Modbus Address Calculator | 40001 to 0-Based Converter

Modbus Address Calculator to convert 40001 to 0-based addressing, 5-digit to 6-digit formats, and protocol-level hex values.

Convert Modbus addresses instantly between 4-digit, 5-digit, 6-digit, and 0-based formats.

Modbus addressing can be confusing because different tools and manufacturers use different formats. This often leads to incorrect readings, off-by-one errors, and wasted debugging time. 

Use a full Modbus Tester tool to verify registers instead of manually converting addresses. If you're testing devices, use a full Modbus Tester instead of manual conversion.

• Modbus Tester 

4-Digit Reference (Legacy Format)

• Example: 0001, 0002

• Used in documentation

• Does NOT include function code

This is just the register number — not the full Modbus address.

5-Digit Modbus Address (Standard Format)

• Example: 40001, 30001

• First digit defines type:

  • 0xxxx → Coils

  • 1xxxx → Discrete Inputs

  • 3xxxx → Input Registers

  • 4xxxx → Holding Registers

This is the most commonly used format in Modbus maps.

6-Digit Extended Address (Modern Systems)

• Example: 400001, 400123

• Used in SCADA systems and advanced tools

• Supports larger address ranges

Function Codes (FC01, FC03, FC06)

Modbus uses function codes to define operations:

• FC01 → Read Coils

• FC03 → Read Holding Registers

• FC06 → Write Single Register

The same register number can behave differently depending on the function code.

0-Based vs 1-Based Addressing (Critical)

• Documentation uses 1-based (40001)

• Protocol uses 0-based (0)

Example:

• 40001 → 0

• 40002 → 1

This mismatch is the most common source of errors.