Duvet specs can look like bedtime maths. Good news: fill power is much simpler than it sounds. It tells you how lofty down is, not how warm the whole duvet will be by itself. A higher number usually means the down traps more air, so you get cosy insulation with less weight. The smart move is to read fill power together with fill weight and tog.
Why this number matters more than you think
Fill power measures how much space a small amount of down takes up once it has fully expanded in controlled test conditions. It is usually shown as CUIN, short for cubic inches. So if one ounce of down expands to 700 cubic inches, that duvet has 700 fill power.
More space means more loft. More loft means more air pockets. And those air pockets are what help hold warmth. That is why higher fill power down usually feels lighter, fluffier and more insulating for its weight. You may also see the word loft used around duvets. In shopper terms, loft and fill power are closely linked.
One important catch: fill power mainly applies to down-filled duvets. If you are comparing synthetic duvets, material type, construction and tog are usually more useful than fill power.
Don't confuse fill power with fill weight or tog
These three terms work together, but they are not the same thing.
Fill power tells you the loft and efficiency of the down. Fill weight tells you how much filling is actually inside the duvet. Tog tells you how warm the finished duvet is designed to feel.
That means a heavier duvet is not automatically warmer, and a higher fill power duvet is not automatically hotter. A 600 fill power duvet can feel very warm if it contains a lot of filling. An 800 fill power duvet can still feel relatively light, or even cooler, if it uses less filling and has a lower tog. Higher fill power mostly improves the warmth-to-weight ratio. In plain English: warmth without the brick-on-your-body feeling. Want to fine-tune warmth across the year? How to adjust your duvet for every season.
Quick guide to common fill power ranges
Need a quick decoder? Start here.
| Fill power | What it usually feels like | Typical shopper fit |
|---|---|---|
| 400-500 | Less lofty and usually a bit weightier for the same warmth | Entry-level down duvets and people who like more substance |
| 550-650 | Good loft with a strong balance of comfort, warmth and value | A reliable all-round choice for many sleepers |
| 700-800+ | Very lofty, light and efficient at insulating | Premium-feel duvets for people who want warmth without extra weight |
Treat these ranges as a guide, not a final verdict. The finished feel still depends on fill weight, tog, the down-to-feather mix and the duvet's construction.
How fill power is measured and what affects it
In lab testing, a cleaned and dried down sample is allowed to loft up in a cylinder under standard conditions. The amount of space it occupies becomes the fill power rating in CUIN. Bigger, stronger down clusters usually create higher numbers because they trap air more effectively.
The rating is influenced by the quality and size of the down clusters, the percentage of down compared with feathers, and how well the filling is processed and distributed through the duvet. Standards can vary slightly between countries, so compare like with like whenever you can.
How to keep a down duvet lofty
Even great down can lose bounce if it stays damp, compressed or badly cared for. A few simple habits help preserve loft.
- Air the duvet regularly so moisture can escape.
- Shake it gently to redistribute the filling.
- Avoid storing it tightly compressed for long periods.
- Follow the care label, and if tumble drying is allowed, dry it thoroughly so the down can re-expand properly.
Frequently asked questions about duvet fill power
What does 700 fill power mean in a duvet?
It means one ounce of that down expands to about 700 cubic inches in standard test conditions. In real-life terms, 700 fill power usually points to lofty, light down with strong insulation for its weight.
Is 800 fill down warmer than 600?
Not automatically. 800 fill down is more efficient at trapping air, so it can deliver similar warmth with less filling. But the actual warmth of the duvet still depends on fill weight, tog and construction.
Is 650 fill power good?
Yes. 650 fill power is generally considered very good for a down duvet. It usually gives you a noticeable step up in loft and lightness without jumping straight into the highest price bracket.
What is the best quality filling for a duvet?
That depends on what matters most to you. If you want the lightest, fluffiest warmth, high-quality down is hard to beat. If you prefer a washable duvet or a non-down option, focus on materials, breathability and tog instead, because fill power will not tell the full story.
Is a 7.5 tog duvet warm enough for winter?
Sometimes, but not for everyone. A 7.5 tog duvet often suits milder bedrooms, hot sleepers or autumn use. If your room runs cool or you like a cosier feel in winter, many people prefer a winter duvet with a higher tog. If you overheat easily, see Best duvet for night sweats.
When you are choosing a duvet, start with tog for overall warmth. Then use fill power to judge how light, fluffy and premium that warmth will feel. For a step-by-step process, read the 5-step guide to choosing a duvet. After that, pick the right dimensions with our Duvet size guide. Ready to compare options? Shop duvets.
