Join two duvets and their togs add up. Enter both to see the combined warmth, and find which pairs make a 13.5, 15 or 18 tog winter duvet.
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Tog is a measure of thermal resistance, which is just how well a duvet slows your body heat escaping. When you lay one duvet on another, the two resistances stack, so the warmth adds up. That is the whole idea behind an all-seasons or 3-in-1 set. You get a light duvet for summer, a medium one for spring and autumn, and the two joined together for winter.
It is the same maths retailers use. Marks & Spencer lists a 9 tog and a 4.5 tog that fasten together to make 13.5 tog, and Slumberdown sells a 10.5 tog and a 4.5 tog as a 15 tog set. Those totals are the sellers' own figures, and two separate duvets stacked at home have no combined rating at all. So treat the result above as an estimate, not a tested reading.
Yes, within reason. Stack any two duvets and the togs still roughly add up, so a spare 4.5 tog summer duvet over your 9 tog gets you to about 13.5 tog for a cold snap. The catch is they will slide apart without poppers or corner ties, and a loose top layer slips off in the night. For a tidy result, a matched all-seasons set is worth it. Whatever you use, both duvets should be the same size as your bed, or you will get cold gaps down the sides.
More tog is not always better. Above about 15 tog, a duvet can feel stifling in a centrally heated bedroom, and an over-warm bed breaks up your sleep. If you wake up hot or sweaty, unclip the heavier layer and sleep under the lighter duvet on its own. For babies and toddlers the rules are stricter and duvets are not used at all under one year, so see our baby sleeping bag tog guide instead.
The most common UK pairings and what each combined weight suits. Any two togs add together the same way, so use the calculator above for your own duvets.
| Duvet 1 | Duvet 2 | Combined | Best for |
|---|---|---|---|
| 4.5 | 4.5 | 9 tog | All year, average room |
| 4.5 | 9 | 13.5 tog | Winter, the classic set |
| 4.5 | 10.5 | 15 tog | Cold or draughty rooms |
| 7 | 7 | 14 tog | Winter |
| 9 | 9 | 18 tog | Very cold rooms, can be too warm with heating |
| 4.5 | 13.5 | 18 tog | Very cold rooms |
Do tog ratings add up? Yes. Two layers stack their thermal resistance, so 4.5 plus 9 is about 13.5 tog.
What two togs make 13.5 tog? A 4.5 tog duvet and a 9 tog duvet joined together. For 15 tog, pair a 4.5 with a 10.5.
Can I combine two duvets I already own? Yes, the togs still roughly add up, but without ties the layers slip, so a matched all-seasons set is neater. Keep both the same size as your bed.
Can a combined duvet be too hot? Above about 15 tog it can overheat a warm room. Drop to the lighter layer if you wake up sweating.
Do both duvets need to be the same size? Yes, both should match your bed size, so a double bed needs two double duvets.