Fill Power is a grading system for goose / duck down. Often abbreviated as FP. Higher fill power will add more loft and warmth to an outdoor garment or sleeping bag at the same given amount of weight. Measured using a cubic inch per oz system. 500 fill power down will fill 500 cubic inches of space per ounce used. 900 will fill 900 cubic inches of space per ounce.

A high fill power down cluster.

What Fill Power is Best?

It all depends on the application. For use off the trail (in town, at home) just about any fill power will do, as weight of the garment or sleeping bag is not a concern. This would be a good time to save the cash over a higher fill power option. For the trail however where every ounce counts, a higher fill power jacket like the REI Co-op Magma 850 will save weight; going with the highest fill power you can get that is still in the budget will get you gear that weighs less but could contain the same loft (and warmth) as a heavier lower fill power down option. To sum things up, in a higher fill product the same amount of garment / bag warmth can be achieved for less weight. Higher fill power is considerably more expensive than lower level down.

For more on backpacking insulation, see our article Backpacking Gear: Down vs. Synthetic Insulation. For more information on jackets, see our article on 3-season backpacking clothing, and for sleeping bags, check out our guide on How the Choose the Best Backpacking Sleeping Bag.

Editor's Note: This Jargon installment originally appeared in Issue 2 of TrailGroove Magazine. You can find the original article here.