Here’s a table of ND filter ratings to easily look up exactly which ND filter you need based on how many stops of light you want to reduce.

Optical DensityND Numberf-stop Reduction
ND 0.3ND 21/21
ND 0.6ND 41/42
ND 0.9ND 81/83
ND 1.2ND 161/164
ND 1.5ND 321/325
ND 1.8ND 641/646
ND 2.1ND 1281/1287
ND 2.4ND 2561/2568
ND 2.7ND 5121/5129
ND 3.0ND 10241/102410