Aperture Area Calculator

The f-number is defined as the focal length divided by the aperture diameter, so the diameter equals the focal length divided by the f-number. A 50mm lens at f/2 therefore has a 25 mm opening, and a 200mm lens at f/2.8 has a 71.43 mm opening.

The opening is treated as a circle, so the area is pi times the radius squared, where the radius is half the diameter. For a 25 mm diameter that works out to about 490.9 square millimetres.

Moving from f/2 to f/4 halves the diameter, and area grows with the square of the diameter, so halving it cuts the area to one quarter. That quartering of light-gathering area is exactly two stops of exposure.

In practice, yes. The f-number is defined against the entrance pupil, which is the apparent size of the aperture seen through the front of the lens. The physical iris blades can look smaller because front elements magnify them.

The f-number is a ratio, not a fixed width, so a longer lens needs a wider opening to reach the same f-stop. This is why a 200mm f/2.8 has a huge front element while a 50mm f/2.8 stays compact.

One-tap buttons cover 24, 35, 50, 85, 135 and 200 mm, matching common prime lengths. You can still type any focal length you like, including zoom values or unusual cine lengths.

Results are rounded to four significant figures for readability, so 50mm at f/2 shows 490.9 mm2 rather than the longer 490.874. The underlying math is exact and uses the full value of pi.

Yes. You can enter decimals such as f/1.8 or f/6.3, and the f-number field steps in tenths. Both the focal length and the f-number must be greater than zero, or the tool asks you to correct the input.

No. The calculation runs entirely in your browser, so the focal length and f-number never leave your device. A public JSON API is available if you want to compute aperture sizes in your own code.