NDCalc

Screenshots are german but its translated to english too.

You know the time without the filter -how long do you need to exp. WITH filter? Sounds easy but tell me quickly how long to exp. with an ND 1.8 if the time without filter is 1/320?

Now you can!

Set Filter and exp. time without the filter and you can see the correct time. In the example above 1/5

Even easier it shows you 5 because this is what you can set on a dSLR. Smaller letters showing you the actual time. For 12500 it shows, that these are 3h, 28m and 20s.

For times of 4+ seconds you get a countdown button to easily stop longer exposures manually with a remote shutter release with alarm and vibration (if supported).