5 TOG sack, a short-sleeve onesie or even just a diaper is enough. For a 1.0 TOG sack in a 68–73°F room, a long-sleeve cotton onesie is a solid middle ground. When you’re using a 2.5 TOG in a cooler room, a footed pajama underneath adds warmth without piling on loose blankets. The goal is always to match the total insulation to the room, not to default to more layers “just in case.”
Why Room Temperature Matters So Much
Overheating is a recognized risk factor for sudden infant death. The CDC advises against letting babies get too hot during sleep and recommends keeping soft bedding like loose blankets out of the sleep area entirely. Sleep sacks exist specifically to solve this problem: they provide warmth without the suffocation risk of a blanket that can cover a baby’s face.
The American Academy of Pediatrics doesn’t specify an exact nursery temperature, but research suggests 68–72°F is comfortable for most babies. Rooms that are comfortable for adults are generally fine for infants too. A cheap room thermometer near the crib takes the guesswork out of choosing the right TOG each night, especially during transitional seasons when temperatures swing between daytime and overnight.
How to Tell if Your Baby Is Too Warm
Cold hands and feet are not reliable indicators. Babies naturally have cooler extremities, and checking their fingers will almost always make you think they need more warmth than they do. Instead, touch the back of your baby’s neck or their chest. If either area feels hot or clammy, the baby is overdressed.
Damp or sweaty hair is another clear sign. Flushed cheeks, rapid breathing, or restless sleep can also point to overheating. If you notice these signs, drop down a TOG level or remove a layer underneath. It’s better to err slightly cool than slightly warm.
Fabric Choices and Breathability
Two sleep sacks with the same TOG rating can feel different depending on the material. Cotton is the standard and works well for most situations. Bamboo viscose has become popular because its fiber structure allows more airflow, and it absorbs moisture about 40 percent better than organic cotton. That makes bamboo a strong choice for babies who tend to sleep hot or sweat easily, even at a moderate TOG. It wicks sweat faster and dries quicker, keeping skin drier through the night.
Merino wool is another option, particularly at higher TOG levels. Wool naturally regulates temperature in both directions, releasing heat when the baby is warm and retaining it when cool. It’s more expensive, but some parents find it useful in homes where room temperature fluctuates overnight.
Getting the Right Fit
A sleep sack that’s too large creates a safety issue. The neck and arm openings should be snug enough that the baby can’t slide down inside the sack, but never tight or restrictive around the throat. If your baby is between sizes, go with the smaller option rather than sizing up. An oversized neck or armhole defeats the purpose of a wearable blanket and introduces the same risks as loose bedding.
The length of the sack matters less. Extra room at the bottom gives the baby space to kick and move, which is fine and even desirable for hip development. It’s the top of the sack, around the chest and shoulders, that needs to fit properly.
Seasonal Strategy for Most Families
If you’re starting from scratch, a 1.0 TOG sack covers the widest range of conditions. Add a 0.5 TOG for summer if your home gets above 73°F at night, or a 2.5 TOG for winter if the nursery dips below 68°F. Families in mild climates can often get by with just one. Families in places with harsh winters or no air conditioning may need all three.
Keep in mind that your baby’s own body heat contributes warmth too. Newborns are less effective at regulating their temperature than older infants, so the stakes are higher in the first few months. By around six months, most babies manage heat a bit better, but the same TOG guidelines still apply. Checking the back of the neck before you go to bed yourself is a quick habit that keeps you calibrated as seasons change.