Calculating Cubic Inches… any ideas?

[Ryan Gardner]

Does anyone know a good way to calculate the cubic inches of a top loader type backpack? I was thinking I could take a pack liner, insert it into the main compartment, fill it with water, then take it out and fill the water into some sort of container where I can calculate the volume.

Would this work? Or is there an easier way to do this?

[Rick Dreher]

Hi Ryan,

I've done almost exactly that, using a garbage bag as a liner. I measured the water going in to avoid having to do so after the fact. I'd only do it with strong packs, as the weight adds up quickly, really stressing the fabric and stitching (e.g., 40 liters, 88 pounds). Definitely don't try it with silnylon packs.

Packing peanuts are safer, and will still give a pretty reliable volume estimate if you compact them the same in the pack as in your measuring container (shake to settle). A tall skinny box makes a pretty good container for measuring net volume. Or, use popcorn and go to the movies afterwards :-)

[Vick Hines]

The liner is a good idea, but something lighter than water will give you a more realistic – and easier – medium to work with. Packing popcorn works, but sawdust, mulch or sand would work just as well. Dump it into a large box, level it, measure the height and multiply, etc.

[Elliott Wolin]

Or, weigh the contents, then measure out a known volume and weigh it, then calculate the volume.

[EndoftheTrail]

I would just take a ruler, do a careful measurement of the pack's height, width, and depth — and multiply all three. Repeat for outer pockets — treating each one as a separate rectangle.

[Roger Caffin]

> I would just take a ruler, do a careful measurement of the pack's height, width, and depth — and multiply all three.
But packs are never box-shaped. They distort, and that alters the volume.

Actually, a lot of this is moot anyhow. Take a mass-market pack and measure the volume with packing peanuts. Now try to relate your measured volume with the catalog!

[Jason Brinkman]

"Packing popcorn works, but sawdust, mulch or sand would work just as well."

Sand is over TWICE as heavy as water! Use foam peanuts.

But if you already have the pack… just put your stuff in and see if it fits.

[Randy K]

Dan McHale has a cubic inch calculating formula on his website. I believe it's on the "secondary links page" or something like that. I cannot vouch for its accuracy, but it seems logical.
roosterk

[Paul Tree]

manufacturer's web site perhaps.

[James D Buch]

Ben2World Wrote:
I would just take a ruler, do a careful measurement of the pack's height, width, and depth — and multiply all three. Repeat for outer pockets — treating each one as a separate rectangle.

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I would do pretty much the same. I have gone in some stores with a ruler and calculator and scoped out the pack size by measurement, and then calculated the volume. It usually compares well to what the manufacturer lists on the info tag.

There are a few cases where shapes taper, but if you remember some of your geometry, you can compensate for that. One relationship is Volume = Area * Height for a fairly general cylinder shape. You can compensate for the area change by computing the top and bottom areas and taking the average of the two.

Actually, knowing the EXACT volume is probably not all that useful anyway. Many folks are happy with 30, 40, 50 or 60 liter sizings. You know that the 30 liter bag could really be 33.27 liters or maybe 29.02 liters. But, they never say that so explicitly.

[Mary Sorensen]

The way I did it was to enter "45 liters to cubic inches" on the Google search bar and it came up as 2746. Or is that too easy?

[John Haley]

As Randy K said… Dan McHale has an excellent calculation formula.

http://www.mchalepacks.com/ultralight/Detail%20Hi%20Rez%20Pages/Pack%20Volumes%20-%20Go%20Figure!.htm

It seems that most manufacturers use different methods for computing pack sizes. Dan's formula makes the number real.

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