A duvet’s tog number measures how well it traps heat, not how thick or heavy it feels in your hands. The rating comes from “thermal overall grade”, and it runs from a bare 1 tog at the light end to beyond 16 tog at the warmest.
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That distinction matters because two duvets of the same tog can feel completely different to hold. It’s why a slim-looking duvet can rate the same as a bulkier one, and why price and weight are poor guides to how warm a night you’ll actually get.
How a lab actually measures tog
Testers lay the duvet flat and apply heat to one side, usually with a hot plate underneath it. They then record the temperature difference between the two sides, which shows how much heat has passed through the material. They repeat the measurement at several points across the surface, then average the results into the single tog figure printed on the packet.
That method is why tog is a genuine measurement of insulation rather than a marketing label. A duvet stuffed with down and one stuffed with microfibre can carry the same tog rating if they hold heat equally well, whatever they weigh or feel like under a hand.
What the numbers mean in practice
Most UK duvets sit between 4.5 and 13.5 tog, though you can find a 1-tog summer quilt or a 16.5-tog winter one without much searching. House Beautiful sets out what each common rating suits:
- 4.5 tog: aimed at people who suffer night sweats, keeping you cool in hot weather.
- 7 tog: a touch more warmth, suited to an air-conditioned room or year-round use layered with a blanket in winter.
- 10.5 tog: warm enough for cold nights without becoming unbearable when the weather turns.
- 13.5 tog: cold-weather bedding for the depths of winter, best rotated out once temperatures rise.
- 15 tog: the highest common rating, meant for very cold rooms or homes with no reliable heating.
Why the same tog can feel different room to room
James Higgins, chief executive of the bedding company Ethical Bedding, points out that a duvet’s effective warmth also depends on where it’s used. A small room with a single window traps heat more easily than a larger, draughtier one, so he suggests a 7-tog duvet rather than a 10.5-tog one for a snug bedroom.
The material of your sheets plays a part too. Higgins notes that eucalyptus bedding regulates temperature while staying soft, which suits someone using a heavier-tog duvet who doesn’t want to overheat. Linen, by contrast, has a loose weave that lets air flow through, so he suggests cold sleepers swap it for eucalyptus.
When people tend to switch
Higgins also has advice on when to change from a lighter duvet to a heavier one. He points to an average temperature drop of around 12 degrees Celsius between September and October in the UK. Between August and September the drop is only one degree. He says that makes October the month the change is felt. On that basis, Ethical Bedding advised switching in late September, ahead of the sharper autumn drop.
That advice is about timing a swap, not about what the tog figure itself measures. The rating stays a fixed reading of how much heat the duvet traps, taken once in a lab. What changes with the season is how much of that trapped heat a sleeper actually wants.
None of the household advice changes what tog measures. It only means the number on the label is a starting point, not a guarantee, once your room, your sheets and your own tolerance for heat get involved.
