modbus crc calculator

What this Modbus CRC calculator does

This tool calculates the CRC16 checksum used by Modbus RTU. Enter your frame bytes (without the CRC), click calculate, and the page returns the checksum in multiple useful formats: combined 16-bit value, low/high bytes, and full frame with CRC appended.

Modbus CRC basics

In Modbus RTU, the CRC helps detect communication errors on serial links such as RS-485. The algorithm uses:

• Initial value: 0xFFFF
• Polynomial: 0xA001 (reversed representation of 0x8005)
• Processing order: each byte, least-significant bit first
• Transmit order: low byte first, high byte second

That last point is the most common source of confusion. Even if your 16-bit CRC appears as 0xC5CD, Modbus sends it as bytes CD C5.

How to use this calculator

1) Enter payload bytes

Paste the Modbus address, function code, and data bytes only. Do not include the existing CRC bytes.

2) Click “Calculate CRC”

The calculator validates the input and computes the checksum immediately in the browser.

3) Append CRC to build a valid RTU frame

Use the CRC Bytes (Lo Hi) output to append bytes in the correct Modbus wire order.

Example

A common request frame is 01 03 00 00 00 0A (read 10 holding registers starting at 0). The CRC is 0xCDC5 as a 16-bit value, and the transmitted bytes are C5 CD.

Troubleshooting wrong CRC values

• You included old CRC bytes in the input data.
• You reversed CRC byte order (Hi/Lo vs Lo/Hi).
• Your source uses ASCII Modbus (LRC), not RTU CRC.
• Input contains an odd number of hex digits.
• Hidden characters from logs/sniffers were pasted in.

Why this matters in real systems

Reliable CRC generation is essential when commissioning PLCs, VFDs, meters, and embedded RTU devices. A single byte-order mistake can make a healthy network look broken. A quick checksum check saves debugging time and prevents unnecessary hardware swaps.