About this tool
ND Filter / Long Exposure Calculator is a free, in-browser tool for neutral-density long exposures. Enter your unfiltered base shutter speed (fractions like 1/125 are accepted), choose one or two ND filters or type the total stops, and it returns the adjusted shutter time. You can also reverse the problem: enter a target long exposure and it solves the base shutter you need. All math runs locally with nothing uploaded.
Each stop of neutral density doubles the required exposure time, so the light-reduction factor is 2 raised to the total stops. A filter's stop rating comes from its factor via stops = log2(ND number): ND8 is 3 stops, ND64 is 6 stops and a 10-stop ND1000 multiplies exposure by 2^10 = 1024. Stacking filters adds their stops together. The adjusted time is base × 2^stops; the reverse mode divides the target time by the same factor.
Use it for silky waterfalls, smooth skies and empty streets: meter without the filter, read the base shutter, then let the tool hand you the exposure time in seconds and minutes so you do not have to count stops in your head.