Communication Protocols · Free Tool

Modbus Register Address Calculator

Convert between Modbus reference numbers (40001, 30001, 10001, 00001) and the zero based address that actually goes in the message, with the matching function codes.

40001 to 0Function codes5 and 6 digitHex address

Modbus reference

Data table

Zero based address

Zero based address (in the message)107

Address in hex0x006B

5 digit reference40108

6 digit reference400108

TableHolding register (16 bit, read and write)

Read withFC 03 Read Holding Registers

Write withFC 06 Write Single Register, FC 16 Write Multiple Registers

How this result was calculated

The first digit selects the table; the rest is the 1 based register number.
Address in the message = register number − 1 = 107

Modbus Reference Numbers

0xxxx coils · 1xxxx discrete inputs · 3xxxx input registers · 4xxxx holding registers
Address in the message = reference − table offset − 1 (40108 → 107)

The leading digit is a documentation convention, not part of the message. The message carries only the function code and the zero based address.

Worked example

Holding register 40108 is read with function code 03 at address 107 (0x006B).

Key insight: most "wrong value" problems on a new Modbus link are an off by one address. If every value looks shifted by one register, try address ± 1 before anything else.