The app's outside answer is for a baby sitting or lying still in a stroller or pram, so this table is its plain answer: below 10°C the outer layer a parent would wear, over a warm sweater, a hat in the cool and mittens in the cold.
What the app suggests in a stroller or pram, from the skin out, for a baby of 6–12 months:
25–30°C: short-sleeve bodysuit and sun hat. Hot — as little as possible, plus a sun hat and shade. 24°C: short-sleeve bodysuit, pants and sun hat. Warm — light single layers. 23°C: short-sleeve bodysuit, pants, socks and sun hat. Warm — light single layers. 20–22°C: short-sleeve bodysuit, pants and socks. Warm — light single layers. 19°C: long-sleeve bodysuit, pants and socks. Mild — add a light mid layer. 15–18°C: long-sleeve bodysuit, pants, socks and light sweater. Mild — add a light mid layer. 14°C: long-sleeve bodysuit, pants, socks, light sweater and warm hat. Mild — add a light mid layer. 13°C: long-sleeve bodysuit, pants, socks, warm sweater and warm hat. Cool — a mid layer plus a hat. 12°C: long-sleeve bodysuit, warm pants, socks, warm sweater and warm hat. Cool — a mid layer plus a hat. 10–11°C: long-sleeve bodysuit, warm pants, warm socks, warm sweater and warm hat. Cool — a mid layer plus a hat. 6–9°C: long-sleeve bodysuit, warm pants, warm socks, warm sweater, jacket and warm hat. Cold — full outer layer and a hat. 2–5°C: long-sleeve bodysuit, warm pants, warm socks, warm sweater, jacket, warm hat and mittens. Cold — full outer layer and a hat. 1°C: long-sleeve bodysuit, warm pants, warm socks, warm sweater, winter jacket, warm hat and mittens. Freezing — everything on, cover the hands. −15 – 0°C: long-sleeve bodysuit, warm pants, warm socks, warm sweater, winter jacket, warm hat and warm mittens. Freezing — everything on, cover the hands.
A footmuff is one of your layers, not an extra on top. It traps warmth around the legs and feet, the parts a blanket usually slips off, so with it zipped up, dress lighter underneath than you might expect. Zip it up on cold or windy days, and skip it when the air feels mild. This is Baby Layers' own step: no source puts a number on it.
In the heat, never cover the stroller or pram with a muslin or blanket: the NHS says it traps the heat. From 25°C the app suggests a single light layer and a sun hat, and shade matters more than any garment. Under six months, keep your baby out of direct sun altogether.
In a carrier your own warmth counts as a layer, so a carried baby wears one layer fewer than the table. Indoors, or in a warm car, bus or train, take the hat and outer layers off.
0–3 months, 10°C: long-sleeve bodysuit, warm pants, warm socks, warm sweater, jacket and warm hat. 3–6 months, 10°C: long-sleeve bodysuit, warm pants, warm socks, warm sweater, jacket and warm hat. 6–12 months, 10°C: long-sleeve bodysuit, warm pants, warm socks, warm sweater and warm hat. 1–3 years, 10°C: long-sleeve bodysuit, warm pants, warm socks, warm sweater and warm hat.
Indoors, or in a warm car, bus or train, take the hat and outer layers off.
Check the neck — warm but not sweaty means just right.