Which ND filter do you need for your drone?
Tell the calculator the shutter speed your drone picks in auto without a filter, and what you want: video at your frame rate, or a slow shutter for blurred water and clouds. It shows the stops you need and the ND filter that gets you there.
How to use it
For video, the usual rule is to set the shutter to twice the frame rate, so 30 fps wants about 1/60 s. In bright light a drone with a fixed aperture would pick something like 1/1000 s, which makes footage look choppy. An ND filter cuts the light so you can use the slower shutter.
Each stop of ND halves the light. ND8 is 3 stops, ND16 is 4, ND32 is 5, and ND64 is 6. The calculator works out the stops from your two shutter speeds, then lists every common strength with the shutter speed you would get.
For stills, a slow shutter of 1/4 s to 1/2 s can soften moving water and clouds. Keep the drone in a steady hover and take several frames, because a slight drift is likely at slow speeds.
Questions
What is the 180 degree rule?
It means setting the shutter speed to about double the frame rate. It gives motion blur that looks natural to the eye.
Do I need a polariser as well?
Many drone filter sets are polarising ND filters. A polariser also cuts roughly a stop and a half of light, so allow for that when you meter.
Which ND strength is best for a bright day?
For 30 fps video in full sun, ND16 to ND64 is common. Use the calculator with your own metered shutter speed to be sure.
More drone tools: Drone Flight Check · Drone Photo Coverage Calculator