| Measures | Down loft / quality |
| Unit | Cubic inches per ounce |
| Higher (800-900) | More warmth for less weight |
| Not the same as | Fill weight (amount of down) |
Fill power measures the loft, or fluffiness, of down — how many cubic inches one ounce of it fills. Higher fill power (such as 800-900) means the down traps more air per gram, giving more warmth for less weight. It rates down quality, not total warmth, which also depends on how much down (the fill weight) is used.
Power vs weight
Fill power is down quality; fill weight is the amount. Warmth needs both — see down insulation and loft.
Where it matters
It rates the down in a down jacket or sleeping bag.
Frequently asked questions
What is fill power?
Fill power is a measure of down's quality and loft, defined as the number of cubic inches that one ounce of the down fills when allowed to fully expand. Common values run from around 550 (lower quality) to 900+ (premium). Higher fill power means loftier, more efficient down.
Is higher fill power warmer?
Higher fill power is more efficient — it gives more warmth per gram and packs smaller — but it doesn't automatically mean a warmer product. Total warmth depends on fill power and fill weight together; a jacket with lots of 650-fill down can be warmer than one with a little 900-fill down.
What's the difference between fill power and fill weight?
Fill power is the quality/loft of the down (cubic inches per ounce); fill weight is the actual amount of down used (in grams or ounces). Warmth comes from both: high fill power makes the down efficient, while fill weight determines how much of it there is.
Sources
- Down insulation — The Mountaineers