VarInt

A VarInt or "Variable Integer" is an integer format used widely in Bitcoin to indicate the lengths of fields within transaction and block data.

A VarInt is a variable length field 1, 3, 5 or 9 bytes in length dependent on the size of the object being defined. The VarInt format is used as it is space efficient over simply using an 8-byte field where variable length objects are used.

Using VarInts

When expressing an integer value less than or equal to 0xFC (252) the value itself can be used.

When expressing an integer value greater than 0xFC but less than or equal to 0xFFFF (65,535), the varint is 0xFDXXXX where XXXX represents the two byte integer

When expressing an integer value greater than 0xFFFF but less than or equal to 0xFFFFFFFF (4,294,967,295), the varint is 0xFEXXXXXXXX where XXXXXXXX represents the 4 byte integer

When expressing an integer value greater than 0xFFFFFFFF but less than or equal to 0xFFFFFFFFFFFFFFFF (18,446,744,073,709,551,615), the varint is 0xFFXXXXXXXXXXXXXXXX where XXXXXXXXXXXXXXXX represents the 8 byte integer

Examples

Value	Size	VarInt
187 (0xBB)	1 byte (less than or equal to 0xFC)	0xBB
255 (0xFF)	1 byte (greater than 0xFC)	0xFD00FF
13,337 (0x3419)	2 bytes	0xFD3419
14,435,729 (0xDC4591)	3 bytes	0xFE00DC4591
134,250,981 (0x80081E5)	4 bytes	0xFE080081E5
198,849,843,832,919 (0xB4DA564E2857)	6 bytes	0xFF0000B4DA564E2857
5,473,425,651,754,713,432 (0x‭4BF583A17D59C158‬)	8 bytes	0xFF4BF583A17D59C158