Temperature Rating: Choose the proper bag for the type of camping you will be doing. The temperature ratings assigned by manufacturers are a rough guide. There is no industry standard method of determining temperature ratings of sleeping bags.

Sleeping bags can be divided into the following four classes based on manufacturers' ratings and bag design:
Summer camping: 40o to 50o F (4o to 10o C)
Spring/Fall camping: 15o to 35o F (-9o to 2o C)
Winter: -10o to 10o F (-23 to -12o C)
Mountaineering: -30o to -10o F (-35o to -23o C)

Shape: Sleeping bags come in three basic designs:
MUMMY BAGS: The mummy bag provides for the ultimate in warmth. They are the warmest simply because there is less space in the bag for your body to heat. The mummy bag provides a hood to help cut heat loss from your head ­ where up to 85% of heat is lost.

SEMI-RECTANGULAR BAGS: The semi-rectangular (also called tapered) bag is a good compromise between theultimate in warmth and extra sleeping room. Tapered bags are very popular withcampers who like extra foot and knee room.

RECTANGULAR BAGS: The rectangular bags are the least efficient and weigh the most, but provide more room and comfort than either mummy or tapered bags. The rectangular bag's wide shape provides generous air circulation, and its full-length zipper allows two bags to be zipped together.

Fill (insulation material): Generally, the more fill a bag has, the warmer it is. However, it is the air trapped inside the insulation itself that keeps you warm. As you would expect, the higher performing fills are more expensive. The following is a list of common fill options.

Down: Down is the warmest of all fills. Because down has tremendous loft it insulates incredibly well despite being lightweight, and can be squeezed into a small compression sack. Down is also a natural material that is complete biodegradable and does not come from a chemical process. Down also takes a long time to dry out if it gets wet. Damp down looses its loft and will not provide warmth until it is dry again.

Hollofil or Quallofil: Hollofil, Hollofil II and Quallofil are all similar products. These fills are frequently found in lower cost sleeping bags. They provide warmth, and maintain loft even if there are a little damp. Hollofil is relatively heavy compared to other synthetic and man made materials and doesn’t compress down as well, taking up more room in your pack.

Polarguard: Polarguard, Polarguard HV, and Polarguard 3D is also close cousins to each other. Able to take punishment better than Hollofil or “Loft,” type materials, Polarguard is bulky like Hollofil requiring more room in your pack and doesn’t compress down very well. The long fibers of Polardguard can tear, leaving a cold spot in your bag and making it useless. Polarguard takes to getting wet better than down and dries reasonable fast. Most bags today use Polarguard 3D which is the lightest, loftiest, and most durable of the three.

Other Man Made Loft Fillings(Lite Loft, Micro-loft, Primaloft, Termolite): These man made materials are similar to natural down. They are much lighter than Hollofil and Polarguard but not quite as light as down. These materials do an excellent job of keeping you warm and cost less than down. They resist getting wet better than down does. The only material that will stand up to the rigor of frequent use better is down.

Size: Getting the appropriate size sleeping bag is an important consideration.
Length: Measure your height when standing from the floor to your chin, add a couple extra inches of space to give your fee a little room at the bottom of the bag. If you will be placing a items like the next day's clothes at the foot of the sleeping bag, add a few more inches to your toe-to-chin height. There is no point in buy a bag much longer than your toe-to-chin height because the extra space just increases weight, pack-space and cost while requiring you to warm a larger amount of dead space in your bag.

Width: You can save weight and pack-space with a relatively narrow bag. Because narrower bags require less fill, they are often less expensive than a comparable, wider bag.