Warm room — a single light layer is enough, no sack needed.
For a baby up to a year, in a 26°C room: short-sleeve bodysuit.
A 26°C room is warm for sleep. A single short-sleeve bodysuit is usually plenty, with no sleep sack on top.
The sack makers' charts still list a 0.5 TOG sack up to about 26°C. Baby Layers goes lighter from 26°C: a baby is safer slightly cool than too warm.
In a room this warm, keep the covers off and let the air move. The Lullaby Trust says that in real heat a nappy alone can be enough.
From 26°C: a light bodysuit and no sleep sack.
26°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 25°C it changes to short-sleeve bodysuit and sleep sack, 0.5 TOG.
At 27°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. A fan can move the air — just not pointed at the baby.
0–3 months: short-sleeve bodysuit. 3–6 months: short-sleeve bodysuit. 6–12 months: short-sleeve bodysuit. 1–3 years: short-sleeve bodysuit. 0–3 months, swaddled: short-sleeve bodysuit.
Runs warm: the answer at 26°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.