Every answer the app and this site give comes from the same set of rules. Here they all are, each with where it comes from.
Three places, one dial
You pick where your baby is, outside, at home or asleep, and set the temperature. Outside it is the weather, at home and for sleep the room. The app answers with a list of layers, from the skin out, and each layer is a starting point with alternatives that work as well.
One piece at a time
Each piece of clothing has its own temperature: socks come on below one, a sweater below another, a jacket below a third. So as the temperature drops, the outfit changes one piece at a time rather than jumping from summer to winter.
Where the guidance gives a number, like the TOG chart for sleep, the app uses it. Where it gives the idea but no degree, like "one layer more than you", the degree is Baby Layers' own step between the numbers it does give, and the app says so.
Age, and babies who run warm or cool
Newborns lose heat faster than adults, so outside and at home the app dresses them as if it were a little colder, easing off by six months. Toddlers run warm and move a lot, so a touch lighter.
A parent can say their baby runs warm or cool, and the answer moves a touch lighter or warmer. Asleep, a warmer adjustment is left out: a baby is safer slightly cool than too warm.
Wind, rain and sun
Outside, the app works out how the day feels, not just the number: wind and rain make it feel colder, sunshine warmer, and it dresses for that. In rain it adds a jacket to keep the layers dry. In a carrier your own warmth counts as a layer, and in a car seat the coat comes off and a blanket goes over the straps.
Sleep follows the chart
For sleep, the sack follows the room at every age, from the sack makers' TOG charts, which the safe-sleep charities point to. The app never suggests a sack warmer than the chart: in a cool room it adds a layer under a 2.5 TOG sack instead, and in a warm room it drops the sack a step earlier than the charts.
Never a hat for sleep, and nothing loose in the cot. These hold at every temperature.
What it doesn't do
The app gives suggestions, never verdicts about a baby. It is general guidance, not medical advice, and it doesn't know your baby: check the neck, warm but not sweaty means just right. It asks for no account, and for your location only if you turn on the forecast, as a rough area.
Every rule
Sleep
From 26°C: a light bodysuit and no sleep sack. A 0.5 TOG sack for a room of 24–25°C. A 1.0 TOG sack for a room of 21–23°C. A 2.5 TOG sack over a long-sleeve bodysuit for a room of 18–20°C. Below 18°C: a sleepsuit under the 2.5 TOG sack. A swaddled newborn wears the swaddle in the sack's place — until the first signs of rolling, around three months. 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.
At home
No hat indoors — a baby cools through the head. Short sleeves from 21°C, covered arms below. Pants below 21°C, lined ones below 15°C. Socks below 24°C, warm ones below 15°C. A sweater below 18°C, a warm one below 15°C.
Outside
One light layer from 20°C, covered arms below. Pants below 25°C (toddlers keep shorts to 21°C), lined ones below 13°C. Socks below 24°C, warm ones below 12°C. A sweater below 19°C, a warm one below 14°C. Below 10°C: one layer more than you'd wear. In rain below 20°C, a jacket keeps the wet off. Below 2°C, a winter jacket. A sun hat from 23°C, or from 20°C in sunshine. A warm hat outdoors below 15°C. Mittens below 6°C, warm ones below 1°C. No coat or snowsuit under the harness — below 10°C, a blanket over the straps instead. In a carrier on a dry day below 14°C, a blanket over the carrier instead of the jacket.
Checks shown with an answer
Under six months, keep your baby out of direct sun — shade, and a wide-brim hat. Never cover the pram with a muslin or blanket in the heat — it traps the heat. Indoors, or in a warm car, bus or train, take the hat and outer layers off. No hot-water bottle or electric blanket, and not next to a radiator or heater. A fan can move the air — just not pointed at the baby.
The site runs the same rules as the app, line for line, and is tested against the app's own answers.