Structure and Encoding

ULID is a 128 bit number represented as a 26 character string. It is a combination of a timestamp and a random part.

Binary Structure

The 48 bits form an Integer value and it's a reprensentation of the timestamp in milliseconds. The maximum value it can hold is 281474976710655 (2^48 - 1). That year 10,889 AD.

80 bits are for the random component.

Encoding

These 128 bits are represented as string using a 5 bit encoding to form 26 characters. This encoding is called Crockford's Base32.

There are no special characters in the encoding. The encoding is case insensitive. Decoding of lower case letters is supported. At the time or encoding, all characters and ecoded as upper case. To avoid mistakes, characters i, l, o and u are not used while encoding.

While decoding, o and O are treated as 0. Characters i, I, l and L are treated as 1. u / U is considered invalid.

Char	Binary	Decimal	HEX
0, o/O	00000	0	0
1, i/I, l/L	00001	1	1
2	00010	2	2
3	00011	3	3
4	00100	4	4
5	00101	5	5
6	00110	6	6
7	00111	7	7
8	01000	8	8
9	01001	9	9
a/A	01010	10	A
b/B	01011	11	B
c/C	01100	12	C
d/D	01101	13	D
e/E	01110	14	E
f/F	01111	15	F
g/G	10000	16	10
h/H	10001	17	11
j/J	10010	18	12
k/K	10011	19	13
m/M	10100	20	14
n/N	10101	21	15
p/P	10110	22	16
q/Q	10111	23	17
r/R	11000	24	18
s/S	11001	25	19
t/T	11010	26	1A
v/V	11011	27	1B
w/W	11100	28	1C
x/X	11101	29	1D
y/Y	11110	30	1E
z/Z	11111	31	1F