You can see how as the filter strength increases, the exposure time is doubled sequentially to compensate for the loss of light. When the light is cut in half, you need to double the shutter speed to maintain the same exposure. Add another ND stop, double the shutter speed again.

ND filters come in many different strengths. Depending on the manufacturer, these strengths may be notated in one of three ways: stops, optical density, or ND factor. Stops (sometimes referred to as exposure value) are fairly straightforward in that they tell you exactly how many stops your exposure will be adjusted by. Optical density is essentially 0.3 x the exposure value. This is the least common labeling system on the market. The ND factor is often listed as ND2, ND4, ND8, and so on. These numbers refer to the amount by which the light is reduced, ie. ND2 halves the light while ND4 reduces the light to one quarter. The chart below shows the translation between the three labeling systems.