Im doing a weekend backpacking trip. I also do some bikeing and hicking. i dont quit know what a good size bag would cover all that, dont want anything to big. I also want to be able to wear it comfortably while im rideing my bike. Any ideas.

Standard reply to this frequently asked question:

You should buy your pack only after you have accumulated everything you plan to put in it, then buy a pack just large enough to hold that amount.

Additional standard reply:

For fair weather (late spring to early fall), if you choose gear from the more lightweight (and expensive) end of the spectrum, you should end up with a carry weight under 25 pounds, including food and water, that will comfortably fit in a 3,000 - 3,500 cubic inch pack. The pack itself shouldn't weigh more than 3 pounds, usually. This also assumes you aren't trying to pack all the comforts of home.

If you go as light as possible, you'll need less pack. If you take the five pound two person dome tent, the synthetic bag, the thick inflatable sleeping pad, etc., you will need a big heavy-ish pack to haul it in.

And then there's the matter of a pack fitting me that doesn't necessarily work for you. Osprey packs for example tends to work for some people but not all. I hate the frames on them, because the frame hates me - makes me gimpy, limpy and sore. You might find it the most comfortable pack in the world. It Just Depends.(tm)

Try on some packs with weight in them, or find a store that will let you bring your gear in and find a pack that it'll fit in.

If you ask me (and you did), there is no such thing as a pack that is comfortable to wear on a bike, even one that is great while walking, especially if you are getting up around 3500 cu in.

another standard reply, borrow one and see how it works.
Jim

and another standard reply...

use the search feature for pack, backpack, size etc.

set the time frame for 6 months

Or better yet, one year.

Also, search the 2005-2008 archives, which have lots of useful info.

All good replies so far. Unfortunately for me, I'm an engineer by training so I like a little more concrete of an answer. I found that answer in what I consider to be the backpacker's Bible, Colin Fletcher's The Complete Walker (Amazon)

Colin Says Pack Volume (in cu.in.) is roughly described by the following forumla:

Pack Volume (cu.in) = (Body Weight + 30) * SQRT(# of Days) * R * S * T * P

Where

R = your resilience
0.75: Sleeps naked on bedrock, hikes barefoot, eats small animals raw
1.00: Sound sleeper, feet always warm, laughs in the teeth of the wind
1.50: Pleasantly normal
2.00: Cold sleeper, comfort hound, serious photographer or gourmet chef
2.50: Feet of ice, special diet, multiple phobias, needs background music
Or any value in between

S = Season
4 = Summer
5 = Spring or Fall
8 = Winter
Adjust accordingly

T = Gear type
0.9: Ultralight
1.0: Light
1.2: Average
2.0: Cast-iron and canvas

and P = party Size
Solo: 1.5
Two: 1.0
Three: 0.7
Four (or more): 0.6

So, for me to go out for a 3-day weekend with my buddy in the Fall, I'd plug something like:

(170+30) * SQRT(3) * 1.25 * 5 * 1.1 * 1 = about 2400 cubic inches, or roughly 40L pack... about right for me and my gear.

I have no idea how to adjust this for use when cycling. I'd rather drop my pack and just ride loops (ala base-camping), or pack my gear on the bike (ala cycle-touring).

Hope this helps.

Neat. I'd forgotten that St. Colin had taken a math detour in CW4.

For me: (180+30)*SQRT 3 * 1.5 * 5 * .9 * 1.5 = 3700 cubic inches. I use a 3600 cubic inch pack, but sometimes add a couple of side pockets for early spring or late fall.

So, that's two results that are pretty close.

If I could only have one pack, it would be in the 3500-4000 cu in range, especially if I could cinch it up for dayhiking loads.

+1 on the 3500 to 4000 sq. in. pack. My current pack is about 3800 sq. in. and it has served me for up to 11 days w/o re-supply. And, without using a bear can, I could have gotten at least twice that amount of food aboard - if I were not adverse to weight.