is there a ratio of pounds per cubic inches for packs?
i want an external with 35-45 pounds of room. how many cubic inches would i need. i want to go 3-5 days and i am the type that likes to bring lots of luxeries like a bottle of wine per day and any good food that wont spoil. i like to treat myself after hiking all day.
i will be hiking mostly NJ,PA,NY. The AT and other local trails that allow camping. No extreme cold, just 20 degrees and above.

Object density is what affects volume as compared to weight. My down sleeping bag takes up a helluva lotta room, but doesn't way as much as that bottle of wine you will be carrying. The foods most hikers carry are very dense and therefore weigh more. If for some resaon you decided to carry gold bars, you wouldn't need a large pack to hold all you could possibly carry, but if you were to carry down sleeping bags, your pack would have to be HUGE to hold everything you could carry.

As posted elsewhere, the general procedure for buying a pack includes buying all your other gear first. Then, take all that gear, plus water, and food, and go to your local outfitter and try out loading up various packs.

I don't recall a rule-of-thumb, but my guess would be 100 cubic inches per pound for an internal pack. Of course with some external frame packs it would be more difficult to have a 'rule-of-thumb' because a lot of gear can be attached directly to the frame.

Youngblood

One hundred cubic inches per pound is the ratio. That puts you at about 3,000 to 4,500 cubic inches. Because you have an external frame you gain about an additional 1,000 cubic inches, because of items such as sleeping bags and or tents are not packed inside.