Sunny 16, in T-shirt form!

The basic Sunny 16 rule is, "On a sunny day set aperture to f16 and shutter speed to the [reciprocal of the] ISO film speed."

For example:

On a sunny day and with ISO 100 film in the camera, one sets the aperture to f/16 and the shutter speed to 1/100 or 1/125 second (on most cameras 1/125 second is the available setting nearest to 1/100 second).

On a sunny day with ISO 200 film and aperture at f/16, set shutter speed to 1/200 or 1/250

On a sunny day with ISO 400 film and aperture at f/16, set shutter speed to 1/400 or 1/500.