Which sleep sack for which room, and what goes under it. The same table the Baby Layers app uses, for babies up to a year.
Match the sack to the room, not the season. Brands vary a little, so your sack's own label wins over this table.
26°C and up: No sack, short-sleeve bodysuit. 24–25°C: 0.5 TOG, short-sleeve bodysuit. 21–23°C: 1.0 TOG, short-sleeve bodysuit. 18–20°C: 2.5 TOG, long-sleeve bodysuit. Under 18°C: 2.5 TOG, long-sleeve bodysuit and sleepsuit.
The safe-sleep charities point to the sack makers' own charts for the TOG per room. Those charts give roughly: no sack from about 27°C, 0.5 TOG for 24–26°C, 1.0 TOG for 21–23°C, 2.5 TOG for 16–20°C, and 3.5 TOG below 16°C.
Baby Layers follows them with two differences, both on the cooler side. From 26°C it suggests no sack at all, a step lighter than the charts. And in a room below 18°C it adds a sleepsuit under the 2.5 TOG sack rather than moving to a warmer sack. The Lullaby Trust says a cooler room can take an extra layer of clothing or a warmer sleeping bag, and the app takes the layer.
Nothing, not the baby's age and not a parent's setting, ever makes the app's sack warmer than the chart. A baby is safer slightly cool than too warm.
TOG is a warmth rating: how much warmth a sleep sack or blanket holds in. The higher the number, the warmer. A warmer room calls for a lower TOG, a cooler room for a higher one.
The chart is the same from birth to a year. For sleep, the app never dresses a newborn warmer than the chart, even though it does outside.
Toddlers run warm, so the app dresses a 1–3 year old a touch lighter, as if the room were a little warmer.
A swaddled newborn wears the swaddle in the sack's place. Swaddling stops at the first signs of rolling, around three months.
No hats or loose blankets for sleep. Check the neck — warm but not sweaty means just right.