Fraction such as 1/125, or plain seconds such as 2
Why reach for an ND filter instead of just closing the aperture
Moving water and clouds only turn into that smooth, blurred look once the shutter stays open for several seconds, but a sunny scene simply does not offer that much time to work with: stop the aperture down to f/22 and drop the ISO to its floor and there is still far too much light hitting the sensor for a multi-second exposure. An ND filter solves the actual problem, which is too much light, without pushing the aperture into the range where diffraction softens the image or forcing an ISO the camera cannot go below. It buys time on the shutter dial that daylight would otherwise never allow.
Reading the number printed on the glass
Filter makers do not agree on one notation, which is where a lot of confusion starts. Some rings are marked with a factor, like ND8 or ND1000, meaning the light is cut to one eighth or one thousandth. Others carry an optical density, like 0.9 or 3.0, where each 0.3 step is one stop. A filter marked 0.9 and one marked ND8 are the same three stops under a different label, and a 10-stop filter shows up in the wild as ND1000, ND1.0K, or 3.0 depending on the brand printed on the ring.
Where the mental math falls apart
Doubling a shutter speed for one extra stop is easy to track in your head, and most people manage two or three stops without a calculator. Past that, the numbers stop behaving the way instinct expects: stacking a 6-stop and a 10-stop filter is not 16 seconds of extra exposure, it is 16 stops of light cut, and the resulting shutter speed comes from multiplying the base time by 2 to the power of 16, not from adding the two filter numbers together. A 1/125 base speed under that stack lands past eight minutes, a jump that catches people who tried to eyeball it in the field.
Metering after the filter is already on the lens
Once a 10-stop or denser filter sits on the front element, the viewfinder and the camera's own meter usually go too dark to read anything useful, live view included on some bodies. The practical workaround photographers use is to meter the scene before the filter goes on, then work out the new shutter speed from the stops the filter removes, which is exactly the conversion this calculator automates so the math does not have to happen by hand on location.