Anyone who has shopped for a sleeping bag has become probably all-too-familiar with the temperature ratings of sleeping bags. Unfortunately, as many people have come to discover, the ratings used on most sleeping bags isn't always right. The reasons for this are many and are explored on this page.
What Temperature Ratings Mean
A sleeping bag temperature rating is at the end of the day, the "best guess" of the manufacturer as to how warm the bag will be for the "average person." They arrive at this number basically by giving bags to testers or employees, who then test the bags in various conditions or in an "ice box."
However, the "average person" is generally a rare person. You see, the reason a temperature rating will NEVER be fully accurate is because people sleep at differing body temperatures. If you are what is known as a "cold sleeper" - you are going to need a significantly warmer bag then someone who is a "hot sleeper" is.
Now, what are hot and cold sleepers? Best way to describe it is to use sleeping at home in your bed as an example in a house that is around 65 degrees or so.
Hot Sleeper - You are someone who sleeps most of the night with few if any sheets on. You may put the sheets on initially, but as the night goes on, the sheets seem to magically lighten up or disappear entirely. Additionally, your body itself will give off a LOT of body heat. If you have a bedtime partner and they complain about how warm you make the bed, then you are probably a hot sleeper.
Cold Sleeper - You are a cold sleeper if, when you go to bed, you sleep with and wake up with every single blanket piled on top of you - and often still aren't entirely as warm as you would like to be. What this means is that you have a slow nighttime metabolism - hence very little body heat being generated. Which is why you need all the blankets.
Sleep Temperature & The Bag You Buy
Once you determine whether you are a "hot sleeper", a "cold sleeper" or somewhere in between, you can now have a better grasp of what temperature rating to get in a sleeping bag.
Now, this is a rough rule of thumb here. This is nothing scientific - just my own experiences. But, I do believe it is a good gage to follow.
Hot Sleepers - Assume the bags temperature rating is probably fully accurate for you.
Cold Sleepers - Assume the bags temperature rating is at LEAST 10 degrees warmer than advertised. Thus, a bag that is rated to 20 degrees will likely only work for a "cold sleeper" down to 30 degrees or so. And 10 degrees may NOT even be enough, either, depending on other factors.
All Others - If you fall somewhere between "hot" and "cold", I would assume the bags temperature rating is at least 5 degrees warmer then advertised.
Other Factors That Effect The Temperature Rating
Getting the right bag to match your sleeping temperature, however, is only half the battle. You see, when the manufacturers come up with the ratings, they make several reasonable assumptions. These are:
Sleeping Pad is Used - A sleeping pad is mandatory. Without a sleeping pad, you are in essence sleeping on the cold ground. No matter how "hot" you sleep or how great your bag is, you'll still be cold. Moreover, the assumption is made you are sleeping on at least a midweight sleeping pad (R Factor of 5 or greater), which provides for substantially greater insulating abilities than a lightweight pad does.
Use of a Hood - If the bag comes with a hood (and most 3-season bags do), the rating assumes the hood is used and drawn up relatively tightly over the user. Remember, about 40-50% of your body heat escapes from the top of your head. If you have little or no hair up "on top", use of a hood (a good fleece hat also works) is crucial to get near the temperature rating of a bag. Even if you have a full head of hair, a hat or use of the hood is STILL needed to truly achieve the bags temperature rating.
Bag is Fully Zipped Up - This seems like common sense. But, all ratings assume you zip the bag up to the very top - not leaving it down a bit like many campers do in order to have a "bit more room" in the bag.
Tight Fit of the Bag Around User - This is another thing people forget about. When buying a bag, buy a bag designed for your height. If you are 5'10, do NOT get a bag that fits people up to 6'6. The reason is because a pocket of cold air will form at the end of the bag, keeping your feet (and thus you) cold all night long.
No Clothes! - Yes, that's right. Temperature ratings assume you sleep in the "buff", or at least in your undies. The reason is because sleeping with clothes on, and in particular cotton clothes, can actually make you colder! The cotton, in combination with the bags insulation, will make you sweat. The cotton will then absorb the water into your clothes - making you wet ALL NIGHT LONG! This will make you cold right along with it. By not sleeping with a shirt or pants on (or using fleece if you do), you can closer achieve the temperature rating. And NEVER sleep in cotton clothes.
In summary, I believe it is ALWAYS better to buy a bag that will provide you with at LEAST an extra 15 - 20 degrees of temperature rating. Doing so will allow you to use the bag in a wider variety of temperatures and also allows you to keep the bag partially unzipped or sleep partially clothed if you want to. Additionally, a warmer rated bag is really crucial if you use a lightweight and thin sleeping pad.
Remember, you can always "open up" a bag that is a bit too warm your environment to stay comfortable. But if you have a bag that is a "bit too cool" you are going to have a VERY long night! So it is always better to buy a bag that is a bit too warm than a bit too cold!
Thus, for general three season use, I suggest getting a bag rated down to a minimum of 20 degrees, preferably 15.
For four season use, don't chance around. Get a bag down to -40 degrees. -20 degrees sounds warm until that arctic blast comes through.