The sunny 16 rule is the go to rule for when you are out and your light meter dies or breaks at one of the most inopportune times. The type of exposure you get from using this method is the same as if you were to use an incident light meter, one that measures the light falling on the subject as opposed to being reflected and bounced off of your subject. If you have a light meter, the incident meter is when you put the white semi-circle attachment over the light sensor.
How does the sunny 16 rule work?
On a completely sunny day, you set your aperture to f/16 and your shutter speed to the same speed as your ISO or the closest speed up. For example, I use ISO 400 film so my camera would be set to f/16 with a 1/500 shutter speed. For ISO 100 you would do f/16 at 1/125 and ISO 200 would be f/16 at 1/250. That's it.
There are some variations of this that you can take advantage of. You can keep the same exposure value but you need to change your aperture because your subject is closer and you need less depth of field. You can do that for example, with ISO 400 instead of f/16 you can do f/11 at 1/1000 shutter speed. You're opening up your aperture one stop while speeding up the shutter by one stop as well to keep the same value as sunny 16. To go one more than that, you can do f/8 with a 1/2000 shutter speed. As long as your camera has the faster shutter speeds, it's all possible.
What if it's not such a sunny day? Well, you can still use the sunny 16 rule with some slight modifications. Is the day just kind of cloudy or is it straight overcast? if its overcast, you would want to keep your shutter speed the same, set to the shutter speed closest to the ISO number, but you would want to open your aperature one or two stops to f/11 or f/8 and shoot away. For sunset, you might want to go three or maybe four stops up to f/5.6 or f/4.
Here are a couple examples of shots I took while using the sunny 16 rule:
For more information on the sunny 16 rule or a very similar looney 11 rule (for taking pictures of the moon)