You open the duvet aisle and every label has a number on it: 4.5, 10.5, 13.5. That number is the tog, and it is the one thing worth reading before you pick a cover, a colour or a price. It tells you how warm the duvet will keep you, and the right number depends mostly on how cold your bedroom is.
Tog is a measure of thermal resistance: how well a layer slows the heat leaving your body. A higher number means better insulation. Duvets get that insulation from trapped air, so a lofty, well-filled duvet holds more of it. Weight is a poor guide, because a heavy duvet is not always a warm one. The tog on the label is the number to read.
Retailers usually group duvets into rough bands: around 3 to 4.5 for summer, around 9 to 10.5 for spring and autumn, and 13.5 to 15 for the coldest nights. Many households keep two duvets, one light and one warm, and swap them with the seasons. Some duvets can be joined with poppers, so a 4.5 and a 9 become a 13.5 in winter.
Set the bedroom temperature, say how warm you usually sleep, then pick a duvet. The sleeper tells you how it feels.
People often choose by calendar: thin in summer, thick in winter. What your body feels is the temperature right at the bed. A sleep-advice rule often quoted is 16 to 18 °C for adults, a little warmer for older people. A bedroom that runs at 20 °C with the heating on needs a lighter duvet than one at 15 °C with the window cracked, even in the same month.
That is why one tog never suits every home. A flat that holds its heat, a draughty rental and a top-floor room in August all want different numbers. If you share a bed with someone who runs warmer or colder than you, two single duvets with their own tog each is a calm fix, and it stops the tug of war over the covers.
Stacking is flexible. Two duvets, or a duvet and a quilt, roughly add up: a 4.5 tog under a 9 tog behaves like something near 13.5. In shoulder months you take one off. A single thick duvet cannot do that, and it is the layer you tend to throw off at 3 a.m.
Tog is only half the choice. Duvets come in set sizes, and a cover that does not fit hangs badly. Measure your mattress width, length and depth, then check the duvet and cover sizes against them. A cover that is too big bunches up, and one that is too small pulls the filling thin at the corners. If you are not sure about the room itself, put a cheap thermometer by the bed for a few nights and read the number, rather than guessing.
Cosy is not about buying more. If your duvet is fine but the room is cold, a thicker curtain or a draught excluder may help more than a new one.