What Fill Power Really Means — 550, 650 and 800 Explained

Fill power measures how much space a given weight of down occupies, expressed in cubic inches per ounce. A 650 fill power down lofts to 650 cubic inches per ounce; an 800 lofts to 800.

In practice a higher fill power gives the same warmth with less down — so a lighter, more compressible bag. It does not mean the bag is warmer. Two bags with the same fill weight but 550 and 800 down will differ in bulk and weight, and only modestly in warmth.

Three other things usually matter more than the number on the tag:fill weight(how many grams are actually in the bag),construction(baffled boxes hold loft better than sewn-through stitching), andhow the bag is used— a mat with a decent R-value is worth more to a cold sleeper than 100 extra fill power.

A realistic buying guide: 550–650 for general retail where price matters, 650–750 for the bulk of technical ranges, and 800+ only where packed size is the headline claim. Above 800 the price rises steeply for a gain most customers will not feel.

We can supply fill-weight and loft test data with each development — write to linda@reliableoutdoors.com.