Every stop doubles exposure time. Meter without the filter, calculate the filtered shutter, then check for light leaks and colour shift on very long exposures.
Stops are the simplest common language
Neutral-density filters may be labelled by stops, optical density or a filter factor. Stops are easiest for exposure planning: one stop halves the light and doubles the required shutter time; six stops multiplies it by 64; ten stops multiplies it by 1,024.
For stills, meter the scene before attaching a very dark filter, enter the unfiltered shutter speed and select the filter strength. Keep aperture and ISO fixed if you want the calculator’s shutter result to apply directly.
Fixed versus variable ND
Fixed filters tend to be predictable and can be stacked carefully, though stacking increases the chance of flare and vignetting. Variable ND filters are quick for video, but their usable range can be narrower than the markings suggest, especially with wide lenses where uneven polarisation may create a dark cross pattern.
Strong filters can introduce a colour cast, and exposures lasting minutes are vulnerable to light entering through the viewfinder or imperfect seals. Shoot a reference frame and inspect the histogram rather than assuming the theoretical value is final.
ND Filter Exposure Calculator
Convert a metered shutter speed into the corrected exposure for any neutral-density filter from one to sixteen stops.
Open the calculator