Fill power measures the quality of down, not how warm a bag or jacket is. Here's what the numbers mean, why fill weight matters as much, and when higher fill power is worth paying for.
Down sleeping bags, quilts and jackets are sold by their fill power: 650, 800,
850, 950. The higher numbers cost a lot more, and it's easy to assume they're warmer. They aren't,
necessarily. Fill power measures the quality of the down. How warm a product is depends on
how much of that down it contains. Understanding the difference is the key to comparing down gear.
What fill power is
Down is the fluffy undercoat of ducks and geese: clusters of fine filaments that trap air. Fill
power is the volume one ounce of down fills when it's allowed to loft fully, measured in cubic
inches. One ounce of 850-fill down fills 850 cubic inches; one ounce of 650-fill fills 650. Larger,
more mature down clusters loft higher, so they reach higher fill powers.
Fill power is loft per ounce. Higher fill power traps more warm air per ounce, so you need less down, and carry less weight, for the same warmth.
Trapped, still air is what keeps you warm, so higher fill power down gives more warmth per ounce.
It also compresses smaller in your pack and springs back faster.
A note on testing: the US and European test methods condition the down differently, and the
European method tends to report a lower number for the same down. When you compare two products,
compare figures from the same market where you can.
Fill power vs. fill weight
This is the part most people miss. Fill weight is how many ounces of down a
product contains. A product's warmth comes from its total loft, which is roughly fill power
multiplied by fill weight:
Warmth comes from total loft, roughly fill power times fill weight. Compare both numbers, not fill power alone.
The two quilts above are close in warmth, but the 850-fill one carries 3 oz less down and packs
noticeably smaller. That's what higher fill power buys you: the same warmth for less weight
and bulk. It also means a 950-fill quilt with very little down can be colder than an
800-fill quilt with plenty of it. When you compare bags or quilts, check both numbers: every product
page on Base Weighted lists fill power and fill weight when the maker publishes them.
Real warmth also depends on design: baffles that stop the down shifting, a draft collar, and a
footbox that doesn't compress. Fill numbers get you close; temperature ratings and design details
finish the comparison (see temperature ratings
explained).
600 to 700: good value and durable; heavier and bulkier for the warmth. Common
in budget bags and everyday jackets.
800 to 850: the sweet spot for most backpackers. Light, compressible, and far
cheaper than the top grade.
900 to 1000: the lightest and most compressible, at a steep premium. It pays
off most in very warm gear, where there's a lot of down: switching a 0°F bag from 800 to 950
fill can save several ounces, while a 40°F quilt saves very little.
Goose down tends to come in larger clusters, which is why the highest fill powers are usually
goose. But fill power is fill power: 800-fill duck down insulates like 800-fill goose down. Don't
pay extra for the word "goose" alone.
Down and moisture
Wet down collapses and stops insulating, and it dries slowly. Over a long trip, a bag also gains
moisture from your body and from condensation, losing loft night by night. Hydrophobic (treated)
down resists moisture longer and dries faster, which helps, but it's no substitute for keeping your
bag dry: pack it in a waterproof liner and air it out on sunny breaks. In consistently wet climates,
synthetic insulation is worth considering. It's heavier for the warmth but keeps insulating when
damp.
Care and storage
Store down gear loose, in a big cotton or mesh sack, never compressed in its stuff sack.
Wash rarely, with a down-specific soap, and dry on low heat with clean tennis balls to break up
clumps. It takes hours; patience restores the loft.
Ethics and certification
Look for down certified to the Responsible Down Standard (RDS) or a similar scheme, which audits
animal welfare through the supply chain. Most quality makers now use certified down.
