base 28 is a positional numeral system with twenty-eight as its base. It uses 28 different digits for representing numbers. The digits for base 28 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, and r.

conversion table

base 28	base 21	base 28	base 21
1	≈ 1	b	≈ b
2	≈ 2	c	≈ c
3	≈ 3	d	≈ d
4	≈ 4	e	≈ e
5	≈ 5	f	≈ f
6	≈ 6	g	≈ g
7	≈ 7	h	≈ h
8	≈ 8	i	≈ i
9	≈ 9	j	≈ j
a	≈ a	k	≈ k