Q:

How to determine the needed volume size when purchasing a backpack?

In addition to sizes for different sizes of people, backpacks also have a volume size, usually given in liters or cubic inches, as a measure of how much stuff the backpack can hold.
How can I determine what volume size I need, for a given time frame, from say a day hike to a multi-week backpacking trip?

A:

What you need is quite individual and the best way to tell what works for you is from your own experience.  Some ultralight backpackers use backpacks that I would personally consider impossibly light and small (in particular in the USA), but they may be happy to lie under a thin single-layer sheet even in bad weather.  Others prefer to carry a bit more for comfort and (perceived or real) safety.  It depends on the climate too.
Try with a 65 litre backpack.  Those are very common.  If it seems tight for a 7-day trip, get something larger for your 14-day trip.  If it seems too large for a 7-day trip, then it may be good enough for your 14-day trip.  Etc.

A:

I always take my 80+25L backpack. Exception might be warm summer weather. It does not matter if I go 1 day or 1 week trip I take large backpack. And here is why:

I can easily take things from my wife\kids\friends and put them to my backpack quickly. I don't want to think if everything I need to take will fit into backpack.
Putting to many thing into small backpack will make it uncomfortable for carry.
If I go for 1 day winter trip I take crampons,ice axe, thermos and down jacket(all this can fit into small backpack). So in case I would need to spend a night without tent and sleeping bag I can put my legs into backpack and wear down jacket.

Weight of such backpack is around 2.5kg. This is the price I pay when I'm taking large backpack.

A:

Typical volume requirements
Pack requirements vary hugely depending on your hiking style, the weather, and on the length and purpose of your trip. But to get you oriented, here are some typical volumes:

For a summer day-pack 15 liters should be adequate.
Lightweight hikers might tackle a thru-hike with a 40-50 liter pack.
Traditionalist hunters, fishers and bushcrafters packing into a base camp
might need 100 litres or even more, depending on their style and
equipment.
While camping in extreme cold requires more volume for bulky sleeping
bags and clothing.

Most people will simply list their equipment and total up the estimated volume of each item. Then add around 1.5 litres for each day of food, and, obviously, 1 liter for each liter of water.
How to select your pack
Unless you intend to build up a quiver of packs for different uses, the key is to err on the large side. Most of the pack's weight is in the frame and straps - the weight of additional fabric for an extra 10 or or 20 liters of capacity is minimal. If the pack has a good compression system you can use it for a very wide range of capacities and still have it carry well. It's also quicker and easier to pack a sack when it's not too tight for capacity. There's very little penalty for a pack that's a bit larger than you need, while a pack that's too small can be a major issue.
Packs will generally fall into 3 categories:

Frameless sacks for light loads
Packs with a lightweight frame designed for loads up to around 15kg/30lbs
Load haulers with substantial suspensions designed for big weights.

At the very most you should be able to cover your general requirements with 1 pack in each class. Though for many people, a single pack with a lightweight frame should cover all their needs.
Finally, there is no standard for measuring pack volume so advertised capacity can vary widely between manufacturers. Again, the answer is to err on the large side. Something I've done in the past is to take my equipment into the store and see how it fits in the pack - a good specialised retailer should be happy accommodate you, particularly if you call ahead and ask.

