Bedroom temperature:
17 °C
You usually sleep
Cold
Average
Warm
Your duvet (tog)
Add a light 4.5 tog duvet or quilt on top
Side view of a bed with a sleeper under a duvet
The duvet gets thicker as the tog rises, and the sleeper's colour shows whether they are cold, comfortable or too warm.
Rule of thumb, not a lab result: it follows the usual retail guidance, with about 1 tog more for every degree colder than 18 °C. Everyone's body, pyjamas and mattress differ, so treat it as a starting point.