Fun w/ packing peanuts

You want your “measuring box” to have about the same shape of the pack.

A cylinder optimizes. An narrow rectangle goes in the other direction. A square box is somewhere in the middle.

I found wine boxes taped end-to-end to be very close to the shape and cross section of a 40 L pack. (for 750 ml bottles.)

Make sure your peanut-filled pack approximates what your “packed pack” looks like.  It will want to turn into a cylinder. You will have to help it stay rectangular.

No matter how you try to eliminate air space in packing peanuts (a product designed by the devil himself – did you and your son have fun picking those up?), I don’t see how they can be more than an estimate of volume.  Did you try using something that packs more densely, like beans or plastic pellets?  Some people have used a plastic bag filled with water, too, then measured the liters held by the backpack.

Also, have you tried contacting ZPacks directly and asking them how they measure volume?  You might try that before claiming they are lying about the product specs.

I have never actually checked the volumes on any of my gear:  if it seems the right size and my gear fits, then it’s good.  I don’t think reputable manufacturers routinely lie about specs.  They might use some “rounding” now and then, but I wouldn’t call that dishonest.

Most manufacturers include the volume of the extended outside pockets too. Trouble is that you often can’t get the same volume in those pockets once the main compartment is full.
BTW, I don’t believe that the shape (cylindrical vs box) makes any practical difference.
To compare , get the largest pot you have, measure the size in litres then fill the pot with peanuts and then use those peanuts to measure the box.

“BTW, I don’t believe that the shape (cylindrical vs box) makes any practical difference.”

Agreed.  The shape of the measuring box doesn’t matter.  I was waaaay overthing it.

Packing peanuts aren’t the best material to use because they flex and compress a bit (as illustrated by the 32-liter versus 35-liter results you got), but otherwise, your methods seem reasonable.  I’m not seeing that slightly different surface-area to volume ratios in different geometries is going to add up to any measurable difference (it could, if the objects were close in scale to the pack or measuring box).  Still, easier to fill a narrow wastebasket a few times: full, full, and one partial full and then your calculations aren’t as sensitive to that one height measurement.

And 32/35 versus 42 is a bigger difference than one method versus another.

One possibility is that they credit themselves with the volume WITHOUT rolling the collar down.  Which doesn’t seem at all accurate, since you’ve never hike with it that way.

Some vendors offer a “40-liter pack” in Small, Medium and Large which each have distinctly different volumes.  For the Arc Blast, Zpacks seems to only vary the frame, not the bag.  But their website clearly claims the main bag is 42 liters, not including outside pockets:

“Volume: 42L main body, 2.5L each side pocket, 8L center pocket, 55 Liters Total”

Ryan’s method is actually pretty similar to the ASTM standard for measuring backpack volume:

https://compass.astm.org/download/F2153.21358.pdf

BPL also used the same methodology in their State of the Market reviews on framed and frameless packs back in 2011 (need subscription to view):

https://backpackinglight.com/lw_internal_frame_packs_part_1a/

https://backpackinglight.com/frameless_backpacks_sotm_part2a_2011/

The conclusion then and probably still valid now is that some manufacturers follow the standard and their quoted volumes are reasonably accurate while other manufacturers don’t follow the standard and use their own definition (e.g. including open mesh pockets) or calculate volume based on measurements.

The big question is how do you work out which manufacturers follow the standard and which ones don’t.

You might like to read our extensive review of internal frame packs:
https://backpackinglight.com/lw_internal_frame_packs_part_1a/
https://backpackinglight.com/lw_internal_frame_packs_part_1b/
https://backpackinglight.com/lw_internal_frame_packs_part_1c/
https://backpackinglight.com/lw_internal_frame_packs_part_2/

The review covers the use of polystyrene balls and peanuts fairly carefully. You might also like to note that this way of measuring pack volumes is actually covered by an ASTM Standard.

Now, as to mfr honesty: I found that most mfrs were fairly honest and followed the Standard, but a few absolutely cheated. The cheat was to measure the volume of each compartment separately, while all other compartments were empty, and to then add up the lot. This sounds like what the OP was describing. In some cases I had some trouble getting the quoted volume while being able to seal the top of the pack: that’s not good either.

I would not buy from any mfr which behaved in such an unethical manner.

Cheers

Just to be clear, the post title was somewhat toungue in cheek. I think Zpacks is a great company and own a number of their products. If I was really serious, I’d have called them something worse than “stinker heads” :)

@Greg- as a couple have commented st this point, certainly if I was measuring the volume in a super skinny tall vessel, I’d think my calculations would be suspect. But a 12×12 box vs 16” cylinder, etc should all yield the same within a very small margin of error.

@Jenny- they were surprisingly easy to cleanup actually! Most were the corn starch-esque style so weren’t too staticky. I did keep finding 1-2 every hour for the rest of the day though. Using something less compressible like dried beans would be ideal but alas, w/ the one small bag we had, I found myself about 39.5L short of dried beans sitting around the house :) I did my best to mitigate the squishy factor of the packing peanuts by making sure NOT to compress them in either the backpack or the measuring vessel.

last, after doing a little more research and measuring, I think I have a reasonable explanation for what I found. Currently on Zpacks website, they list the body dimensions of the ArcBlast as 12.5”x7”x30”. Multiply that out and you get 2625 cu in = 43L. As I measure my pack, I get 12”x6”x29” (w/ the 29” being to the very top of the roll top) which yields 34L so right in line w/ my calculations. So it seems that Zpacks is definitely calculating the volume w/ the rolltop basically open, and either I have an older model that is slightly smaller than the current, or Zpacks is taking some liberties in their panel measurements. I am going to reach out to try and verify that.

about using peanuts…
recently I filled a bean bag with…small beans.
To avoid having beans all over the place I did it in the bathtub.
Tip : put the plug in first.

Forgot to mention two things along the lines of mfr stated info versus the “real world” info. There is precedent for how Zpacks is doing this. Think of any cooking pot you’ve used… Evernew lists their pots based on volume to the rim. so the 1.3L is really more like a 1.1L (1.2L if you’re really careful).

And most of the tent manufacturers list the weight of their tents without “accessories” (guy lines, stuff sacks, etc). Another topic I’ve been posting about a lot lately is my MLD Cricket in DCF. The stated weight is 9oz, and buried in the description it states the “base weight does not include guy lines or stuff sack”. Once I added the lengths I wanted and stuck it in a sack, total weight jumped up to 11.4oz (a 26% increase). At least MLD stated as such though.

Other than having the discussion and hearing other’s experience in this realm, the point of posting this isn’t to defame Zpacks, but simply to point out that the stated volume of packs isn’t always the actual volume. It would be nice if manufacturers would state HOW they’re coming up with their numbers so consumers can be better informed. If anything, I’d think that would be better for business… I personally am a big fan of manufacturers who are open and forthcoming with info.

If you use corn starch peanuts, you can leave them in the pack as your meals on your next trip.

Evernew lists their pots based on volume to the rim.
I have no objection to this. It is clear, simple and easily verified by the average consumer. Any other specification would be open to abuse.

the stated volume of packs isn’t always the actual volume.
I take very strong objection to this. Claiming a 32 L pack is a 42 L pack is simply a lie. This is compounded by the difficulty the average consumer would have trying to verify the measurement. Any mfr behaving in this way should revisit their own ethics.

Cheers

Roger- IF it turns out that my pack IS identical to the current version, I’ll share your concern and have some choice words for Zpacks. But at least based on the length/height/width measurements they give for the current pack, it appears mine might be an older smaller version. Like I said, I’ll be reaching out to them and trying to research that further independently. I’ll update this thread when I confirm one way or another.

As mentioned, the values Zpacks lists for the main bag dimensions are 7″ x 12.5″ x 30″. If we calculate the volume of that 3D rectangle, we get 2625 cubic inches or 43L.

But you would only have that much volume if you had the pack wide open at the top which is obviously inappropriate. If you wanted to simply close the pack you’d consume some material off the height to form the top of the pack, even ignoring the material contained in the rolls. The pack is listed at 7″ deep, so if we assume that is the depth at the top (which Zpacks implies it is, since they list the depth at 7″ without specifying that it’s less in some areas) then we need those 7″ of fabric for the top which reduces the height by 3.5″ on each side. So we get a 7″ x 12.5″ x 26.5″ rectangle, which is 38L.

But a double roll is really the minimum to hold the pack securely shut, so if we assuming two 3/4″ rolls that’s another 1.5″ off the height, or 7″ x 12.5″ x 25″, which is 35.8L – and which is also about the volume measured here.

So my best guess is that Zpacks length/width/height dimensions are reasonably accurate, and Ryan’s volume is reasonably accurate, while Zpacks volume spec may be inappropriate because it appears to be the volume if the pack were wide open at the top and filled to the brim. The volume spec should be the maximum volume you can enclose within the compartment since being able to close your pack is a reasonably expectation.

This thread has me thinking that I’m glad I went with the Zpacks Arc Zip instead.  I ordered mine without the top opening but I still have the straps to compress the top if I don’t need the volume.  No volume lost due to the need to close it!

Couldnt you just put a garbage bag in your pack and pour nalgenes to get your volume?

Am i missin sumpin?

That’s it!

Fill it to the brim with water. Pour the water into a cardboard box :-)

BTW, does all your gear fit into the pack?

Yeah, you are missing something – like the weight of 42 L of water (42 kg). That amount of weight might burst the pack or it might just fall over and spill across the floor. Water is a LOT heavier or denser than the normal contents of a pack.
Speaking from experience, you understand :)

Cheers

@Jeff- I actually tried to do it w/ water… it’s a pain. 35L is ~9 gal @ 8lb/gal = 70lb of sloshy ridiculousness. If my toddler hadn’t been hanging on me, esp if I had a second adult to help, it would be easier, but def WAY harder than the packing peanuts.

@Nick- does my gear fit? Kinda… it generally feels like a stretch, esp if it’s colder or I have any fly fishing stuff. Of course I strap my CCF sleeping pad to the outside to save that volume. To a large extent, this thread is academic (I just like to know) and/or a public service announcement (help others understand true pack volumes). Im not necessarily trying to solve a problem per se… unless it turns out I really have been trying to pack 55L of stuff in a 45L pack :)

Yeah, you are missing something – like the weight of 42 L of water (42 kg).

Use motor oil. Only about 36 kg for that volume.

I contacted Zpacks and have a resolution to the discrepancy of my calculated volume vs the stated volumes of a 55L ArcBlast. Turns out, my pack is actually a rare 45L pack that was only made for a short time. Prior to ~2015/16, the ArcBlast was offered in 45L, 52L and 60L versions, but it was a very different style than currently offered. The hipbelt and shoulder straps were sewn directly to the pack vs the removable hipbelt and adjustable shoulder strap system, the “string” that forms the trampline was dyneema cord instead of webbing, etc etc. In 2016, Zpacks revised the design to what you see today (and the style my pack is), but for a short time, continued to offer the 45L, 52L and 60L options. My pack is from that short era.

SO, my apologies to Zpacks for calling them lying stinkerheads in this regards.

HOWEVER, the reason I thought my pack was a 55L, is that is how it was listed in the Bargain Bin when I purchased it. When I contacted Zpacks, they confirmed the listing for me. Since I didn’t want a 45L, I asked about returning it. Unfortunately, i’ve had it for well past the one year “warranty” period so that’s not an option. Since this was definitely they’re mistake, they offered a discount on one of the current bargain bin packs, which I’ll likely take them up on. So, we’ll see what happens at this point.

Hi Ryan

Given a (hopefully) satisfactory resolution to the mystery, would it be appropriate to amend the title of the thread to something like ‘Fun w/ packing peanuts’? You cannot do this of course, but I can.
Please note: I will not do this unless asked by you.

Cheers

Yea that’d be just fine. Thx for offering