A 1.5 TOG sack sits between the 1.0 and 2.5 TOG sacks the app names. The Baby Layers app doesn't give it as its answer, but for a room of 19–21°C it shows that a 1.5 TOG sack works too.
21°C: short-sleeve bodysuit. 19–20°C: long-sleeve bodysuit.
The app's answer names one of three sacks, 0.5, 1.0 or 2.5 TOG, the steps of the makers' charts the safe-sleep charities point to. A 1.5 TOG sack sits between the 1.0 and the 2.5.
For a room of 19–21°C, the answer still names its own sack, and a line under it says a 1.5 TOG sack works too. Those rooms stay on the cooler side of the gap, because a baby is safer slightly cool than too warm.
It never comes up where the app keeps the 2.5 TOG sack and adds a sleepsuit under it, which for a baby up to a year is below 18°C.
Another number on the label, like 0.2 or 1.2? Go by the nearest row of the TOG chart, and pick the lighter sack if unsure.
The sack makers' charts differ most on the ratings in between: some brands give 1.5 TOG a row of its own, and others don't make it at all. The safe-sleep charities point to those charts, and your sack's own label wins over any table.
0–3 months: 19–21°C. 3–6 months: 19–21°C. 6–12 months: 19–21°C. 1–3 years: 18–20°C.
Warmer than 21°C: a 1.0 TOG sack over short-sleeve bodysuit.
Cooler than 19°C: a 2.5 TOG sack over long-sleeve bodysuit.
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.
Check the neck — warm but not sweaty means just right.