Cool room — add a sleepsuit under a 2.5 TOG sack.
For a baby up to a year, in a 61°F room: long-sleeve bodysuit, sleepsuit and sleep sack (2.5 TOG).
At 61°F the room is at the cool end, so Baby Layers adds a sleepsuit under the 2.5 TOG sack, over a long-sleeve bodysuit.
Below 61°F the makers' charts move to a 3.5 TOG 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 extra layer: it stops at 2.5 TOG and adds the sleepsuit under it, never a warmer sack than the chart.
Keep it to the sack: no blanket on top, however cool the room feels.
Below 64°F: a sleepsuit under the 2.5 TOG sack.
61°F is inside the 61–68°F the Lullaby Trust recommends for a baby's room.
At 59°F the answer stays the same.
At 63°F the answer stays the same.
Every night, whatever the room: Never a hat for sleep — a baby cools through the head. A sleep sack, never a loose blanket. Safer slightly cool than too warm: nothing makes the sack warmer than the chart. No hot-water bottle or electric blanket, and not next to a radiator or heater.
0–3 months: long-sleeve bodysuit, sleepsuit and sleep sack (2.5 TOG). 3–6 months: long-sleeve bodysuit, sleepsuit and sleep sack (2.5 TOG). 6–12 months: long-sleeve bodysuit, sleepsuit and sleep sack (2.5 TOG). 1–3 years: long-sleeve bodysuit, sleepsuit and sleep sack (2.5 TOG). 0–3 months, swaddled: long-sleeve bodysuit, sleepsuit and swaddle.
Runs warm: the answer at 61°F stays the same.
Runs cool: asleep the app never goes warmer than the chart, so the answer stays the same. If the chest or neck feels cool, a layer under the sack helps.
Check the neck — warm but not sweaty means just right.