Sunny 16 Calculator

How it is calculated

The Sunny 16 rule says that in bright sun at f/16 the shutter time is about 1/ISO seconds. For other apertures the tool scales this by the square of the f-number ratio: denominator = ISO × (16 / N)², where ISO is the film or sensor speed and N is the f-number. The shutter speed is then shown as 1/denominator s, rounded to a whole number.

Opening the aperture by one full stop (for example f/16 to f/11) halves the needed time, which is exactly what the square in the formula does. Both inputs must be greater than zero, otherwise there is no result. The rule is only valid for bright, direct sunlight with distinct shadows; overcast or shade needs a slower shutter.

Common mistakes

• Applying the rule in overcast light or shade; Sunny 16 assumes bright sun, so the result will underexpose there.
• Entering the focal length or a stop count instead of the f-number (for example 2.8 for f/2.8).
• Forgetting that a smaller f-number means a wider aperture and a much faster shutter: f/4 is not slightly different from f/16 but 16 times faster.
• Treating the output as exact; the tool rounds the denominator, and real shutters offer standard steps such as 1/1000 or 1/500, so round to the nearest available speed.
• Ignoring ND filters or exposure compensation, which must be added by hand to the result.