This article focuses on choosing a multi-day/expedition pack, which is generally considered to be a pack with a capacity of over 50L. The major difference (other than increased capacity) between these packs and their smaller counterparts is that they contain a contain an internal frame to stiffen the pack and transfer weight from the shoulders to the hips.

Intended Usage
The first thing to consider when you start to shop for a new pack is what you intend to use it for. If you're planning a hostel-hopping trip across Europe your needs will be quite different than if you're planning to trek through northern Canadian wilderness. This is the first question any sales-person will ask, so you should have an answer in mind before you set out shopping.

Personal Fit
The most important thing about choosing a backpack is making sure that it fits properly. Most quality backpacks are highly adjustable, and you may find yourself thinking "it's okay that it doesn't seem to fit quite right, I'll just adjust it when I get home." Strap adjustments can help with pack comfort quite a bit, but the simple fact is not every pack will work for every person.

Pack length is probably the most important consideration when fitting a pack. If your pack is too long, you'll find your hip belt sliding down constantly and the bottom of your pack may even rub against your buttocks as you walk. A pack that is too short will tend to "ride high", increasing the weight borne by your shoulders, leading to back pain.

To address the length issue, many backpacks come in long, regular and short models. Do not buy the long model just for the extra 2L of capacity! It is also important not to choose a length based on your height alone. Human beings are proportioned differently, and women especially may find that their torso length is not properly reflected by their overall height. Try each size and have the sales person help you adjust the shoulder straps and hip belt.

Reputable backpacking stores will have weights on hand to put in the pack to allow you a more realistic try out. You'll find that a pack that feels great empty may lie entirely differently on your back with some weight in it. Do not buy a backpack without walking around for at LEAST 20 minutes with at LEAST 25lbs in it.

Another fitting consideration is width, particularly in the shoulders. Shoulder straps that lie too narrow will cramp your neck and restrict blood flow. Shoulder straps that are too wide will tend to slide down your shoulder when you lean over to one side. Also, look for a pack that allows you to move the straps in and out easily, as this is something you can do throughout the day to minimize sore shoulders.

Choosing a Capacity
Think about the trip where you'll be carrying the most stuff and then add on those few extra items you're forgetting. Will it all fit in your new pack? I recommend laying out all your gear before you go shopping to get a rough idea of how much stuff you have. Check out our camping gear packing list if you need more information.

I generally recommend that people go with a larger pack than they may think is necessary, as it makes packing easier and it's also less likely that you'll need to resort to lashing things on to the outside of your bag.

While more space is generally better, a pack that is excessively large means you're carrying unnecessary weight and bulk. Another common beginner's mistake is buying the hugest backpack available, packing it completely full, and then discovering there's no chance of carry that amount of weight all day. Additionally, if you're flying somewhere, an over-sized pack you could result in unnecessary excessive baggage charges.

Here are some guidelines for backpack capacity and appropriate usages:
50L-60L:

• Long weekend group camping trips
  
• Shorter bare-bones solo camping trips
  
• Extended "hostel-hopping" trips

60L-75L:

• 3+ day solo camping trips
  
• 5+ day group camping trips
  
• Shorter winter camping trips

75L+:

• Camping trips over a week
  
• Extended winter camping trips
  
• Trips where special equipment is required (eg rock climbing or science expeditions)
  
• Leading a group (Eg High school teachers who need to carry extra first aid equipment, food and backup gear)

Construction
You'll be depending on your pack to hold up through all kinds of abuse, so it's important to make sure it's well made and sturdy. Here are some things to look for:

• Durable fabric type, like Cordura or packcloth. A higher denier count means that material is more durable, at the expense of being heavier. Newer fabrics can be quite strong even at low denier counts. Different materials also have different water resistance properties and maintenance requirements.
  
  
• Check all seams to make sure they are double stitched with beefy thread. Pay special attention to seams connecting straps to the rest of the backpack, as a lot of stress will be put on these parts.
  
  
• Check for high quality, metal zippers. Zippers should be covered by material flaps to prevent water from seeping into your pack.
  
  
• Check the inside of the pack as this is where sacrifices are often made on cheaper packs. Inside seams should be covered with material, as loose threads will cause the material to fray during use.
  
  
• Look for urethane coatings (or other waterproofing materials) on seams and frequently exposed parts. Don't expect this type of waterproofing to hold up to dumping your canoe, but it should keep your gear dry during a normal rainy day.

I hope this helps with choosing a pack. If you have anything to add, or you would like to recommend a certain pack, please use the comments tool below!