Fill Power x Fill Weight Conversion Rule of Thumb?

[jacob_dbu]

If I'm comparing a 650-FP sleeping bag with an 850-FP sleeping bag, and the 850-FP has XXXg of down fill, then what is the fill weight required in a 650-FP bag to have the equivalent warmth?

Assuming all else is equal..

[adrenalated]

Warmth comes from loft. The same volume of 650FP and 850FP will be close to the same warmth.

The units for fill power are in^3/oz. In other words, 650 fill power means 1oz of down fills a volume of 650 in^3.

So lets so you have a 850 FP bag with 10oz of down. That means in total, you're filling 8500in^3 of volume (10oz X 850in^3/oz = 8500in^3).

To fill that volume with 650 FP, you divide 8500in^3 / 650in^3/oz = 13.1oz of down required.

[Lindahl]

The same volume of 650FP and 850FP will be close to the same warmth.

Yes but...

Warmth comes from loft.

Some testing I’ve seen over at backpackinglight suggests that loft is only loosely correlated to warmth. A high-lofting 800 fp jacket is approx the same warmth as a jacket with the same fill weight of 800 fp with lower loft, as long as the down is not overly compressed by the pattern. I think I saw something like 80% compression from ‘natural’ loft before you lose performance.

Therefore you could easily have a 800 fp that is less lofty but warmer than a 650 fp garment. And vice versa.

The conclusion was that overstuffing to an extent is superior because it yields lower weights (less fabric), and less cold spots.