Fill power explained
Updated August 2026
Fill power is the most quoted and least understood number in bedding. It is a measure of loft per ounce — a quality rating for the down itself — and it does not tell you how warm a comforter is. Five checks, then the arithmetic.
1. What the number measures
Fill power is the volume, in cubic inches, that one ounce of down expands to fill when it is allowed to loft freely under a light weighted disc in a standardized cylinder. An 800 fill power sample means one ounce reached 800 cubic inches. That is the entire definition. It says nothing about how much down is in your comforter, only about the quality of each ounce.
Higher fill power comes from larger, more mature clusters — typically from older birds in colder climates. Larger clusters trap more air per gram, so they insulate more efficiently by weight. Down alternative has no fill power rating; polyester fiber is measured by GSM instead.
2. Worked example: two 40 oz comforters
Loft is fill power multiplied by fill weight. Run the multiplication on a queen comforter holding 40 oz of down.
| Comforter | Fill weight | Fill power | Total loft | Result |
|---|---|---|---|---|
| A | 40 oz | 600 | 40 × 600 = 24,000 in³ | Baseline warmth |
| B | 40 oz | 800 | 40 × 800 = 32,000 in³ | 33% more loft — warmer, but bulkier |
| C | 30 oz | 800 | 30 × 800 = 24,000 in³ | Same loft and warmth as A, 10 oz lighter |
Comforter C is the one that explains why people pay for high fill power. To reach the same 24,000 cubic inches of trapped air that comforter A needs 40 oz to produce, 800 fill power down only needs 24,000 ÷ 800 = 30 oz. That is a quarter less mass over your body for identical warmth. The result is a comforter that drapes around your shoulders instead of tenting over them, and one that packs down noticeably smaller.
Read the table the other way and the trap becomes obvious: comforter B is not "the same but better." At equal fill weight, more fill power means more loft, which means a thicker and hotter comforter. Buyers who upgrade fill power without reducing fill weight often end up sweating.
3. Fill weight is the warmth spec
Warmth tracks total loft, and total loft is set by both numbers together. Since fill power is fixed by the down you bought, the lever manufacturers actually pull is fill weight. On a queen: 20–30 oz is a summer weight, 35–45 oz is all-season, 50 oz and above is winter. A 700 fill power comforter with 22 oz of down is a summer blanket; a 550 fill power comforter with 55 oz is a winter slab. Fill power alone predicts neither.
Construction can cancel the advantage entirely. High fill power down compressed inside sewn-through channels never reaches the loft it was rated for, which is why baffle boxes matter above roughly 35 oz — the geometry is laid out in baffle box vs. sewn-through.
4. Which ratings are worth paying for
| Fill power | Verdict | Who it suits |
|---|---|---|
| Below 550 | Skip | Usually immature cluster cut with crushed feather |
| 550–600 | Good value | Guest rooms, cool climates, tight budgets |
| 600–700 | Very good — the sweet spot | Most buyers, best price per unit of loft |
| 700–800 | Excellent | People who want warmth without weight |
| 800+ | Specialty | Diminishing returns indoors; priced for expedition gear |
Between 600 and 700, the cost per cubic inch of loft is at its lowest. Above 800 you pay a steep premium for a difference you feel mainly when you pick the comforter up. If you are choosing between a fill power upgrade and a construction upgrade at the same price, take the baffle boxes.
5. How the number gets inflated
Fill power is measured under two different standards, and the older European (Lorch) method typically reports 5–10% higher than the US (IDFB) method on the same down. A "750 EU" sample is roughly 700 by US testing. Sellers also quote the rating of the best material in a blend, so "800 fill power goose down" can legally describe a comforter whose fill is 75% down and 25% feather — the feather has no fill power at all. Insist on both the fill power and the down-to-feather ratio; 80/20 or better is the line worth holding. Whether the down is worth choosing over polyester at all is a separate question, and certification claims that ride alongside these numbers are unpacked in our certification guide.
Red flags
- Fill power quoted with no fill weight. Half a spec, and the half that flatters the seller.
- Fill power on a down-alternative product. The rating does not apply to polyester.
- "Up to 800 fill power." The word "up to" is doing all the work.
- No down-to-feather ratio. Undisclosed usually means well under 80/20.
- Fill power above 900 at a supermarket price. That material genuinely costs what it costs.
Where to go next
Run a listing through the full sequence in the comforter buying guide, or jump to shortlists: down comforters · down alternative · winter weights · lightweight. Already own one that has gone flat? Start with how to fluff a comforter. More spec breakdowns in all guides.