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Sunny 16 Calculator

Find the shutter speed for a bright sunny day.

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. When the shutter would be slower than 1 second it is shown in seconds instead, for example ISO 25 at f/128 gives 2.6 s.

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.

Worked example

Take ISO 200 at f/8. The aperture factor is 16 / 8 = 2, and squared that is 4. Multiply by the ISO: 200 × 4 = 800. The recommended shutter speed is 1/800 s. As a check, at f/16 the same ISO 200 would give 1/200 s, and f/8 is two stops wider, so the time is four times shorter.

Try this example →

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.

About this calculator

This calculator applies the Sunny 16 rule, a photography rule of thumb stating that on a bright sunny day with the aperture set to f/16, the correct shutter speed is approximately the reciprocal of the ISO value (ISO 100 → 1/100s, ISO 400 → 1/400s) — and from there it derives the equivalent shutter speed for any other aperture by applying the standard stops-of-light relationship between aperture and shutter.

Film photographers without a working light meter, street and travel photographers who prefer working by feel rather than metering every shot, and photography students learning the exposure triangle (aperture, shutter speed, ISO) use this as a quick manual-exposure starting point.

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Comments (1)

  • Editorial team

    The tool answers for full sun only. For other light, the usual Sunny 16 table opens the aperture instead: about f/11 for slight overcast, f/8 for overcast, f/5.6 for heavy overcast and f/4 in open shade. Enter that f-number to get a matching shutter time. It is a convention photographers use, not something this tool computes, so bracket a frame or two if the scene matters. What light do you shoot in most, and which film or ISO do you carry?

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