Your rucksack is an important item to any hiking trip, but with such a bewildering array to choose from, which one do we need? To be honest, we could write a book over all sorts of excellent brands, we certainly have our own personal preferences, but the simple answer is you need to find what you feel you need for the hike or what you are aiming to do for your trip or adventure.
Your rucksack choice is a personal thing!
So, to start, there are mainly four size ranges to look out for, and this is normally determined by what style of day out you are having. This is a simple overview of different capacities and their most appropriate uses.
10L to 25L
Mostly used for trail running or fast walking and long-distance walking, but can also be used for a small hike when just carrying a drink, food and waterproofs or anything else lightweight that you feel is needed for your outing.

25L to 45L
This sort of rucksack is typically used for an all-day walk but also has the capacity to support an overnight stay in a hut, bothy or B&B etc. The incremental capacity is helpful for items you need for cooking, sleeping and of course, your food. If you can afford it, and are competent in self-sufficiency outdoors, the purchase of Ultra-light gear in this size of rucksack is a great space/weight saving option.

45L to 65L
This big boy is starting to take on serious meaning, rucksacks with this much capacity are designed for 1 to 3 days hiking for staying outdoors . This one takes all the gear required for the trip, Food, clothing, tent, sleeping bag/mat etc, etc.

65L to 120L
Used for big adventures when you are staying or going on a trip for a long time , Mostly used in high mountain trips when warm and cold gear is required and extras for all sorts of conditions. One of the drawbacks with this much capacity can be the tendency to over pack, adding to the weight carried, its at that planning moment you really need to be ruthless about desirable and essential items for the trip!

In our experience, despite the vast array of choice, we tend to find that the most widely used is the 25L to 45L , this you will find, is an adequate an all-rounder and used by hikers who get to learn what they need to take from years of experience enduring different types of weather and trips.
We all have our own items and gear we like to take, but the first rule before your trip is planning what are the essentials that you need for your trip, anything else after that is a plus! (See Outdoor preparation, what to carry in your rucksack).
Things to look for when selecting your rucksack of choice.
ABS system (adjustable back system ) which you can adjust to the length of your back for a comfortable fit.
Air flow system, which is a space between your back and rucksack that keeps air circulated and keeps sweat away from your back, this is typically a mesh net system.
Compartments and pockets, Look for one that has a compartment for a hydration bladder, side pockets ,mesh side and front pockets, lid top pocket and some hip belt pockets. You will quickly determine where you want to stow your most accessible accessories (jelly babies)!
Rain cover, normally on most rucksack fixed underneath of rucksack. (remember a rucksack is never 100% waterproof, so therefore be prepared to use drybags for your gear or even plastic bags to keep things dry).
Hip Belt, helps to evenly move the weight to your hips and back .
Chest strap, keeps the rucksack steady while walking over uneven ground etc.
Walking poles/ice axe Loops for fixing them safely to rucksack.
The above details are the minimum aspects that most decent rucksacks will offer, the best way to find and fit a rucksack, is to visit a good outdoor shop who will be prepared to help ‘fit’ your choice appropriately to your frame.
Our choice, Chris is a big Osprey/Berghaus/Mountain Equipment fan, and for me My choice is either Deuter/Lowe alpine or for lightweight days Mountain equipment.