base 21 is a positional numeral system with twenty-one as its base. It uses 21 different digits for representing numbers. The digits for base 21 could be 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, a, b, c, d, e, f, g, h, i, j, and k.

conversion table

base 21	base 15	base 21	base 15
1	≈ 1	b	≈ b
2	≈ 2	c	≈ c
3	≈ 3	d	≈ d
4	≈ 4	e	≈ e
5	≈ 5	f	≈ 10
6	≈ 6	g	≈ 11
7	≈ 7	h	≈ 12
8	≈ 8	i	≈ 13
9	≈ 9	j	≈ 14
a	≈ a	k	≈ 15