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Sleep sacks: which TOG to choose, and why the month does not matter
TOG is chosen for the room temperature, not the season. The table, and what must never be added on top.
Paul · September 21, 2026 · 5 min read
The sleep sack is the most important item in the layette, because on its own it replaces the sheet, the blanket and the duvet, none of which belong in a baby’s cot before age two.
It is also the most commonly mis-chosen, for a simple reason: people buy it for the month of birth, when the only useful reference is the temperature of the room.
The table, one line per range
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Above 24 degrees: TOG 0.5
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18 to 20 degrees: TOG 2, the most common range and the default sack
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Below 18 degrees: TOG 3, with a bodysuit and a thicker sleepsuit
Why the room rather than the season
A well-insulated flat heated to 21 degrees in February has nothing in common with an old house at 17 degrees on the same day. The month is identical, the sleep sack needed is not.
A room thermometer costs little and settles the question for good. The recommended comfort range sits around 18 to 20 degrees, which makes TOG 2 the default for most homes.
A sleep sack that is too thick in a heated room does more harm than good: the overheating risk is real, and it rises precisely when a high TOG and strong heating are combined.
How to check you got it right
Feel the back of the baby’s neck, not their hands or feet. A damp neck means too warm, a cool neck means add a layer of clothing, not of bedding.
A newborn’s extremities are naturally cooler than the rest of them. Cold feet are not a reliable signal, and yet they are everybody’s first instinct.
What never goes on top
A blanket, a duvet, a pillow or a cot bumper, whatever the temperature. According to ameli.fr and the guidance from the French association of ambulatory paediatrics republished by sante.fr, none of it belongs in the cot before age two.
If it is cold, the answer is a higher TOG or a thicker sleepsuit under the sack, never a blanket added over it. That is exactly the problem the sleep sack exists to solve.
Size matters as much as TOG
The neck opening must be narrow enough that the baby cannot slip down inside the sack. That is the one place where an oversized sleep sack is a genuine problem rather than a comfort issue.
Plan for two so you can wash one, three if you would rather never chase the laundry. And expect to buy again at the next change of season, both for the TOG and for the size.