Fill power at a glance
- Entry level
- 550 to 650 FP
- Good backpacking
- 700 to 800 FP
- Premium
- 850 to 950 FP
How fill power is measured
Down insulates by trapping air in the structure of its clusters. Fill power tests how much air a given weight of down can hold: an ounce is placed in a cylinder, lightly compressed, and the volume it occupies in cubic inches is the number. 850 fill power down expands to 850 cubic inches per ounce.
Why it is not a warmth rating
It's a measure of quality per gram, not of total warmth. A bag stuffed with a lot of mediocre down can be exactly as warm as one with a little excellent down. What higher fill power buys you is the same warmth for less weight and less packed volume, which is why it commands the price it does.
What the bands mean in practice
Practical bands: 550 to 650 is entry level and heavy but perfectly warm, 700 to 800 is where most good backpacking bags sit, and 850 to 950 is premium territory where you're paying steeply for the last few ounces. There's also a testing wrinkle worth knowing, since the European and American methods produce slightly different figures for identical down, with the US number typically reading a little higher.
Why US and European numbers differ
The two regions condition the down differently before measuring it, and the American method generally returns a slightly higher figure for identical down. The gap is real but modest, and it means a European 800 and an American 850 can describe the same material.
It matters when you are comparing a European brand against an American one on paper. If both publish fill weight as well, compare total loft instead and the testing difference mostly washes out.
Fill power in jackets against sleeping bags
In a sleeping bag, high fill power buys packed size as much as warmth, because a bag is one of the two or three bulkiest things in the pack. In a jacket the calculation is different: jackets carry far less down, so the absolute weight saved by going from 650 to 850 is small, and the price difference is not.
The exception is a jacket you carry every day and rarely wear, where packed size is the whole point. For a piece you actually live in around camp, a heavier 650 fill jacket with a generous fill weight is usually the better buy.
What this means when you're choosing
- Fill weight published alongside it, without which the number says little
- 700 to 800 for most backpacking, where the value sits
- Whether the figure is the US or European test, which read slightly differently
Work it out
Compare two bags
Fill power on its own tells you nothing about how warm a bag is. Multiply it by fill weight and you get loft, which is the number worth comparing.
Total loft
8,800 cubic inches
Fill power times fill weight is the volume the down actually occupies, and volume is what traps air. It is the only number that lets you compare two bags across different fill powers, and it says nothing about draft collars, baffle design or fit.
The common mistake
Reading fill power as a warmth rating
Fill power tells you the quality of the down, not how much is in the bag. A 900 fill jacket with very little down in it is a summer piece. Fill weight is the number that says how much is actually there.
Common questions
Is 800 fill power warmer than 650?
Not by itself. It's more efficient, meaning it delivers a given warmth for less weight and packs smaller. A 650 fill bag with more down in it can be warmer than an 800 fill bag with less.
Is high fill power worth the money?
It's worth it when weight and pack space genuinely matter to you, which is why long-distance hikers pay for it. For car camping or occasional weekends, 650 fill does the same job for a lot less.
Where this is defined
- Testing regulations for down and feather, International Down and Feather Bureau