VarInt

A VarInt or "Variable Integer" is an integer format used widely in Bitcoin to indicate the lengths of fields within transaction, block and peer-to-peer network 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. VarInt values are almost always represented in little endian.

Using VarInts

When expressing an integer between zero and 0xFC (252) the value itself can be used and expressed in uint8_t format.

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 in uint16_t format.

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 in uint32_t format.

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 in uint64_t format.

References with Examples

Value Range Example Value	Size Range Example Size	VarInt Format Example VarInt
0 <= Value <= 252 (0xFC) e.g. 187 (0xBB)	1 byte 1 byte	uint8_t 0xBB
253 <= Value <= 65,535 (0xFFFF) e.g. 255 (0xFF) e.g. 13,337 (0x3419)	1 - 2 bytes 1 byte but greater than 0xFC 2 bytes	0xFD + number as uint16_t 0xFDFF00 0xFD1934
65,536 <= Value <= ‭4,294,967,295‬ (0xFFFFFFFF) e.g. 14,435,729 (0xDC4591) e.g. 134,250,981 (0x80081E5)	3 - 4 bytes 3 bytes 4 bytes	0xFE + number as uint32_t 0xFE9145DC00 0xFEE5810008
4,294,967,296 <= value <= ‭18,446,744,073,709,551,615 (0xFFFFFFFFFFFFFFFF) e.g.198,849,843,832,919 (0xB4DA564E2857) e.g. 5,473,425,651,754,713,432 (0x‭4BF583A17D59C158‬)	5 - 8 bytes 6 bytes 8 bytes	0xFF + number as uint64_t 0xFF57284E56DAB40000 0xFF58C1597DA183F54B