How big for a 72 hour pack

[Walkman]

I'm going to buy a new pack for my 72 gear at some point. Right now I have a AF CFP-90, but I'm changing systems.

How big in cubic inches (since that's the way they are listed online) should I be looking at?

[Eeyore]

This is what I carry my 72 Hr gear in: http://www.botachtactical.com/camelbaktalon.html

Dimensions: 17 in x 14 in x 10 in (432 mm x 254 mm)
Volume: Cargo – 1820 cu in (29.8 l) Total: 2000 cu in (32.8 l)
Total Volume – 2730 cu in (44.7 l)
Weight: Empty Reservoir – 3.80 lbs (1.70 kg)
Filled Reservoir – 10.10 lbs (4.50 kg)

[Walkman]

This is what I carry my 72 Hr gear in: http://www.botachtactical.com/camelbaktalon.html

Question, do you keep your 24hr gear in a separate pack then transfer what's needed for the mission into the 72, or do you have a separate pack? The reason I'm asking is that I was thinking of going with a buttpack for the 24 hour gear, then keeping 72 hour gear in the backpack. That was I can grab either on my way out the door and not have to transfer basic first aid/survival stuff from one to the other. That being said, if 1800 cu in fit 24hr gear + the added 72 hour stuff, maybe I could go with something smaller.

Nice pack though. Does that smaller pack on the back detach?

[Eeyore]

I keep them separate, but I don't transfer anything between the two. My 24 hours is just 24 hour, my 72 hour has both the 24 hour and 72 hour stuff in it. Two completely packed set-up's and I grab whichever one I need. It's nice because I don't need to worry about forgetting anything.

I did the butt pack, pistol belt and suspenders thing, but it was just too uncomfortable.

Now I use the Camelbak Transformer for my 24hr http://www.botachtactical.com/camtran.html

On the Talon the outside pouch does not detach, it is held with a flap of fabric on the bottom, you could probably modify it pretty easily though.

On the Transformer, everything detaches, it's very modular. I like it for my 24 hour because if I'm just sitting on a flight line, I detach the pouches that have the stuff that I don't need.

[Walkman]

For the buttpack I wasn't going to go pistol belt. I've been looking at some made specifically for ES use. Nice padded waist straps, etc.

Good point on keep each as it's own unit. I might have to think about that myself. That way I can use a 72 pack that has a waist strap as well for better LB.