While working on a WordPress plugin to display a photos exposure information I created an exposure calculator that is able to convert exposure values as well as show you the proper value when a neutral density filter is added.
You can find it on CodePen.
I haven’t found a lot of uses for it other than using the ND calculator. I have a 10 stop ND filter that I love to use to slow down water shots to a silky smooth blur. Sometimes it is difficult to figure out what the actual exposure should be with the filter. What I do is meter without the filter, add the settings to the calculator, set the filter density and click the button. The proper exposure value with the filter is then output giving me the values to put in the camera.