Buying a duvet can be rather complicated. As well as selecting the duvet material and size, you’ll also want to pay attention to things like fill power, loft, tog, weight, and warmth. Here’s a primer on what all these terms mean and why they matter when choosing a duvet.
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What is duvet fill power?
Fill power is a measure of the amount of space (volume) taken up by down in a duvet. I’ve also seen this called the loft or fluffiness of the duvet, though this can be a bit misleading as a duvet can have high fill power without actually being ‘lofty’, i.e., tall. More on this in a moment.
Fill power is only used for down and there’s no equivalent single measure for the insulating capacity of other types of fiber fill (such as cotton, kapok, polyester, or wool).
Why does fill power matter?
In general, high fill power is a great indicator of quality and insulating capacity. I’ve slept under some very fluffy, lofty down duvets and it’s incredible how light they can feel while still being super cozy.
A duvet with high fill power is also very breathable as the loft comes from down with lots of air chambers. This means you’re also less likely to overheat or get sweaty as the duvet can disperse heat and humidity effectively.
High fill power duvets are better able to fluff up too. A quick shake in the morning and a quality duvet can bounce back from its night’s work. Fluffing your duvet (and pillows) each morning can also help to keep them in good shape generally, so they last longer.
Here’s a rough guide to which fill power works for summer, winter, and all year round:
| Fill power | Season |
|---|---|
| 400 or below | Lightweight, best for summer or very hot sleepers |
| 400 to 600 | Good all year round! |
| 600 to 800 | Best for colder seasons and climates |
| 800 and up | Best for winter and very cold sleepers |
How do you measure fill power?
Fill power ranges from 450 to 900 for down and is written as CUIN, or cubic inches per ounce.
Most often, fill power information is only given for higher end down duvets. Products with less fill power won’t advertise the fact that they provide little insulation.
Typically, high fill power relates to a duvet with 600 CUIN or above. The average duvet has fill power of around 450-500.
At the highest end, a duvet with fill power above 800 provides incredible insulation and feels super fluffy and lofty. Such duvets also come with a high price tag, though, and those made with a lot of lofty down are almost always going to prove too hot for most sleepers in most climates.
Interestingly, I learned recently that it’s rare for duck down to have a fill power higher than 750. For that, you’ll need goose down.
Who measures fill power? Is it accurate?
Having never owned a down duvet (being vegan and all), I had to dig into the research to learn about down. Honestly, it’s fascinating. And, unsurprisingly, as with thread counts, manufacturers have ways to manipulate fill power measurements.
I learned that there’s no global standard for measuring fill power, although most down for technical outdoor wear uses a rating based on the Lorch Fill Power standard per the International Down and Feather Laboratory (IDFL) in Switzerland.
In the U.S., it’s more common, however, to see fill power based on a similar standard. This U.S. standard gives higher numbers, though, than the European measure.
For instance, the exact same down could have the following fill power depending on the standard the manufacturer cites:
| European standard fill power | U.S. standard fill power |
|---|---|
| 500-600 | 530-640 |
| 600-700 | 640-750 |
| 700-800 | 750-860 |
| 800+ | 860+ |
Naturally, it’s rare for bedding companies to state which measure they’re using. Assume, then, that for down duvets sold in North America, the fill power is the U.S. standard, given that this makes the product look better.
Like greenwashing, some down duvet manufacturers effectively power-wash their duvets. Watch out for marketing that uses the U.S. measure to inflate the fill power while also highlighting the use of ‘European’ down. The latter suggests to consumers that the EU standard applies, not the U.S. standard.
What is fill weight for a duvet?
To further complicate things, a duvet may also have a fill weight as well as fill power. The weight is a measure of grams per square meter (gsm) and tells you how heavy the duvet is, as well as how soft or firm it will feel.
Note that the gsm figure given for duvets is not a measure of the heaviness of the entire duvet. If the weight of the full product matters to you, look for that specific measurement in addition to gsm.
Fill weight doesn’t necessarily indicate how warm a duvet will feel. In fact, you could buy a duvet with a high fill weight but low fill power and find the duvet heavy but poor at keeping you cozy.
In contrast, low fill weight but high fill power can mean a higher quality duvet that is light, airy, and insulating.
Is fill weight the same as tog?
Fill weight is not the same as tog. I grew up in the UK and remember the tog scale being very common, both for duvets and for things like baby snowsuits and sleep sacks. The tog scale ranges from 1 to 15, with higher numbers indicating greater warmth.
The tog scale doesn’t just apply to down and feather products, it can also show up on synthetic duvets and clothing and on those made with wool, cotton, or even hemp.
Tog doesn’t apply to silk. As such, if you’re looking for a silk duvet, you’ll want to check for the fill weight in gsm. That said, you can use the following as rough equivalents for silk gsm to tog ratings:
- 250 gsm – 2-4 tog
- 400 gsm – 7-9 tog
- 600 gsm – 10-13 tog.
For duvets, a tog rating of 1-7 is usually good for younger children and for summer, or for hot sleepers all year round. For winter, a tog rating of 10.5 or more is usually best to stay warm.
If you just want to buy one duvet though, a tog rating of 10 is a good bet, unless you live somewhere with extreme temperatures. In winter, you can always add a blanket if the 10 tog duvet feels too cold. In summer, you can always throw off the duvet and just use a top sheet.
Look, too, for an all-season duvet, sometimes called a 3-in-1 duvet. This typically comprises two duvets, one with a low tog and one with a slightly higher tog. You can use the lower (say, 4.5) tog duvet insert for summer and the higher (say, 9) tog insert for spring and fall. In winter, you clip the two duvets together inside the same duvet cover for a warmer, effectively 13.5 tog duvet.
The all-season, two duvets approach also means you have a convenient spare duvet at certain points of the year, in case of house guests. This helps to save money, resources, and space as you won’t need an extra duvet in storage.
Putting it all together: fill power, fill weight, tog, and more
After looking through hundreds of listings for duvets, here’s what I’ve come to expect for the fill weight, tog, and use of duvets:
| Fill power (EU standard) | Fill weight | Tog | Best for | Typical weight for a King size duvet |
|---|---|---|---|---|
| 400 or less | 80 gsm | 2.5 | Hot summer; for toddlers; very hot sleepers all year round | 400 g |
| 400-500 | 110 gsm | 4.5 | Regular summer; older children; hot sleepers all year round | 565 g |
| 500-600 | 150 gsm | 7 | Spring and fall; average sleepers | 750 g |
| 600-800 | 200 gsm | 10.5 | Winter; colder sleepers | 1500 g |
| 800 and up | 350 gsm | 15 | Extreme cold! | 2000 g |
Bear in mind, though, that the fill power, weight, and tog don’t always align as above. In fact, some lighter duvets may actually be warmer!
It seems counterintuitive but a lighter duvet can actually be warmer than a heavier duvet. This is because a high fill power duvet with less down overall can provide much better insulation than a heavier duvet made with more down with lower fill power.
For optimal sleep, you want the temperature in your bedroom to be around 65 degrees Fahrenheit or 18 Celsius. If you don’t have good heating and air conditioning, however, you may find that you have greater need for a winter and a summer duvet (or, ideally, an all-season one that clips together).
In contrast, very well insulated homes (which we love for energy efficiency!) or ones with very effective heating and A/C may mean you never need a duvet with a tog rating above 9.
Finally, note that while quality products are best in terms of sustainability, this doesn’t always mean you should choose a high fill power, heavy, or high tog duvet. Everyone is different and what works for you may not work for someone else.
For instance, you may like more weight on you while you sleep, but be a hot sleeper. A low fill power, high fill weight duvet could be perfect for you, but the weight and poorer insulation could prove uncomfortable to someone who sleeps cold but dislikes heavy bedding.