Fill power is used to measure the warmth of the goose-down and duck-down insulation in down jackets, down sleeping bags, and down quilts. Expressed as a number, fill power measures the ability of down to trap the warm air heated by your body in a down jacket or down-insulated sleeping bag. For example, a down sleeping bag insulated with 900 fill-power down sleeping bag will be warmer, lighter weight, more compressible, and much more expensive than a 650 fill-power down sleeping bag, if both are insulated with the same amount of down by weight.
Unfortunately, the warmth of synthetic insulation is not measured using fill power, which makes apples-to-apples comparisons difficult. But as a rule of thumb, the synthetic insulation found in insulated jackets and synthetic sleeping bags is equivalent to 550 to 600 fill-power-down insulation. meaning that it requires more synthetic insulation by weight to give you the same warmth as a higher fill power down.
This is a useful comparison when it comes to buying insulated jackets and sleeping bags, especially if cost is a consideration. It means you can buy a much less expensive 650 or 750 fill-power down jacket or down sleeping bag that is warmer, lighter weight, and more compressible than an expensive top-of-the-line synthetic jacket or synthetic sleeping bag. The manufacturers of synthetic insulation such as Primaloft (Polartec), Coreloft (Arc’teryx), VerticalX (Outdoor Research), and others don’t go out of their way to explain this, but I thought you should know, especially if you have budgeting constraints.
But the superior weight-to-warmth ratio and cost aren’t the only reasons to choose 650 or 750 fill power down over synthetic insulation, but they are important factors to consider depending on your location and activity level, as shown below.
| DOWN INSULATION | SYNTHETIC INSULATION | |
|---|---|---|
| Best For | Cold, dry weather | Cold and damp weather |
| Activity | Stationary activities | Start-stop activities |
| Material | Goose or duck down | Polyester fibers |
| Pros | Highly compressible | Dries quickly |
| Cons | Loses insulation ability when wet | Heavier, Not as compressible |
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