Bedding
Duvet Warmth Explained: What Tog Means and How to Read Fill Power and Fill Weight
Tog rates how a duvet holds heat; fill power and fill weight describe the fill. Here is how the three numbers fit together, and how to match a duvet to the season.
By the Snug Casa editors · ·
The short answer
Tog measures how well a duvet holds heat: the higher the number, the less warmth escapes through it. Down and feather duvets often skip tog entirely and print two other numbers instead — fill power and fill weight. All three describe the same thing: how warm the duvet sleeps. The practical method is to match tog to your bedroom’s coldest months, then fine-tune with blankets instead of buying one duvet for every season.
What tog actually means
Tog is a unit of thermal resistance, on a scale that runs from about 1 to 15. A 1 tog duvet is the thinnest summer layer; a 15 tog is deep-winter weight. Two details matter more than the scale itself. First, tog rates the duvet, not the bed — the same 10.5 tog sleeps differently in a 60°F room than a 70°F one. Second, a high tog does not mean heavy. Warmth comes from trapped air, and a lofty duvet can insulate better than a heavy one.
Tog by season
The usual conventions:
- 2.5–4.5 tog: warm sleepers, heated bedrooms, and summer.
- 7–10.5 tog: spring, fall, and the single-duvet compromise most households land on.
- 12–15 tog: cold bedrooms and genuine winter.
Many all-season duvets are a pair — a 4.5 and a 9 that clip into roughly a 13.5 for winter and separate for summer. That two-duvet system beats one heavy duvet kicked off by 2 a.m. If your bedroom stays in the mid-60s year-round, a mid-range tog plus a blanket in January usually beats a 13.5 tog you only need six weeks a year. Our guide to layering a bed covers how to stack those extras.
Fill power and fill weight, translated
For down and feather fills, read two numbers on the label. Fill power is loft: the volume, in cubic inches, that one ounce of fill expands to fill. Six hundred and up is good; 700 to 800 is the high end — the same warmth at a lower weight. Fill weight is the total amount of fill, usually in grams — how much material is actually inside.
Warmth comes from the combination. High fill power with high fill weight is the warmest build; high fill power with low fill weight insulates without pinning you down — better for warm rooms. A label that lists only grams tells you quantity, not quality. Synthetic fills skip fill power altogether; there, compare grams per square meter and the tog rating instead. Our duvet inserts roundup shows how these numbers look on real labels.
The duvet cover changes the math
A cover is one more layer, and it is not neutral. A crisp cotton percale adds very little; flannel or a dense sateen can add the equivalent of a tog or two. That is useful leverage: a 10.5 tog under a flannel cover can carry you through a cold snap, then shed the cover when it passes. Breathability matters too — linen releases heat well, which is why linen covers pair nicely with warmer duvets. The trade-off between the two fabrics is covered in linen vs. cotton sheets. A cover also keeps the duvet clean longer — most duvets only get washed occasionally.
The one mistake to skip
Buying a single 13.5 tog and living under it all year. The cheaper fix: one mid-range duvet, one blanket, one cover you can swap by season.
FAQ
Does a higher tog mean a heavier duvet? +
No. Tog rates thermal resistance, not weight — warmth comes from trapped air, not mass. A lofty down duvet with a 700 fill power can sleep warmer than a dense synthetic duvet of the same weight, which is why the two numbers should be read together.
What tog works for a single year-round duvet? +
The usual compromise is 7 to 10.5 tog, which covers most of the year in a heated bedroom. Add a blanket for the coldest weeks instead of sleeping under a 13.5 tog all summer. If your bedroom swings hard between seasons, the two-duvet clip system is the better fix.
Does a duvet cover change how warm the duvet feels? +
Yes. A crisp percale cover adds almost nothing, but flannel or a dense sateen can add the equivalent of a tog or two. That makes the cover a useful seasonal lever: a mid-range duvet plus a flannel cover can carry a cold snap without a dedicated winter duvet.
