I have a North Face Big Shot backpack that I purchased around 2003. It was a great general purpose backpack that I used for day hikes, rock climbing, and traveling. Unfortunately time and the abuse I've inflicted on the pack have taken their toll and the pack needed to be replaced. I've replaced it with a REI Trail 40 pack since I didn't like the current evolution of the North Face Big Shot pack.

I'm curious if I've gained or lost any space with my replacement pack. While I know I can try to cram as much stuff into both packs to see if one pack fits more, I'd prefer to have something more quantifiable than an "X shirt increase." So what is the best way to get a proper volume measurement on an old pack? I tried to google the older pack but I kept finding newer versions of my pack. The different newer versions all hovered around 2,000 cubic inches, which is quite a bit smaller than the 3,000 cubic inch volume I seem to recall. My memory isn't perfect though so maybe my old pack really is smaller than I recall.

Here is a pic of my old pack for reference. While I currently want an answer for this specific pack I'd prefer a more general solution. Also while I'm sure it would be the most accurate I don't know if I'd be able to handle the math needed to calculate the volume based on the complex curves and angles that make up most packs.

North Face Big Shot pack

  • I'm wondering if you need to make a distinction - do you literally want to know the volume it can hold (in which case you can use whichever "fill and measure" approach mentioned below you want), or do you want to know what number it was marketed as (in which case you need to understand the specific method used by the manufacturer, which we may not be able to provide)?
    – dwizum
    Mar 28, 2018 at 17:43
  • @dwizum in my initial google search I was looking for the marketing number since that is the easy spec to research on the internet, but what I really want is the actual volume so the "fill and measure" approaches are appropriate answers.
    – Erik
    Mar 28, 2018 at 17:47

6 Answers 6

  1. Put a garbage bag in the pack, with the open top of the garbage bag sticking out of the pack.

  2. Pour water into the garbage bag one liter at a time, keep track of how many liters of water you use.

  3. Use Google to convert the number of liters to any measurement you prefer.

  4. Empty the water from the garbage bag.

P.S. You might want to do this outside or in a bathtub.

EDIT There have been some concerns in the comments about the backpack holding a full load of water.

Related: Why are backpacks sized in liters?. While it is unlikely that a backpack will be filled with water, it is likely it will be filled with school books, tools or canned food, all of which are roughly equivalent or greater in density to water. It would be a poorly designed backpack that is not able to carry weight equal to its volume of water without failing. While I do suggest volume testing occur someplace water-safe, if you really have fears the bag will not survive the test, I can't see taking the pack to school or the trail.

  • 3
    This is probably the most precise method, however given how much water weighs, I foresee problems using this method with larger packs. Mar 28, 2018 at 17:32
  • 3
    Rather than garbage bags I suggest rubble sacks; they're tougher and more likely to take the weight where they aren't well supported by the pack
    – Chris H
    Mar 28, 2018 at 20:01
  • 2
    +1, but make sure it's not one of the bags that have small holes in the bottom..!
    – berry120
    Mar 28, 2018 at 20:19
  • 3
    +1 for the method, I have tried this and it works fine, and good to see @berry120 back with us :-)
    – WedaPashi
    Mar 29, 2018 at 4:35
  • 2
    To reiterate what @CharlieBrumbaugh pointed out, a 3000 cubic inch pack filed with water would weigh over 100 lbs. However, 3000ci seems like a very big pack - that's like 13 gallons/50 liters...
    – JPhi
    Mar 29, 2018 at 15:52

There's zero industry standardization for calculating the volume of a bag, and there are numerous factors that affect the total volume - like how many pockets/compartments are in the bag and, and the overall non-uniform shape of the bag.

Most companies just do an approximation by stuffing a bag full, then measuring its length, width, and height and doing a simple calculation for volume (L x W x H = V), and rounding to the nearest Litre/5 Litres (1,000cm3 = 1 Litre).

You can quickly measure the approximate volume of your bag by stuffing it with pillows or a sleeping bag, then taking your three measurements with a tape measure. You don't have to be exact, because not a single bag on the market is, which is why you can fit more stuff in some brands of 30L bags than you can into other's.

From your picture I'm guessing that's a 20-30L bag. Odds are you'll measure the volume of your bag and it'll come out to some odd measurement like 26.87L, in which case your bag would probably have been marketed as either a 25L bag, or a 30L bag.

You can take the time to try and figure out the exact volume of your bag, but what is that really going to benefit you aside from satisfying your curiosity? All that matters is how much stuff you can fit in it. The sizes in Litres are just meant to help you get you into the ballpark of what you're looking for in size when shopping for bags.

  • @Erik It's pertinent for this site, but if you want to get exact then you could probably post this on Math.SE.
    – ShemSeger
    Mar 28, 2018 at 17:32
  • 5
    @Erik, Oh they will suggest other maths to use, I gurantee it.
    – ShemSeger
    Mar 28, 2018 at 18:13
  • @ShemSeger I guarantee we at Math.SE could make it awful, triple integrals or worse. But this seems more practical.
    – Charles
    Mar 29, 2018 at 3:54

You need to fill the backpack with something and and then measure the volume of that filler.

It would probably be easiest to to stuff the backpack full with t-shirts until you can't fit anymore, then take them out and stuff them into a shoe box which is a nice cubic shape. Then you multiply the height x width x length to get the cubic size.

I am also seeing ping pong balls, beans and small plastic balls used, where one fills the backpack with them, dumps them out and then calculates the volume of the filling. At least one site claims 20mm plastic balls are the standard.

  • 1
    That makes sense. So I was just missing the last step where I stuff the t-shirts in an easily measurable/calculable container.
    – Erik
    Mar 28, 2018 at 16:45
  • Golf balls would work well, and if you go to a driving range, they probably have large buckets of balls you could "borrow" for the experiement.
    – JPhi
    Mar 29, 2018 at 15:46
  • +1 But a shoebox isn't a cube, it is a rectangular parallelepiped. Also, to reduce the error, try to get a box that is much larger than a shoebox. Try styrofoam peanuts. Fill the box, count the peanuts, repeat with your pack.
    – ab2
    Mar 29, 2018 at 22:20
  • @ab2 styrofoam peanuts might compress in the pack
    – Erik
    Mar 30, 2018 at 0:35
  • 1
    @ab2 Archimedes would have stuck the whole thing in a bucket of water then inflated a bag inside and measured the displaced water to find the answer Mar 30, 2018 at 1:32

If you have some dry-bags handy, seal them up full of air and check how differently they fit in each bag respectively. It won't give you an exact measure, but it should be a good enough (and easy enough) way to solve your problem.


One option is to take the backpack to an outdoors store like REI or LL Bean and compare it to backpacks that list their volume.

  • And risk being accused of theft if you walk out holding an item they sell.
    – Loduwijk
    Jan 23, 2020 at 20:49
  • @Loduwijk, all decent shops have better methods of detecting theft (or else you wouldn't be able to walk in wearing your rain jacket). Feb 3, 2020 at 18:55
  • @TobySpeight Depends on if it is raining. If I wore a rain jacket in on a sunny day I'd worry that someone is suspicious of me. Same thing if I brought a backpack into a place if they sell the same one and mine still looks new. Maybe it's just me, but I worry about being accused of stuff.
    – Loduwijk
    Feb 4, 2020 at 2:38

While water works and is precise, I strong suggest doing this outside or in the bathtub.

A much easier way is to get a bag of packing peanuts, either by scrounging from your local receiving department, or buy a bag at a shipping supply company. You may even be able to borrow some from your shipping department.

This won't be as precise. The peanuts don't fill all the voids as well as water.

Measure using a white 5 gallon pail. Calibrate the pail with 1 liter container, putting the marks on the outside. A strong light on the inside will show you the level inside the pail. After you have done the calibration, you can transfer the marks to the inside. Nominal 5 gallon pails will usually have between 19.6 and 21 liters volume. (Gotta love standards -- so many to choose from)

Fill the pail with peanuts. Dump into the pack. Repeat until full. Note the amount left in the pail. If you have a 20 liter pail, and you did 3 pails and have 8 left over in the pail, you have a 3 * 20 + (20 - 8) = 72 liters.

If you are in an outdoor club you can do this as an evening activity, and see how companies lie.

You may want to track exterior pockets separately.

Compare to doing your best estimate using L x W x H with it hold a sleeping bag.


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