base 13 is a positional numeral system with thirteen as its base. It uses 13 different digits for representing numbers. The digits for base 13 could be 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, a, b, and c.

conversion table

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