In the book “Analysis of Lost Person Behavior” author William Syrotuck analyzed over 200 rescue cases. He found that 11% ended in fatalities, and three quarters of those people who died generally did so from hypothermia, within the first 48 hours of becoming lost.
So, to look at these statistics in a more positive way:
If you’re lost, you have an approximately 90% chance of surviving.
If you can stay alive for 2 days and minimize the chances of getting hypothermia, your survival odds increase to around 97%.
Therefore, carrying some kind of emergency shelter is an excellent idea.
But what, exactly does that mean? A one person tent, a tarp, a bivy sack, maybe even a bothy bag, which is kind of a British invented combination of all three? Or something more minimal?
If you have a reasonable chance of spending a night out, you probably want something fairly substantial, like a bivy sack and tarp. But, if you're after something to keep in your hiking day pack for emergency use only, something lighter which takes up less space would probably be a good call.
For emergency use when hiking, consider the combination of a heavy duty garbage bag and a good quality mylar space blanket.
The garbage bag is great for rain and weather protection, but doesn’t maintain body heat very well. A space blanket is the opposite, fairly good for reflecting back your body heat, but fragile and not so great as weather protection. Combine both of them and get the best of both worlds, for a low cost and light weight.
The garbage bag can double as a rain cover for your pack, and is easily converted into a sort of poncho for hiking if you cut a hole for your neck and arms. If you need it as more of a survival shelter, you can just cut a small slit in the side end for your head, see the video below.
Be sure and get the heavy duty “contractor clean-up bags”, that are huge and at least 3 mil thick. At the big box hardware stores, you usually need to buy a humongo bag of 50 of these, so if you know anyone who is a builder you could ask them for a couple of extras. Or, they’re available on Amazon as a more reasonably sized box of 20.
The very inexpensive kind of mylar blankets will work in a pinch, but they are very fragile. It's better to pay a more and get a much better quality blanket that will probably last you through the night. SOL makes some nice ones, called Heat Sheets. They cost just about $10 for a 2 pack, and are recommended. (If you're much over 6 feet tall, you may want to get the 1-2 person model.)
Here is a nice short video from the excellent MedWild YouTube channel on making a quick emergency shelter with a garbage bag. Note that he cuts just a small hole in the side of the bag, which lets you breath, but keeps your head protected.