One more layer than you, several thin layers rather than one thick one, and a hat and mittens outdoors.
The NHS and the American Academy of Pediatrics both suggest one more layer than an adult in the cold, and the AAP adds several thin layers, a hat and mittens. It is how the app builds a winter outfit, one piece at a time as the temperature drops.
Outside, degree by degree
For a baby of 6–12 months on a still, dry day. A jacket comes on below 10°C (50°F), when a parent would wear a coat, and it is a winter one below 2°C (36°F). Mittens join below 6°C (43°F).
Newborns
Newborns hold heat less well, so the app dresses them as if it were a little colder, easing off by six months. A toddler who runs about is dressed a touch lighter.
Wind and rain
Wind and damp pull warmth away faster than a still, dry day at the same temperature. The app dresses for how it feels, and in rain it adds a jacket to keep the layers dry.
In the car
No puffy coat or snowsuit under car seat straps. Buckle the harness snug over thin layers, then lay the coat or a blanket over the top.
Coming inside
Take the hat and outer layers off indoors, or in a warm car, bus or train. The NHS and the Lullaby Trust both say so: a baby cools through the head.
Winter nights
Match the sleep sack to the room, not the season. No hat for sleep and no loose blankets, however cold it is outside. If the room is cool, a layer under the sack helps, and no hot-water bottle or electric blanket, and not next to a radiator or heater.