Down fill meanings

What does 750 fill mean in comparison to 800 fill and 600 fill and so on? Is it the amount that per square inch? Heat retention?

Without getting into the testing details, it's how many cubic inches one ounce will occupy.

Larger number more or less insulation?

The larger number basically means the same insulation for less weight. Insulation is related to the volume of dead air space created by the down (or other insulator); the more space created, the more insulation you get. Down that fills 900 cu. in. per ounce provides roughly half again the insulation volume and warmth as does an equal weight of 600 fill down.

There are lots of threads on this subject. Just do a search using the term "fill power" (be sure to include quotation marks) and go back a few years. Tons of information.

Large number = lighter weight.

Two 15-degree bags will be equally warm, whatever the insulation (assuming the "15-degree" rating is objectively determined, not just marketing hype or wishful thinking.) For example, a synthetic 15-degree bag will be just as warm as a 600 fill or a 900 fill down bag. However, it will weigh a bunch more than either of the down bags, and the 900 fill down bag will weigh less than the 600 fill down bag.

Large number also = lighter wallet. That 900 fill bag will weigh less, but it will also be significantly more expensive. As in, "That's OK, honey, I'll get the mail" so you can get to the credit card statement before she sees what you did. <img src="/forums/images/graemlins/tongue.gif" alt="" />

The most pertinent part of the discussion.

Ain't that the truth! <img src="/forums/images/graemlins/shocked.gif" alt="" /> <img src="/forums/images/graemlins/grin.gif" alt="" /> Sometimes the 'density' of the 650 fill down products can feel 'as warm' within reason to the 800 fill products. i site jackets, vests, etc here, as a stiff wind blowing through 800 fill down may be felt sooner than through the heavier/bulkier 650 fill down IME.