A 1.0 TOG sack over a bodysuit is a good start.
For a baby up to a year, in a 21°C room: short-sleeve bodysuit and sleep sack (1.0 TOG).
At 21°C a 1.0 TOG sack over a short-sleeve bodysuit is a good start. If the bodysuit you have is long-sleeved, that works too.
The makers' charts give 1.0 TOG for a 21–23°C room, and Baby Layers follows them.
This is a little warmer than the 16–20°C the Lullaby Trust recommends for a baby's room, so one light layer under the sack is enough.
A 1.0 TOG sack for a room of 21–23°C.
21°C is warmer than the 16–20°C the Lullaby Trust recommends for a baby's room. Lighter layers help, and cooling the room a little, if you can, helps most.
At 20°C it changes to long-sleeve bodysuit and sleep sack, 2.5 TOG.
At 22°C 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.
0–3 months: short-sleeve bodysuit and sleep sack (1.0 TOG). 3–6 months: short-sleeve bodysuit and sleep sack (1.0 TOG). 6–12 months: short-sleeve bodysuit and sleep sack (1.0 TOG). 1–3 years: short-sleeve bodysuit and sleep sack (1.0 TOG). 0–3 months, swaddled: short-sleeve bodysuit and swaddle.
Runs warm: the answer at 21°C 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.