Convert tog to GSM for any filling, or answer three questions to find exactly the right tog for your bed.
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A quick reference for tog, the season it suits, and the rough GSM you'll see on the label. GSM varies by filling, so treat these as a guide.
| Tog | Best for | Feels | Down GSM | Synthetic GSM |
|---|
The short version, so you can buy with confidence.
Tog measures warmth, how well a duvet traps heat. It runs from about 1 (cool summer) to 15 (deep winter).
GSM (grams per square metre) measures the weight of the filling. More GSM usually means warmer, but only within the same filling type.
That's the catch: a 200 GSM down duvet can be as warm as a 400 GSM synthetic one, because down traps far more heat per gram. So always convert with your filling selected above.
No single formula works for every duvet, because warmth-per-gram depends on the fill power and construction. This tool uses the typical conversion ranges manufacturers publish for each filling, which gets you within the right band for shopping.
If a product lists both numbers, trust the label, use this to sanity-check it.
A 9-10.5 tog duvet is the classic all-rounder for a UK bedroom. If you want true year-round flexibility, look for an all-seasons (3-in-1) set, a 4.5 tog and a 9 tog that popper together into a 13.5 tog for winter.
Keep it lower than an adult's. A 4.0-4.5 tog duvet suits a child's bed for most of the year, with up to 7 tog in a cold winter room. For babies under 12 months use a tog-rated sleeping bag instead of a duvet.
Two duvets can share the same tog and still feel nothing alike in the hand. Tog measures warmth, not weight, so a 10.5 tog down duvet can weigh a third of a 10.5 tog hollowfibre one and keep you exactly as warm.
Down and feather trap more heat per gram, which is why they cost more, pack down smaller, and feel lighter on the bed. Synthetic fillings like microfibre and hollowfibre need more material to reach the same tog, so they're heavier and cheaper, and usually the better pick if you want something machine-washable or you're prone to allergies. Comparing two duvets? Convert both to GSM with the right filling selected above; it's the only fair way to see which is genuinely warmer.
As a rough rule for a UK bedroom: 3-4.5 tog for summer, 7-10.5 for spring and autumn, and 12-13.5 for winter. A 9 or 10.5 tog is the classic "leave it on all year" choice if you only want one, you just adjust what you wear and how the heating's set.
If your room swings a lot between seasons, an all-seasons set is the clever buy: a 4.5 tog and a 9 tog that popper together to make 13.5 for the coldest months. Three duvets in one, without the storage faff.
If you wake up kicking the duvet off, go a tog band lower than the season norm, a 7 tog in winter often beats a stuffy 13.5, especially with the heating on. Natural fillings like down and wool breathe better than synthetic too, shifting heat away instead of trapping it.
Cold sleepers can do the reverse: go up a band, and look for a higher fill-power down. It's warmer for its weight, so you stay toasty without being buried under something heavy.
Same tog, very different duvet. Here's how the common fillings stack up, so you know what you're actually paying for.
| Filling | Warmth for weight | Feel | Care | Best for |
|---|---|---|---|---|
| Goose down | Excellent | Light, fluffy | Specialist clean | Warmth without weight |
| Duck down | Very good | Light, soft | Specialist clean | Down feel, lower price |
| Feather & down | Good | Heavier, some quills | Dry clean | Natural feel on a budget |
| Microfibre | Moderate | Soft, down-like | Machine washable | Allergies, easy care |
| Hollowfibre | Lower | Firmer, springy | Machine washable | Budget, durability |
| Wool | Good | Heavier, breathable | Spot clean | Hot sleepers |
| Silk | Good | Light, drapey | Spot clean | Allergies, warm sleepers |
Buying a duvet really comes down to four things. Get these right and the rest is detail.
1. Start with your room, not the calendar. A modern, well-heated bedroom rarely drops below 16-18°C even in winter, so most people are warmer in bed than they think. If your room holds its heat, lean to the lower end of the season's tog range; if it's a draughty older house or you sleep with a window open, lean higher.
2. Factor in how you sleep. If you routinely wake up too hot and shove a leg out, drop a whole tog band, a 7 tog in winter with the heating on beats a 13.5 you fight all night. Cold sleepers do the opposite. Couples often settle on a mid tog and fine-tune with layers, or use the two-duvet trick.
3. Pick the filling for your life, not just the warmth. Down gives the most warmth for the least weight but costs more and usually needs specialist cleaning. Microfibre and hollowfibre are cheaper, machine-washable and hypoallergenic, at the cost of a bit more weight. Wool and silk breathe brilliantly and suit hot sleepers.
4. Consider an all-seasons set. If you'd rather buy once, a 3-in-1 (a 4.5 tog and a 9 tog that popper together into 13.5) covers summer, spring, autumn and winter from a single purchase.
Fill power is the quality measure for down, how many cubic inches an ounce of it fills when it lofts up. Higher fill power (say 600 to 800+) means fluffier down that traps more air, so you need less of it to reach a given tog. That's why a pricey high-fill-power down duvet can feel feather-light yet toasty, while a cheap one feels heavier for the same warmth.
It's the single biggest reason two down duvets at the same tog can cost wildly different amounts. If you're paying up for down, fill power is the number worth checking, arguably more than the tog itself.
No, tog is only about warmth, not quality. A well-made 4.5 tog summer duvet can be far nicer than a cheap 13.5. Quality comes from the filling, the fill power (for down) and the construction. Baffle-box or channel stitching stops the filling shifting into cold spots. The tog number tells you none of that.
A 9-10.5 tog all-rounder is the safe bet for a guest room, you won't know whether your visitor runs hot or cold, or which season they'll turn up. A washable synthetic filling is practical here too, since guest bedding gets used in bursts.
Roughly every five years, though a good down duvet cared for well can last longer, and a cheap synthetic less. If it's gone lumpy, lost its loft or you're waking up sniffly, it's had its day. A washable duvet protector buys you extra years.
Not necessarily warmer, but better at regulating temperature. Wool and silk wick moisture and let heat escape, so they feel less stuffy, which is why hot sleepers and people who sweat at night often prefer them to a same-tog synthetic.