Down fill power is one of the most broadly misunderstood concepts in the outdoor industry, even by those who sell down-filled products. Through my work in specialty retail I overheard countless misguided explanations of fill power from people impressing companions with their knowledge, retail staffers, and a whole stack of others who should've known better. I think that what happens is people try to "run" with an incomplete understanding, and fill in their gaps of knowledge with supposition. Let's take a nuts n' bolts look at fill power.
The fill power numbers are those that you'll frequently see on a hang tag or even embroidered onto a product, usually a number ranging from 500 to 900. Maybe you'll see a "625" or a "900+" or a "750..." but really, why do any of them matter? What do they mean? It's super simple, but also super critical to understand this core concept... in fact, it's so important I'll set it apart:
The fill power number represents how much space one massed ounce of that down occupies.
By extension, the weight of one ounce (as weighed on a scale) of 550-fill down will occupy 550 cubic inches. One ounce of 900-fill down will occupy 900 cubic inches.
The significance, the difference between those fill powers, relates to that old proverbial question about a glass half-full... Given the same weighted amounts, 500-fill down won't fill as much space as 900-fill down. In fact, an ounce of 900-fill down fills nearly twice as much space as 500-fill down. But that doesn't mean that 900-fill is better! That's so important I'll say it again:
900-fill down is NOT BETTER than 500-fill down.
It IS different, and it CAN be better for SOME situations... but sometimes 900-fill down is a WORSE choice than 500-fill down. Similarly, 900-fill down is NOT WARMER than 500-fill down... but this is where things can get confusing for people.
If you have equal weights of two down samples, the 900-fill sample will fill nearly twice as much space. Given that the purpose of insulation is to create dead air space, the 900-fill sample will create a lot more dead air space. An ounce of 900-fill down is warmer than an ounce of 500-fill down, BUT IN PRACTICAL TERMS other variables will almost always be more significant in understanding how warm something will be.
I say this because each fill power rating has its own sort of specialized niche. An automotive analogy: A 3/4-ton pickup can't accelerate or handle like a Corvette, and the Corvette can't haul a load of lumber.
The reality is that 900-fill down won't be used in applications better-suited to 500-fill down, or vice versa. The advantage of higher fill powers is that you can use less of them to achieve the same amount of warmth of lower fill powers... Higher fill powers allow you to use less weight and less bulk to achieve the same amount of warmth. This DOES NOT MEAN that a 1/2"-thick 900-fill jacket will be as warm or warmer than a 2"-thick 500 fill jacket. The amount of loft, the poofiness, is what determines warmth. It's the thickness of the jacket, not the fill power of the down, that ultimately determines how warm the jacket will be.
It is the THICKNESS of the jacket, not the fill power of the down, that determines how warm a jacket will be.
If I made two jackets as examples (maybe this is something we can do for a future video?) and filled both of them to 1-inch thick, using 500-fill down in one jacket and 900-fill down in the other jacket, they would be equally warm. Re-read that part! EQUALLY WARM! The difference would be that the 900-fill jacket would use less down to get that thickness, so it would be lighter weight and would pack smaller.
And that, friends, is the reason to use higher fill powers... you can create lighter weight things that stuff into smaller balls. It wouldn't make a lot of sense to use 850 fill inside a nylon shell that could be made with a lighter-weight nylon.
Lower fill powers, frankly, are better for routine use. The products made with lower fill powers will cost less... both because the down itself is less expensive, and because the materials enclosing it are also less expensive. Lower cost aside, the lower fill powers are also more versatile, more resilient. They don't require as much fussing or attention. Here at Backcountry Bonsai we ran some experiments on the effects of humidity on down, and found that the lower fill powers retained their loft better than higher fill powers under humid conditions. Anecdotal experience also indicates that higher fill powers are more susceptible to losing loft and/or matting under more extreme or prolonged conditions.
Heavier shell material will actually compress higher fill-power down, whereas it will not have the same effect on lower fill powers... We believe that, by extension, and in consideration of our humidity testing, lower fill power downs are overall more durable. (Gross comparison, what's easier to break... one toothpick, or ten of them in a bundle?) So a lower fill power product can cost less, work better under worse conditions, and last longer... the trade-off being that it does so at the expense of some additional weight and bulk.
You will also see down billed as "grey" or "white," and you should know that color has NO effect on insulating properties. In our experience working with material that weighs less than an ounce per square yard, we prefer grey down simply because it is less "contrasty" behind the rich fabric colors we prefer. (White down is more visible through the material, and the whiteness somewhat "fades" the material color.) Hey, pop quiz: What weighs more: A ton of feathers, or a ton of bricks?
I hope you said they weigh the same! In a similar way, 700-fill down is 700-fill down... regardless of whether that down was sourced from duck or goose. "Goose down" has a certain cachet, I'm guessing because of some historical basis, but if all other factors are equal, duck and goose down of the same fill power are of the same quality, same warmth, same weight, and same compressibility. True eider down, the undisputed king of all downs and easily costing 10 times more than "standard" 900-fill downs, is duck down.
One last thing we should address: all of the downs we're talking about are hypoallergenic. Do you have allergies? Horrid allergies? Cool. You'll be fine with down. What people are allergic to isn't down itself, but the dust and dander and extraneous oils & dirts associated with FEATHERS. Feathers, by the way, are not down. Feathers have quills... you know, like those super-old-school feather pens in the ink well. Feathers poke. Feathers can wreak havoc on allergies. A lot of upholstery and even home bedding is filled with feathers... they're tougher, they "give way" less, so they maintain more cushion. (If you had a pillow filled only with 900-fill down, when you laid your head down there wouldn't be anything left but two layers of nylon between your head and the mattress.) I had a featherbed for a short while... could NOT sleep on it, because of all the allergens associated with feather products. When you start talking about down, though, from fill powers 500 and up... you're not talking about feathers anymore. The down we're talking about, even the 500-ish stuff, is pretty high end. You're gonna love it.