CRC Calculator
Text is converted to UTF-8 bytes.— standard check string
Result
Algorithm Reference
Result is calculated from the current input. Check is the published verification value for the standard input 123456789. RefIn and RefOut are separate input and output reflection settings. Click a row to apply that algorithm.
CRC Lookup Table
CRC-16 / MODBUS · Poly 0x8005 · Init 0xFFFF · XorOut 0x0000
CRC Source Code
CRC-16 / MODBUS · Poly 0x8005 · Init 0xFFFF · XorOut 0x0000C · 56 lines
/*
* Model: CRC-16 / MODBUS (16-bit)
* Poly: 0x8005, Init: 0xFFFF, XorOut: 0x0000
* CRC Table C Source Code & Lookup Table Generator
*/
#include <stdint.h>
#include <stddef.h>
const uint16_t crc_table[256] = {
0x0000, 0xC0C1, 0xC181, 0x0140, 0xC301, 0x03C0, 0x0280, 0xC241,
0xC601, 0x06C0, 0x0780, 0xC741, 0x0500, 0xC5C1, 0xC481, 0x0440,
0xCC01, 0x0CC0, 0x0D80, 0xCD41, 0x0F00, 0xCFC1, 0xCE81, 0x0E40,
0x0A00, 0xCAC1, 0xCB81, 0x0B40, 0xC901, 0x09C0, 0x0880, 0xC841,
0xD801, 0x18C0, 0x1980, 0xD941, 0x1B00, 0xDBC1, 0xDA81, 0x1A40,
0x1E00, 0xDEC1, 0xDF81, 0x1F40, 0xDD01, 0x1DC0, 0x1C80, 0xDC41,
0x1400, 0xD4C1, 0xD581, 0x1540, 0xD701, 0x17C0, 0x1680, 0xD641,
0xD201, 0x12C0, 0x1380, 0xD341, 0x1100, 0xD1C1, 0xD081, 0x1040,
0xF001, 0x30C0, 0x3180, 0xF141, 0x3300, 0xF3C1, 0xF281, 0x3240,
0x3600, 0xF6C1, 0xF781, 0x3740, 0xF501, 0x35C0, 0x3480, 0xF441,
0x3C00, 0xFCC1, 0xFD81, 0x3D40, 0xFF01, 0x3FC0, 0x3E80, 0xFE41,
0xFA01, 0x3AC0, 0x3B80, 0xFB41, 0x3900, 0xF9C1, 0xF881, 0x3840,
0x2800, 0xE8C1, 0xE981, 0x2940, 0xEB01, 0x2BC0, 0x2A80, 0xEA41,
0xEE01, 0x2EC0, 0x2F80, 0xEF41, 0x2D00, 0xEDC1, 0xEC81, 0x2C40,
0xE401, 0x24C0, 0x2580, 0xE541, 0x2700, 0xE7C1, 0xE681, 0x2640,
0x2200, 0xE2C1, 0xE381, 0x2340, 0xE101, 0x21C0, 0x2080, 0xE041,
0xA001, 0x60C0, 0x6180, 0xA141, 0x6300, 0xA3C1, 0xA281, 0x6240,
0x6600, 0xA6C1, 0xA781, 0x6740, 0xA501, 0x65C0, 0x6480, 0xA441,
0x6C00, 0xACC1, 0xAD81, 0x6D40, 0xAF01, 0x6FC0, 0x6E80, 0xAE41,
0xAA01, 0x6AC0, 0x6B80, 0xAB41, 0x6900, 0xA9C1, 0xA881, 0x6840,
0x7800, 0xB8C1, 0xB981, 0x7940, 0xBB01, 0x7BC0, 0x7A80, 0xBA41,
0xBE01, 0x7EC0, 0x7F80, 0xBF41, 0x7D00, 0xBDC1, 0xBC81, 0x7C40,
0xB401, 0x74C0, 0x7580, 0xB541, 0x7700, 0xB7C1, 0xB681, 0x7640,
0x7200, 0xB2C1, 0xB381, 0x7340, 0xB101, 0x71C0, 0x7080, 0xB041,
0x5000, 0x90C1, 0x9181, 0x5140, 0x9301, 0x53C0, 0x5280, 0x9241,
0x9601, 0x56C0, 0x5780, 0x9741, 0x5500, 0x95C1, 0x9481, 0x5440,
0x9C01, 0x5CC0, 0x5D80, 0x9D41, 0x5F00, 0x9FC1, 0x9E81, 0x5E40,
0x5A00, 0x9AC1, 0x9B81, 0x5B40, 0x9901, 0x59C0, 0x5880, 0x9841,
0x8801, 0x48C0, 0x4980, 0x8941, 0x4B00, 0x8BC1, 0x8A81, 0x4A40,
0x4E00, 0x8EC1, 0x8F81, 0x4F40, 0x8D01, 0x4DC0, 0x4C80, 0x8C41,
0x4400, 0x84C1, 0x8581, 0x4540, 0x8701, 0x47C0, 0x4680, 0x8641,
0x8201, 0x42C0, 0x4380, 0x8341, 0x4100, 0x81C1, 0x8081, 0x4040
};
uint16_t calculate_crc(const uint8_t *data, size_t length) {
uint16_t crc = 0xFFFF;
for (size_t i = 0; i < length; i++) {
#if 1
uint8_t idx = (uint8_t)(crc ^ data[i]);
crc = (crc >> 8) ^ crc_table[idx];
#else
uint8_t idx = (uint8_t)((crc >> 8) ^ data[i]);
crc = (crc << 8) ^ crc_table[idx];
#endif
}
return crc ^ 0x0000;
}CRC Algorithm Identifier
Already know the CRC output for this data but not which algorithm produced it? Enter the known result below — every one of the 55 catalog algorithms is recalculated against the current input data and matched against it automatically.
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All 55 algorithms are checked against the current input data.
CRC Patch Finder
CRC-16 / MODBUS · Input data: 9 bytes inherited · Uses the calculator's algorithm and data
Why CRC Calc
CRC Calc is a focused toolkit built for the exact question engineers ask a dozen times a day: what checksum does this data produce under a specific CRC model? Rather than bundling a general-purpose hashing utility with CRC as an afterthought, every part of this site — the calculator, the 55-algorithm catalog, the lookup table and code generators, the patch finder, and the algorithm identifier — is built around the parameters that actually define a CRC: width, polynomial, initial value, input and output reflection, and final XOR.
Everything runs entirely in your browser. Text, hex, decimal, binary, Base64, and uploaded files are parsed and processed on your device; nothing is transmitted to a server, logged, or stored, which makes the tool safe to use with proprietary protocol data or firmware images. The algorithm catalog documents published parameters and check values for well-known CRC-8, CRC-16, and CRC-32 variants — from MODBUS and XMODEM to Ethernet's CRC-32/ISO-HDLC — so you can confirm you're matching an existing implementation rather than guessing at undocumented defaults.
Use the calculator to verify a single value, or the Compare table to see how the same input behaves across every preset at once. The lookup table and code generator turn that same configuration into a precomputed table or a ready-to-paste C, Python, or JavaScript implementation; the patch finder brute-forces the two bytes that make patched data hit a specific target CRC; and the identifier works backward from data and a known result to name which of the 55 catalog algorithms produced it. Whether you're debugging a mismatched checksum, forcing a checksum for testing, or reverse engineering an unknown protocol, CRC Calc aims to answer the question directly instead of sending you back to a spec sheet.
Supported CRC parameters
Custom CRC calculations can be configured using width, polynomial, initial value, input reflection, output reflection, and final XOR value. These parameters allow the calculator to reproduce many standard and proprietary CRC implementations used in embedded systems, serial communication, industrial protocols, file formats, and network applications.
- Width
- Defines the CRC register size, such as 8, 16, or 32 bits.
- Polynomial
- Specifies the generator polynomial used by the CRC algorithm.
- Initial value
- Sets the value loaded into the CRC register before processing data.
- RefIn and RefOut
- Control whether input bytes and the final CRC result are reflected.
- Final XOR value
- Applies a final XOR operation to the CRC register before displaying the checksum.
CRC-8, CRC-16, and CRC-32
CRC-8 produces an 8-bit checksum and is commonly used in compact embedded and sensor protocols. CRC-16 produces a 16-bit checksum and is widely used by protocols such as Modbus, XMODEM, and CCITT-based communication systems. CRC-32 produces a 32-bit checksum and provides stronger error detection for larger files, archives, Ethernet frames, and storage formats.
CRC lookup table and source code generator
A CRC lookup table replaces repeated bit-by-bit polynomial operations with precomputed values. This can significantly improve checksum performance in firmware, embedded software, and high-throughput applications. The generated table and function code can be copied directly into C, Python, or JavaScript projects and adapted as needed.
CRC patch finder
The patch finder brute-forces the two-byte value that, inserted or appended at a chosen offset, makes the CRC of the patched data equal a target you specify. This is useful when a save file, firmware image, or other checksummed format rejects an edit because its stored CRC no longer matches — appending the found bytes restores a valid checksum without needing to reverse the CRC algorithm by hand.
CRC algorithm identifier
The identifier works in the opposite direction: given input data and a CRC result you already have, it checks that pair against all 55 catalog algorithms and reports every one that reproduces the result. This helps when a protocol or file format's documentation doesn't state which CRC variant it uses, since matching width and polynomial alone is not enough to confirm the model.
Frequently asked questions
What is a CRC checksum?
A cyclic redundancy check, or CRC, is an error-detection method used to identify accidental changes in transmitted or stored data. The sender calculates a CRC value, and the receiver calculates it again to verify that the data has not been corrupted.
Which CRC algorithm should I use?
Use the algorithm required by your protocol or file format. For example, Modbus commonly uses CRC-16/MODBUS, while other systems may require CCITT-FALSE, XMODEM, KERMIT, CRC-32, or a custom parameter set.
Why do CRC calculators sometimes return different results?
CRC results depend on more than the polynomial. Width, initial value, input reflection, output reflection, final XOR value, input encoding, and byte order must all match. A difference in any one of these settings can produce a different checksum.
Can I calculate the CRC of a binary file?
Yes. Select the file input option and choose a binary file. The calculator reads the file as bytes and computes the selected CRC directly in your browser.
Are uploaded files sent to a server?
No. CRC calculations and file processing are performed locally in the browser. Your files and input data are not uploaded to the website server.
What is the standard CRC check value for 123456789?
The ASCII string 123456789 is commonly used as a standard CRC test input. The expected result depends on the selected algorithm. For example, CRC-32/ISO-HDLC produces 0xCBF43926, while CRC-16/MODBUS produces 0x4B37.