base 31 is a positional numeral system with thirty-one as its base. It uses 31 different digits for representing numbers. The digits for base 31 could be 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, p, q, r, s, t, and u.

conversion table

base 31	base 13	base 31	base 13
1	≈ 1	b	≈ b
2	≈ 2	c	≈ c
3	≈ 3	d	≈ 10
4	≈ 4	e	≈ 11
5	≈ 5	f	≈ 12
6	≈ 6	g	≈ 13
7	≈ 7	h	≈ 14
8	≈ 8	i	≈ 15
9	≈ 9	j	≈ 16
a	≈ a	k	≈ 17