Learn: QR Code Structure Guide

A module is one square cell in the QR grid: the smallest black-or-white unit the symbol is built from. So yes, visually it is one little square. In the data region, a module usually carries one written bit after masking, but many modules in a QR code are not payload bits at all. Finder patterns, timing patterns, alignment patterns, format information, version information, separators, and the dark module are all made of modules too, but they serve structural jobs instead of carrying message content.

Before a decoder can talk about payload bytes, ECC bytes, or format bits, it has to complete a more basic job: turn an image into a clean QR module grid. In practice that means decoding the image file, locating the symbol, estimating the grid geometry, sampling each module, reading the format and version metadata, undoing the mask, and only then reconstructing codewords.

That is why debugging rows about image dimensions, quiet zone, contrast, and module pitch are not cosmetic. They describe the evidence the decoder had available before payload parsing could even begin.

A QR image starts as some encoded file bytes: PNG, JPEG, SVG, or another image format. The decoder first interprets that file and turns it into a raster image, meaning a rectangular pixel grid with a width and height in pixels. File byte length and pixel dimensions are related but not interchangeable. A tiny SVG can scale into a large crisp raster, while a large JPEG can still decode into a blurry image with weak module edges.

After decoding the file, the scanner has to decide which pixels count as dark modules and which count as light background. That step depends on contrast. Thresholding is the step where the decoder draws that dark-versus-light boundary. If dark modules are not dark enough, or the background is not bright enough, thresholding becomes unstable and the recovered grid can drift or fragment before payload decoding even starts.

Module pitch is the estimated pixel width and height of one QR module in the recovered image. A healthy scan usually has enough pixels per module to distinguish dark and light cells cleanly. If pitch is too small, compression and resampling can blur neighboring modules together. If horizontal and vertical pitch differ too much, that often hints at distortion, cropping, or perspective issues.

The quiet zone is the blank margin around the QR symbol. Beginners often think of it as wasted space, but decoders use it to separate the symbol from nearby graphics, text, or borders. If the dark region nearly touches the image edge, the scanner may misread the symbol boundary or fail to detect the finder structure cleanly at all.

Three identical 7x7 concentric square patterns placed at the top-left, top-right, and bottom-left corners of every QR symbol. They allow any scanner to detect that a QR code is present, determine the symbol orientation, and establish the module-grid coordinate system before any data is read.

Three finder patterns create a unique L-shaped arrangement. Any three squares in those positions unambiguously define position and orientation; the missing fourth corner tells the decoder which way is up. A fourth pattern would create 180-degree rotational ambiguity.

Each finder pattern is a 7x7 grid with three concentric layers.

• Layer 1: 7x7 dark outer border
• Layer 2: 5x5 light ring
• Layer 3: 3x3 dark center block

Any horizontal or vertical scan line crossing a finder pattern produces dark:light:dark:light:dark module widths in the ratio 1:1:3:1:1. This ratio is what scanners search for, and it survives changes in scale, angle, and rotation.

Each finder pattern is surrounded by a 1-module-wide white separator on the sides facing the interior of the symbol. This prevents the finder from visually merging with nearby data modules. Separators are always white regardless of masking.

Beyond the required 4-module quiet zone outside the symbol boundary, the internal separators act as an internal quiet buffer that keeps the finder patterns distinguishable from nearby data modules.

Two alternating dark/light strips, one horizontal and one vertical, let the scanner determine module-grid density and compensate for perspective distortion or skew in the captured image.

• Horizontal timing: row 6, columns 8 through size-9.
• Vertical timing: column 6, rows 8 through size-9.

The first module of each timing strip is always dark. After that, the strip alternates dark, light, dark, light, and so on. The strip length is size-16 modules.

Row 6 and column 6 are fixed across all 40 versions. Because they sit between the finder patterns, scanners know exactly where to look before they know anything version-specific about the symbol.

When a QR code is photographed at an angle, modules appear warped. By counting transitions along both timing strips, the decoder can estimate the perspective transform and realign the grid before reading payload data.

Small 5x5 concentric patterns placed throughout the data region of larger QR codes. They provide extra reference points for correcting warping and local distortion across the full symbol.

• Border: 5x5 dark outer ring
• Inner Ring: 3x3 light ring
• Center: 1x1 dark module

• Version 1: no alignment patterns.
• Version 2 and above: at least one alignment pattern, with count increasing by version.
• Maximum: Version 40 uses 46 alignment patterns.

The specification defines a list of center coordinates for each version. All combinations of (row, col) from that list are used as alignment centers, except any that would overlap a finder pattern or its separator.

If a computed center would place a 5x5 alignment pattern inside a finder corner region, that center pair is skipped. That is why the number of patterns is always lower than the raw square of the center-list length.

Large QR symbols can accumulate noticeable lens distortion and paper warping by the time the scanner reaches the middle of the code. Alignment patterns let the scanner continuously recalibrate instead of trusting a single global transform.

Every module that is not part of a function pattern - finder, separator, timing, alignment, format information, version information, or the dark module - is part of the data region. These modules carry the full payload stream: mode indicator, character count, encoded data, error correction, and remainder bits.

Before placement, the final bitstream follows a fixed structure. Beginners should read it left to right as: what mode am I in, how long is the payload, what are the payload bits, and what extra bits are needed to finish the QR structure cleanly and safely. A terminator is the short end marker that says the payload is over. Pad bits are zero bits added only to reach the next byte boundary. Pad codewords are full filler bytes added after that if the symbol still has unused data capacity.

1. Mode Indicator: 0001 numeric, 0010 alphanumeric, 0100 byte, 1000 Kanji
2. Character Count: variable length by version range and mode
3. Encoded Data: the actual payload bits
4. Closure: terminator, byte padding, pad codewords, ECC, then remainder bits

For a short byte-mode example in versions 1 through 9, the character count field is 8 bits long. That means the encoder writes the 4-bit byte-mode marker, then an 8-bit byte count, then the UTF-8 bytes for each character.

For most versions, data is split across multiple Reed-Solomon blocks before ECC is computed. The final stream interleaves data blocks first, then ECC blocks, then appends any remainder bits. That spreads local damage across many blocks instead of destroying one block completely.

• Arrange all data blocks side by side.
• Interleave codeword 1 from block 1, block 2, block 3, and so on.
• Repeat for codeword 2, codeword 3, and the rest of the data codewords.
• Then interleave ECC codewords the same way.
• Append any remainder bits at the very end.

Remainder bits are the final leftover data-region modules that remain after every data codeword and every ECC codeword has already been assigned. They are not payload bits, not padding codewords, and not error-correction bytes. They are simply extra module positions that exist because the usable data-region area of some QR versions is not an exact multiple of 8 bits.

QR encoding works in bytes for data and ECC codewords, but the matrix is a grid of individual modules. After the function patterns are reserved, the remaining writable modules sometimes total a count like 8n + 3, 8n + 4, or 8n + 7. The QR specification still defines codeword capacity in whole bytes, so those extra 3, 4, or 7 modules cannot become another codeword. They are filled with zero-valued remainder bits instead.

That last subtraction is the whole idea. If a version has exactly enough writable modules for whole bytes, the remainder-bit count is zero. If it has a few extra modules beyond the byte-aligned codeword capacity, those modules become remainder bits.

The page already lists version 20 as having 8,324 data modules, written as 1040 CW x 8 + 4 rem. That means the version can carry 1040 total codewords across data and ECC, which consumes 8,320 modules. Four writable modules are still left over, so the encoder writes four zero remainder bits into them.

The important takeaway is that remainder bits are a property of the version's final writable-module count, not of the message, ECC level, or mask pattern. Once you choose a version, the remainder-bit count is fixed.

Remainder bits are always zero. There is no choice here and there is no optimization step. After all interleaved data and ECC codewords have been placed, the encoder writes zeroes into the remaining eligible modules. A decoder does not interpret them as payload and does not feed them into Reed-Solomon decoding.

In the logical stream order, remainder bits come after everything else:

• mode indicator
• character count field
• encoded payload bits
• terminator and zero padding to byte boundary
• pad codewords
• ECC codewords
• remainder bits

This matters because remainder bits are outside the Reed-Solomon-protected byte stream. They do not belong to any block, they are not interleaved as codewords, and they have no effect on payload recovery.

The matrix walk does not need a special path for remainder bits. The encoder performs the same normal zig-zag traversal used for all writable modules. It places interleaved data and ECC bits first. If the traversal still has eligible modules left after the final codeword bit is consumed, those last modules receive zero remainder bits in the same traversal order.

A decoder reconstructs the sequence of writable modules, demasks them, and reassembles codewords until it has read the version's defined number of total codewords. If a few modules remain after that count, the decoder knows they are remainder bits and stops treating them as codeword data. They are effectively ignored after structural validation.

• Remainder bits are not the same thing as the Reed-Solomon remainder that generates ECC codewords. The names are similar, but they describe different parts of the process.
• Remainder bits are not pad codewords. Pad codewords are full bytes inserted before ECC generation; remainder bits are a few trailing modules after all codewords are done.
• Remainder bits do not increase capacity. They exist precisely because there are not enough leftover modules to form a full extra byte.
• Remainder-bit count depends on version only, not on the message contents.

Format information tells the decoder two things it must know before reading the payload: the error correction level and the data mask pattern used across the symbol.

The format string is 15 bits total: 2 ECC bits, 3 mask bits, and 10 BCH error-correction bits. BCH is a short binary error-correcting code used here to protect small metadata strings like format and version information.

• Bits 14-13: ECC level
• Bits 12-10: mask pattern
• Bits 9-0: BCH redundancy

The mask choice is not stored somewhere else in the QR symbol. It lives inside the 15-bit format string itself. The three orange cells below are the mask-pattern bits, so this is the exact place where the selected mask option is represented.

The BCH correction bits are computed with generator polynomial x^10 + x^8 + x^5 + x^4 + x^2 + x + 1. The full 15-bit string is then XORed with the fixed mask 101010000010010 so that all-zero and all-one format strings do not occur.

Masking does not encrypt the message. It only flips selected data modules so the final symbol avoids patterns that are visually awkward for scanners, such as long runs, heavy clumps, or finder-like stripes in the wrong place.

Both copies encode the same information. If one copy is damaged, the other can still be decoded.

• Copy 1 sits next to the top-left finder pattern.
• It occupies row 8 columns 0-5, row 8 column 7, row 8 column 8, and column 8 rows 7-0.
• Copy 2 is split between row 8 near the top-right finder and column 8 near the bottom-left finder.

In Copy 1, bit 14 (MSB, most-significant bit) begins at row 8, col 0 and bit 0 (LSB, least-significant bit) ends at col 8, row 0. In Copy 2, bit 14 begins near col 8, row size-7 and the later bits continue near the top-right strip.

Present only in versions 7-40. Version information encodes the version number as an 18-bit string so decoders can confirm symbol size without counting modules, which matters when large symbols are distorted or partially obscured.

The version block is 18 bits total: 6 data bits for the version number and 12 BCH correction bits. The BCH generator used is x^12 + x^11 + x^10 + x^9 + x^8 + x^5 + x^2 + 1, and the code can correct up to 3 bit errors in the version block.

• Bits 17-12: version number
• Bits 11-0: BCH redundancy

Small symbols are easier to size from the finder patterns and timing lines alone. By the time the decoder sees version 1 through 6, it can infer the grid size reliably enough that a dedicated 18-bit version block would waste valuable space. Versions 7 and above are large enough that explicit confirmation becomes worth the reserved modules.

Both copies encode the same 18 bits. The string is arranged as a 6x3 block read column-by-column, top-to-bottom, left-to-right.

• Copy 1: top-right block, rows 0-5 and columns size-11 through size-9.
• Copy 2: bottom-left block, rows size-11 through size-9 and columns 0-5.
• Copy 2 is effectively the transposed mirror of Copy 1.

Once the payload and ECC bytes exist, the QR encoder must pour those bits into the remaining empty modules. It does that by sweeping right-to-left through 2-column-wide vertical strips, alternating travel direction up and down.

Imagine codeword 0 begins with bits 1 0 1 1 0 0 1 0. Bit 7 goes into the first visited data module, bit 6 goes into the second, and so on. Only after every bit of every interleaved codeword is placed are any leftover remainder modules filled with zero bits.

Within each 2-wide strip, the right column is placed first and the left column second. At a given row, the sequence is right-current, left-current, right-next, left-next.

Codewords are placed most-significant-bit first. Bit 7 of codeword 0 goes into the first visited data module; bit 0 of the final codeword is placed just before any remainder bits.

After all data and ECC codewords are placed, some versions still have a few leftover modules. Those are filled with zero-valued remainder bits and do not carry payload information.

Zig-zag placement distributes data across the full area of the symbol. Combined with block interleaving, that means local physical damage corrupts many different blocks a little instead of destroying one block completely, which is exactly what Reed-Solomon correction wants.