I've recently started making my own bike luggage (and am designing some accessory pouches to extend my lightest backpack). To compare to what I can buy and have owned in the past, I'd like to measure the volume.

The simple frame bag I made can be approximated as a cuboid with rounded corners, giving me about 8l - but this will increase if stuffed as the cross-section will be more rounded. The toptube bag, however, tapers in height and width, and has an arbitrary curve, while a tool pouch is an irregular pentagon from the side and tapers.

So I'd like an actual measurement. These aren't fully waterproof, though made from waterproof fabric, so I can't pour water in (it would flow out at the seams and zips, and the openings aren't necessarily on top). What can I use, that's cheap, easy to get out again (so not sand; anyway that's too heavy), and ideally not wasteful?

  • These aren't fully waterproof. But are they waterproof enough to get a reasonable estimate by quickly pouring in water from prepared containers holding a known quantity? Commented Jun 29, 2021 at 15:50
  • @WeatherVane not at the moment, if I seal the seams
    – Chris H
    Commented Jun 29, 2021 at 18:51
  • 2
    If I understand your use-case correctly, you could pour measured volume of water into polythene bags and put them into the bag.
    – WedaPashi
    Commented Jun 29, 2021 at 19:28
  • 1
    @WedaPashi balloons filled with water, and not massively, are softer and stretchier than polythene bags and will deform better to fill the space. Unless you meant a single bag per compartment, as in user3067860's answer
    – Chris H
    Commented Jun 30, 2021 at 8:17
  • 1
    @WedaPashi thanks, I thought about using a polythene bag, but they usually have holes in them, and it might be awkward to expel any air and seal, and they don't stretch and mold like rubber does. I wish I had thought of the bin liner solution. Commented Jun 30, 2021 at 11:13

4 Answers 4

  1. Put a large trash bag (bin liner) into your bag. (Double bag it if you feel the need.)
  2. Fill the trash bag with water, either using a measured container to fill it until you reach the top of your bag, or fill the bag first and then measure while or after you pour it out.
  3. Use the water to water your plants, let the trash bag air dry and use it as a trash bag.

This will be considerably easier if you have someone to help you hold the bag upright while you fill it. Also, tipping the water out may be a bit heavy. But using a fluid will get you the best measurement, and liquid is easier to measure than gas.

  • 1
    But a gas is a fluid! :p Commented Jun 30, 2021 at 3:01
  • 1
    For a full size backpack I'd use something heavier duty like a rubble sack or survival bag, but this is simple and effective. Some of my more awkwardly shaped bags might be harder this way.
    – Chris H
    Commented Jun 30, 2021 at 6:01
  • 4
    You can do all this with packing peanuts instead of water. Except your plants might not appreciate it Commented Jun 30, 2021 at 15:07
  • 5
    You can weigh the empty and full bag - that's likely easier and more precise than the volumetric way
    – cbeleites
    Commented Jun 30, 2021 at 17:09
  • 1
    @ChrisH: It's easy to use your bathroom scales! First weigh yourself, then don your backpack and weigh the two of you combined.
    – TonyK
    Commented Jul 1, 2021 at 14:45

In addition to the suggestions above, how about using dried beans or similar (e.g. chickpeas, kidney beans, etc)? Once you have successfully measured your pack, you can rinse them and cook with them.

  • +1 but I suspect that rice is easier/cheaper to buy in bulk and it has smaller grains
    – 0xFEE1DEAD
    Commented Jun 30, 2021 at 22:08
  • 1
    @0xFEE1DEAD True. Smaller grains help with getting an accurate volume, but they are more likely to get stuck in corners and be a pain to clean up. Pick your poison...
    – avid
    Commented Jun 30, 2021 at 22:39
  • 1
    I buy dried pulses in 2kg bags anyway, so this is quite reasonable. With the selection I have, chickpeas would be the way to go. Rice (and lentils) would get caught in the seams, and the smaller grains wouldn't be much help in assessing volume compared to chickpeas which are small enough and near enough spherical, so should pack reasonably repeatably. It would take a long time to eat enough for a big pack, but for my smaller ones, this is an option. (also @0xFEE1DEAD)
    – Chris H
    Commented Jul 1, 2021 at 7:29
  • @0xFEE1DEAD Can you buy it at the 0xDEADBEEFCAFE? Commented Jul 1, 2021 at 18:05

Buy some rubber party balloons and a funnel. Pour a measured amount (say 0.25 L) of water into each, such that they are not stretched very much, squeeze out any air, and tie off the neck. Pack them into the bag so they distort and nestle snugly. Handle carefully so as not to rupture. It might be easier to control the filling and tying if each balloon is stood in a plastic tub, such that there is a loose fit after the water is poured.

To be clear: the balloons would not be tightly filled with water as they would be with air. They are filled only enough so they remain flabby, and so that the rubber can stretch and their shape can mold to whatever they are next to, or into a square corner, such that there is no air between any of the balloons, or the walls/corners of the bag.

  • 1
    Water balloons aren't actually that great at filling. The tension needed to keep it "balloon" shaped keeps it..balloon shaped....so it is closer to sphere packing. Commented Jun 29, 2021 at 22:21
  • 1
    @user3067860 the point is, not to have the rubber under much tension, so that they are not sphere-shaped. They are not strong enough use in the same way as if filled with air. The idea is so that they will 'pack' without any interstices. Commented Jun 30, 2021 at 5:21
  • You could also use ziploc bags full of water to much the same effect, and I would expect them to pack more densely than the balloons might. Commented Jul 1, 2021 at 18:00
  • @MichaelSeifert please see my edit and other comments about the (lack of) inflation or rigidity. Polythene doesn't stretch much, and IMO some rubber bladders will pack better. The balloons are used as bladders. Commented Jul 1, 2021 at 18:10

Pine bark mulch is a pretty good substance for your purposes.
A bag of dry mulch is inexpensive (less than three dollars for a 2 cubic foot (57 L), 40 pound (18 kg) bag on Amazon) or you can get it at any garden center. Also, you probably have a friend or a friend of a friend who could use it after the experiment.

Addendum: A bag of dry mulch weighs about 40 pounds (18 kg) and is two cubic feet (57 liters). Bagged pine bark mulch is easy to pour, dry (assuming the bag is intact), clean, and not dusty. (Shredded hardwood bark mulch does not pour easily.) You may want to swab out the inside of your bag after you have poured the mulch out, but that goes quickly.

  • 1
    A bag of mulch is usually 2 cubic feet. Two cubic yards of mulch is 54 cubic feet, and would weigh 1000 lbs or more! Commented Jun 29, 2021 at 17:23
  • @Nuclear Hoagie Thanks, and corrected.
    – ab2
    Commented Jun 29, 2021 at 17:30
  • 4
    You can line your backpack with a trashbag and dump the mulch into that instead of directly into the backpack, and then pull it out avoiding most of the mess inside. Commented Jun 29, 2021 at 20:51
  • @DanIsFiddlingByFirelight I've stolen your comment a bit and made it an answer--except skip the mulch and just use water. Commented Jun 29, 2021 at 22:09
  • 3
    But it is compressible, which makes it not ideal to measure volume.
    – Ant
    Commented Jun 30, 2021 at 12:14

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.