The Lullaby Trust recommends a room of 16–20°C (61–68°F) for a baby's sleep.
It is a range, not a number to hit exactly. What your baby wears is what adjusts to the room: the warmer the room, the less your baby likely needs.
26°C (79°F) and up: just a short-sleeve bodysuit, no sack. 24–25°C (75–77°F): a short-sleeve bodysuit under a 0.5 TOG sack. 21–23°C (70–73°F): a bodysuit under a 1.0 TOG sack. 18–20°C (64–68°F): a long-sleeve bodysuit under a 2.5 TOG sack. Under 18°C (64°F): a sleepsuit plus a vest under a 2.5 TOG sack.
These are starting points, not rules. The back of the neck tells you if you've got it right.
Why not warmer
The NHS says overheating raises the risk of sudden infant death syndrome. A baby is safer slightly cool than too warm, so the app never suggests a warmer sack than the makers' chart for the room, whatever the baby's age.
If the room is warm
Fewer layers: a short-sleeve bodysuit under a light sack, and no sack at all from 26°C (79°F).
A fan can move the air in a warm room, as long as it isn't pointed at the baby, Red Nose says. Head and face stay uncovered.
If the room is cool
Below 16°C (61°F), the Lullaby Trust says to add an extra layer of clothing or choose a warmer sleeping bag. The app adds a layer: a sleepsuit under the 2.5 TOG sack below 18°C (64°F).
No hot-water bottle or electric blanket, and the cot not next to a radiator, heater or fire, the NHS says. Never a blanket on top of the sack.
Room by room, degree by degree
The whole table the app uses, with a page for every degree.