A table of ND filter strength in stops, decimal density (i.e. 0.3), and Hoya’s x-factor/Cokin’s ND#.

Stops	Decimal Factor (B+W, Formatt/Hitech, Lee, )	ND#/#x Factor (Cokin, Hoya)
1	0.3	2
1.5	0.45
2	0.6	4
2.5	0.75
3	0.9	8
4	1.2	16
6	1.8	64
9		400
10	3.0	1,000
13	4.0	10,000
20	6.0	1,000,000