Pack Sizes

When my youngest son was about 4, I was in the market for a new backpack. In those days, my thinking was that if big was good, the bigger was better.  I envisioned myself going on treks 7 days at a time with no resupplies.  With this in mind, I was proud to purchase the Kelty Red Cloud 6400 (Big Red). When I bought it, my son could literally ft inside of it.  He is now in his twenties and that is no longer a good idea.  Neither, it seems, is getting the biggest pack you can just because you can. One of the properties of a gas is that it will expand to fill its container.  This is also true of packing.  If you have enough space, you will find something to fill it with.  This led to me giving up trail comfort in favor of camp comfort which is not always a wise exchange.  I used to think that the strange looks I got was because folks were impressed with the behemoth on my back.  Age has taught me that the look was actually more like "Wow. What an idiot.".  This is especially true of thru-hikers that I would see on the Appalachian Trail.   The pack has been good to me, but 20 years of carrying more that any sensible person would has taken its toll on Big Red.

I am now in the market for a replacement, so I am planning to downsize (it would be hard not to).  Packs sizes are often measured by volume using either cubic inches or liters.  No one measures packs by quarts or gallons, but I thought it would be useful in the chart below to give an idea of how much they hold.  Conversions are rounded to the nearest integer.

Bag Type	Cubic Inches	Liters	Gallons	Quarts
Drawstring/Cinch Bag	693	11	3	12
School Bookbag	1343	22	6	23
Weekend	2746	45	12	48
Multiday	3967	65	17	69
Extended Trip	4577	75	20	79
Big Red	6400	105	28	111

It seems reasonable that anyone thru-hiking the AT might have a good idea what a good pack size should be.  Here is what a survey revealed from hikers that did that in 2019.

Thru-hikers packs had an average (mean) capacity of 52 liters, but anything between about 43 and 61 liters was common. Thru-hiker pack capacities ranged from 25 to 80 liters.
The top three most common sizes were 55, 60, and 65.

I will post back here after I make my final choice, but I am pretty sure it will be less that 105 liters.

edit: I decided to go with the Osprey Aether AG 70.  At 70 liters, it is no thru-hiker pack, but it is closer to it than it is to Big Red.