Fill power is the loft of the down in a bag or jacket, measured as the cubic inches one ounce of down expands to fill. Higher fill power means fluffier, higher-quality down that traps more warmth per gram. It is one of the most misunderstood specs in the outdoor world, so here is what it does, and does not, tell you. Across the down bags we track, fill power runs from about 550 to 1000.
This is the key idea. A 900-fill-power bag is not warmer than a 650-fill bag. It reaches the same warmth with less down, so it weighs less and packs smaller. Two bags rated to the same temperature can use very different fill powers: the high-fill one is lighter and pricier, the lower-fill one is heavier and cheaper, and they sleep about the same. So fill power tells you about weight and packability at a given warmth, not how warm the bag is.
Warmth comes from the total loft, which is fill power times the amount of down (fill weight). A bag with a lot of 650-fill down can be exactly as warm as one with less 850-fill down, just heavier. That is why you cannot judge warmth from fill power alone, and why we rank down bags on measured warmth-to-weight, where high fill power actually shows its advantage, rather than on the fill number itself.
Higher fill power costs more because top-grade down is scarcer. The payoff is weight and packed size, so it matters most on gear you carry far and stuff often: an ultralight quilt or a bag for long-mileage trips. For car camping or occasional use, mid-range fill power (down around 650 to 750) delivers most of the warmth for much less money. Roughly, 800-plus fill is the ultralight tier, 650 to 750 is the value sweet spot, and below 600 is usually budget or synthetic-adjacent.
Fill power is measured by a couple of different lab methods (US vs. EU), which can shift a rating by 30 to 50 points, and a few brands round up. Treat small differences (say, 800 vs. 850) as close to a wash and worth chasing only if you are optimizing hard for weight. The bigger, more reliable signal is the bag's published warmth-to-weight, which folds fill power and fill weight into one comparable number.
No. Higher fill power means the bag reaches a given warmth with less down, so it is lighter and packs smaller, not warmer. Two bags of the same temperature rating sleep about the same regardless of fill power.
Only marginally. A 50-point difference shifts weight and packed size slightly and is partly measurement noise between labs. Chase it only if you are optimizing hard for weight; otherwise it is close to a wash.
Fill power is the loft (quality) of the down; fill weight is how much down is in the bag. Warmth comes from both together, so a lot of low-fill down can match less high-fill down, just heavier.
For weight-conscious backpacking, 800-plus fill power is the ultralight tier and worth the premium on gear you carry far. For general use or a budget, 650 to 750 delivers most of the warmth for much less.
This guide explains published standards and industry specs, illustrated with figures drawn from the 500-plus products in our database. It is educational reference, not a substitute for matching gear to your own conditions. We take no sponsorships and run no paid placements.