base 18 is a positional numeral system with eighteen as its base. It uses 18 different digits for representing numbers. The digits for base 18 could be 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, a, b, c, d, e, f, g, and h.

conversion table

base 18	duodecimal (base 12)	base 18	duodecimal (base 12)
1	≈ 1	b	≈ b
2	≈ 2	c	≈ 10
3	≈ 3	d	≈ 11
4	≈ 4	e	≈ 12
5	≈ 5	f	≈ 13
6	≈ 6	g	≈ 14
7	≈ 7	h	≈ 15
8	≈ 8	10	≈ 16
9	≈ 9	11	≈ 17
a	≈ a	12	≈ 18