Meter without the filter, enter that shutter speed and the filter strength, and get the long-exposure time to use with the filter on.
Charts for 3, 6, 10, 15 and 20-stop filters are linked below, and the long-exposure timer counts the exposure down for you.
Formula
New shutter = base shutter × 2^stops. Filter factor = 2^stops (ND8 = 3 stops, ND64 = 6 stops, ND1000 ≈ 10 stops). Optical density = 0.3 × stops. See the 10-stop chart and other charts below.
FAQ
What ND filter do I need for waterfalls?
In shade, a 3-stop ND usually gets you to 1/2–1 s. In bright light, 6 stops or more.
What is a 10-stop ND for?
Multi-second to multi-minute exposures in daylight: smoothing seascapes, streaking clouds and removing moving people.
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