F-Stop Calculator
Enter a focal length and an aperture diameter to get the f-number (f-stop) — and see why a smaller f-number means a wider opening and more light.
What is the f-number?
The ratio behind every aperture setting
The f-number, also written as the f-stop, is the ratio of a lens's focal length to the diameter of its entrance pupil — the effective opening that lets light in. This f-stop calculator turns those two measurements into the value you see on a lens as f/2 or f/8. It is the number that controls how much light reaches the sensor and how shallow the depth of field will be, which is why it sits at the heart of every exposure decision in photography.
Enter a focal length and an aperture diameter in the same unit to get the f-number instantly, shown as f/N.
The f-number is simply the focal length divided by the diameter of the entrance pupil.
N = focal length ÷ aperture diameterBecause it is a ratio, the units cancel out — millimetres over millimetres leaves a pure number. A 50 mm lens with a 25 mm entrance pupil gives 50 ÷ 25 = 2, written f/2. Halve the opening to 12.5 mm and the f-number doubles to f/4, letting in a quarter of the light.
Suppose you have a 50 mm lens and the entrance pupil measures 25 mm across.
Take the focal length
The focal length is 50 mm — the value printed on the lens barrel.
Divide by the aperture diameter
50 ÷ 25 = 2 — the focal length divided by the 25 mm entrance pupil.
Read the f-stop
The f-number is 2, written f/2 — a wide, fast aperture that gathers plenty of light and gives a shallow depth of field.
A smaller f-number means a wider aperture: more light reaches the sensor and the depth of field is shallower, so backgrounds blur into a soft bokeh. A larger f-number means a narrower aperture, less light, and a deeper depth of field that keeps more of the scene sharp. Because the f-number is the ratio of focal length to opening diameter, the amount of light depends on the area of that opening — so each full stop down the standard scale (f/1.4, f/2, f/2.8, f/4, f/5.6, f/8) roughly halves the light, the steps spaced by a factor of the square root of two. That is why an f/2 lens is called "two stops faster" than an f/4 lens: it lets in four times as much light. Reading your result, f/2 is a bright, low-light-friendly aperture, while f/8 is the kind of mid-range setting a landscape photographer reaches for to keep a whole vista in focus.
The formula is exact, but keep a couple of practical points in mind for accuracy.
Entrance pupil, not the physical hole
The aperture diameter here is the diameter of the entrance pupil — the apparent opening seen through the front of the lens — not the raw width of the physical iris. The two can differ because lens elements magnify the iris. This calculator also gives the geometric f-number; the T-stop, which accounts for light lost to transmission, is always a little larger.