What does down fill power actually tell you?
Fill power measures loft, not warmth by itself. A higher number means better insulation per ounce, but the jacket's total fill weight and construction decide how warm you will be.
What the number measures
Fill power is the volume, in cubic inches, that one ounce of down lofts to fill. Higher fill power means the down traps more air per ounce, so it insulates better for its weight. A 800-fill jacket and a 600-fill jacket can be equally warm; the 800-fill one just gets there with less down and packs smaller. The number says nothing about how much down is actually in the jacket.
Why fill weight matters more than you think
Two jackets with identical 750-fill down can feel completely different if one holds three ounces of it and the other holds eight. Warmth comes from total loft, which is fill power times fill weight, minus what the construction wastes. When comparing jackets, look for the fill weight on the spec tag first and the fill power second. A high fill power with a tiny fill weight is a spring jacket wearing a winter number.
Construction beats specs
Stitch-through baffles are cheap to make and create cold lines wherever the stitching compresses the down. Box baffles and trapezoidal chambers keep the loft even but add cost and weight. Fit matters too: a jacket so trim it compresses the down, or so roomy that air pumps in and out as you move, will underperform its specs. Try it on over the layers you will actually wear.
How to choose for your use
For stop-and-go town use in real cold, prioritize fill weight and a windproof shell. For hiking and ski touring, prioritize a higher fill power so the warmth packs small, and make sure the jacket vents well because you will overheat. For damp climates, consider hydrophobic-treated down or a synthetic alternative, since wet down loses its loft and with it most of its insulation.