What To Look For
Though hydration packs are largely similar from pack to pack and company to company, there are some subtle differences that you want to be aware of when shopping. Look at it as a two-part shopping process: the hydration system and the pack itself.
Hydration packs come in a variety of sizes, usually 35 oz., 50 oz., 70 oz., 72 oz. and 100 oz. You may also see packs listed as 1 liter, 1.5 liters, 2 liters and 3 liters. The first purpose of the hydration pack is to provide you with a portable water source, so get one large enough for your average trip. If you'll be taking multi-day trips with limited natural water supplies or long trips through the desert, opt for a 100-oz. pack. Otherwise, use your judgment as to how much water you need on your average trip. It's best to opt on the large side so that you are sure to carry enough water.
For ultralight applications or sports like running where you prefer to keep as light a load as possible, consider the weight of the pack. In these cases, you may even opt for a pack that is essentially a bladder with straps.
If you plan to use the pack for winter sports, be sure that it was designed for winter use and has an insulated water tube to protect it from freezing.
Most common bladders use a screw-on lid for filling purposes. Some bladders also use a top seal, which makes them easier to clean and fill. Microbial design can help ward off bacteria and algae that can grow in the bladder.
Make sure that the bite valve is easy to operate, especially if you plan to use it with your hands tied up (i.e. biking or climbing). It should also be easy to lock so there's no chance of water spilling out.
Hydration packs come in a variety of sizes and designs. Packs range anywhere from no storage space at all, to 90 cubic inches in slim packs that are good for storing basic items like keys and a map, to sizable day packs around 2,000 cubic inches.
Extra compartments will help keep your gear organized and accessible.
Packs will be put under all kinds of rigorous conditions so be sure that they are built strong and appear durable.
Packs should feel comfortable and fit your body in a way that will make them functional for the activities you'll be doing.
Common Pitfalls
No hydration pack meets every demand. Consider closely the specific activity or activities you'll be using it for most and purchase your pack accordingly. You can always buy additional packs or kits later for different uses. If you buy a 4,000 cubic-inch backpack with bladder and spend 75 percent of your time taking three-hour day hikes, you'll be lugging a lot of unnecessary weight. On the other hand, if you'll be doing mostly backpacking, don't waste time with a separate hydration pack and opt for a multi-day pack with built-in hydration sleeve. You don't need to buy something specifically labeled "hydration pack," since many backpacks on the market are designed to integrate hydration.
Where To Buy
Since hydration packs are pretty simple gear items and mainly require comparing basic characteristics like size and compartments, you can purchase them comfortably online. Two of the top outdoor sports retailers, REI and Backcountry, offer a variety of hydration packs for all types of uses. On the off chance that you get a hydration pack that is too large, small or otherwise ineffectual, both retailers offer exceptional return policies. If you are having trouble visualizing what size pack that you need, or want to try a few packs on to gauge comfort, any outdoor or general sporting goods store should carry the packs. Specialty shops like bike shops and ski shops are also good places to shop.
Cost
Cost is largely related to the size of the pack. A very basic hydration pack with 36-oz. bladder and little storage will run as low as $25. Larger day packs will run between $100 and $150. Of course, many more expensive backpacks include hydration bladders or sleeves in which to place a bladder, so these could run you upward of $500, but more likely in the $150 to $300 range. Separate bladders usually run between $15 and $30.
