Key takeaways
- Fill power measures down quality/loft: the cubic inches one ounce of down fills.
- Higher fill power = loftier down = warmer for its weight and more packable.
- Common ratings: ~550 (entry), 650–750 (mid), 800–900+ (premium).
- Fill power is down QUALITY; fill weight is the AMOUNT of down — both affect total warmth.
What fill power measures
Fill power is a measure of the quality and loft of down, defined as the volume in cubic inches that one ounce of down fills when lofted. An 800-fill-power down lofts to 800 cubic inches per ounce. Higher fill power means higher-quality down that traps more air per unit of weight.
Why it matters
Loftier down traps more warm air for less weight, so a higher fill power means warmer, lighter, and more compressible gear. An 800-fill jacket can be warmer and lighter than a 650-fill one using the same weight of down — which is why premium down products tout high fill-power numbers. Common ratings:
- ~550 — entry level.
- 650–750 — mid-range, good value.
- 800–900+ — premium, lightest and most packable.
Comparing two down jackets, a buyer notes one is 800-fill and the other 650-fill at similar warmth — and realizes the 800-fill version achieves it with less down, making it lighter and more packable, worth the higher price for a weight-conscious trip.
Fill power vs fill weight
Crucially, fill power is down quality; fill weight is the amount of down. Both drive total warmth: high fill power makes the down efficient, but you still need adequate fill weight. A jacket with high fill power but little fill weight packs tiny but won’t be very warm. Both apply to down jackets and sleeping bags (especially mummy bags).
The bottom line
Fill power rates the quality and loft of down — how many cubic inches an ounce fills — so higher fill power means warmer for the weight and more packable. It's the headline number on premium down gear (550 entry to 900+ premium). Just pair it with fill weight in your thinking: fill power is the down's efficiency; fill weight is how much is actually in there.
Frequently asked questions
What is fill power?
Fill power measures the quality and loft of down — specifically, the number of cubic inches that one ounce of the down fills when allowed to loft. An 800-fill-power down lofts to 800 cubic inches per ounce. Higher fill power means the down is higher quality and traps more air per unit of weight.
Why does higher fill power matter?
Because loftier down traps more warm air for less weight, a higher fill power means a warmer, lighter, and more compressible garment or sleeping bag. So an 800-fill jacket can be warmer and lighter than a 650-fill one using the same weight of down — which is why premium down gear advertises high fill-power numbers.
What's the difference between fill power and fill weight?
Fill power is the quality (loft) of the down; fill weight is the actual amount (mass) of down inside the item. Both matter for total warmth: a high fill power makes the down efficient, but you still need enough fill weight for warmth. A light jacket with high fill power but little fill weight may be very packable but not very warm overall.
Sources
- Down & fill power — The Mountaineers
- Sleeping bag insulation — American Hiking Society