The sunny f/16 rule if used correctly will always give an accurate exposure and requires that if working in full sun, one sets the lens aperture to f/16 and the shutter speed is set to the reciprocal of the ISO. In practice, for example, if you are shooting at an ISO of 100 in full sun; simply set the aperture to f/16 and the shutter speed to the reciprocal of the ISO, which is 1/100 second. As you can clearly see the sunny f/16 rule functions without the need to consult a light meter.
Some other examples are:
200 ISO = f/16 @ 1/200 sec
800 ISO = f/16 @ 1/800 sec
When using ISO 100 for example you can of course use any combination of shutter speed and aperture as long as it converts back to f/ 16 at 1/100. The following three examples will again yield identical exposures:
100 ISO, f/8 @ 1/400 sec
100 ISO, f/4 @ 1/1600 sec
What I mean by this, is that if you start at 1/100 sec and f/16 and want to increase the shutter speed to 1/800 sec for some reason, all you need to remember is that you have gone up 3 stops in speed (100-200, 200-400, 400-800) and must compensate by opening the aperture by the same three stops to f/5.6 (f/16-11, f/11-8, f/8-5.6).
If you are really mathematically challenged I suggest you count the clicks of the exposure dials, remembering that if you open the aperture a given number of clicks you must compensate by increasing the shutter speed by the same number of clicks. Remember, equal and opposite!
This trick generally works quite well, but be mindful that if the scene you are photographing is heavily shadowed then you must do a bit of interpretation and use some exposure compensation. Have a play with it and you will master the technique in no time.
Don’t forget to use the histogram to assist in your quest for exposure excellence.