Two sleeping bags can both say “850 fill power” on the label and keep you at genuinely different temperatures. That’s not a contradiction, and it’s not either brand lying. Fill power and warmth aren’t the same measurement, and mixing them up is one of the most common mistakes in choosing a down product.
What fill power actually measures
Fill power measures loft quality, specifically how much volume a given amount of down fills once it’s allowed to loft fully. Higher fill power means the down traps more air per gram, which is a genuine quality measurement. It says nothing on its own about how warm a finished product will be, because it isn’t measuring quantity, only quality.
What actually determines warmth
Warmth comes from total loft, and total loft depends on fill power and fill weight together, how much down is actually in the product, not just how good that down is. A generous amount of decent 700FP down can out-loft, and therefore out-warm, a stingy amount of excellent 900FP down. This is the trap in reading a spec sheet: two products both proudly listing “850FP” can be genuinely different in warmth if one uses considerably more fill weight than the other.
Where the real efficiency gain comes from
Higher fill power does earn its premium, just not in the way it’s often marketed. Because higher-quality down lofts more per gram, you need less fill weight to hit the same warmth target, which is where the actual weight saving comes from, not from the fill power number itself. That’s also where diminishing returns set in. Above roughly 900FP, the extra loft per gram becomes marginal while the cost climbs steeply, so you’re paying an increasing premium for a shrinking real-world benefit.
Reading a spec sheet properly
Fill power alone tells you the quality of the down. Fill weight alone tells you the quantity. Only together do they tell you anything meaningful about how warm the finished product will actually be, and a temperature rating (ideally one that states whether it’s tested or estimated) is a more direct answer to “will I be warm enough” than either number in isolation.
Where our own numbers sit
Our sleeping bag uses 800g of 850FP down, our quilt uses 380g of the same 850FP down. The fill power is identical between them. The fill weight is very different, which is exactly why one is a summer-weight quilt and the other is a three-season bag, despite sharing a down quality spec. 850FP was a deliberate mid-premium choice for us: strong warmth-to-weight performance without paying the steep cost curve that starts above roughly 900FP for a real-world benefit that’s genuinely marginal at that point.
The takeaway
Check fill weight and fill power together, never one alone. A single number on a down spec sheet tells you almost nothing about how warm you’ll actually be at night.
Over and Out