I was wondering, how do you determine how much fabric to use on a pack liner (or stuff sack) and how that contributes to the size of the finished product?

Height is straight forward, width isn't as much. I feel like I need some pie =) har har.

So for example, trying to make a pack liner for a pack that measures 24"H X 11"W X 8"D

Start with maybe 30 inches of fabric so I can roll it down a few times and account for hems. For the width of fabric though...

Should I just be squishing my pack flat, measuring that, and making the liner to conform to that width? I suppose that might make sense, but wanted to double check.

> Start with maybe 30 inches of fabric so I can roll it down a few times and account for hems. For the width of fabric though...

Well, a pack liner needs to match the internal dimensions of the pack. So, for a 24x11x8" pack, you need four side panels, 2 off 24x11 and 2 off 24x8. Then you need a base piece 11x8. Since packs are generally rectangular-ish rather than circular, you're better off using a rectangular base piece with rounded corners. Which means that no pie is needed...

Then you need to allow seam allowances.

Of course, you're better off making the main body with a single seam, from one piece that's 24x(11+11+8+8), i.e. you just need to add up the lengths of the perimeter 'faces' of the pack to get the 'width' of fabric.

As you suggest, the 24" depth is probably better increased by a few inches to allow for a roll-top closure. Since this roll-top will form the top face (nominally 11x8), you will need to extend by an absolute minimum of 4" (8"/2) to close the top on each side. Allowing a 1" roll and a three-fold closure, that will need at least another 1+3".

It's really a bit like deciding how much paper you'll need to wrap a parcel.

Or have I missed something more subtle in your question...?