How accurate are manufacturer volume numbers?

Not accurate. Marketing at work. All specs are spun.

Wildly inaccurate — esp. the wider-known brand names!! Same with weight specs and warmth ratings.

The boutique UL gear makers tend to be much more honest — likely because they know that many of their customers are UL fanatics who will verify!

Never judge a pack by its stated size, Just make sure your gear will fit.

Maybe they should make a standard for the measuring process, just like sleeping bags. At least that would give consumers a better idea on what to look for. Ive seen tons of variation between brands.

The difficult aspect to making sure things fit/measure up is that often I am buying online and waiting to "try it out" after I buy it when it arrives.

I love the cottage industry that the internet has spawned but, at the same time, I detest buying without having the opportunity to know what on earth I'm buying.

Were measurements accurate you could narrow down your choices based upon them. If there is little, or no, reliance upon what a manufacturer asserts then it's basically a roulette wheel. Very frustrating.

Thanks.

in the recent BPL tests the packs that were closest to their claimed volume were jansport and montbell

the least closest were ULA, golite, GG and rei

part of it is a lack of standard measurements or adherence to such … the other problem is that its basically a marketing statement

one company produces a "50L" pack … so another company does as well

There is an ASTM standard for measuring volume, but adherence (and then accuracy) is sometimes lacking. BPL article

Adherence is lacking, as is protocol.

When measuring a Mariposa Plus you immediately notice that the pack is constructed as a cylinder.

When measuring ULA Ohm you notice that the pack is constructed a rectangle at the bottom, which can transform into a cylinder at the top. But, the way I pack it, the top retains its rectangular section. (I like to keep things close to my back.) So, the way it gets measured can give you more volume than the way I use it.

The same can be said of the Mariposa Plus. It has a large volume, but you may or may not be able to use it Effectively, if your pack items force it into a rectangular cross section.

So even with a "standard" the resulting volumes can still be misleading.

But, packing peanuts or ping pong balls will always yield a better result than marketing BS.