# QR code bit stream walkthrough — from text to modules

> A QR bit stream is built as mode indicator, character count, encoded data, a 4-bit terminator, then pad bytes 0xEC and 0x11 to fill capacity. Codewords are split into blocks, extended with Reed–Solomon check codewords, interleaved, placed in a zig-zag from the bottom-right corner, masked, and topped with format information.

Source: https://useqr.app/docs/spec/qr-code-bit-stream-walkthrough · Last reviewed 2026-08-21 · UseQR is free forever, MIT licensed, no signup.

---

## The pipeline at a glance

```
segments → terminator → pad bytes → blocks → RS codewords → interleave
        → remainder bits → zig-zag placement → mask → format/version info
```

Each stage is mechanical; run them in order and any two correct encoders produce the same
codewords for the same input, version, and
[error-correction level](/glossary/error-correction-level). (Divergence enters only at
[mask selection](/docs/spec/mask-selection-penalty-scores).)

## Stage 1: the data bit stream

Each segment is a 4-bit [mode indicator](/docs/spec/data-encoding-modes), a character
count field (8–16 bits wide depending on mode and version), then the encoded characters.
After the last segment come the closing formalities:

1. **Terminator** — up to four `0` bits (fewer if capacity runs out exactly).
2. **Pad to a byte boundary** with `0`s.
3. **Pad bytes** — the alternating pair `11101100 00010001` (**0xEC 0x11**) repeated
   until the version's data capacity is exactly full. The values are not arbitrary:
   they are balanced bit patterns chosen so padding never creates the uniform areas the
   [penalty rules](/docs/spec/mask-selection-penalty-scores) exist to punish. Spot
   `EC 11 EC 11` in a hex dump and you are looking at a QR code's slack space.

## Stage 2: codewords, blocks and error correction

The stream is cut into 8-bit [codewords](/glossary/codeword) and split into the block
structure the standard fixes for this version and level. Each block gains its
[Reed–Solomon check codewords](/docs/spec/reed-solomon-error-correction), and the blocks
are then [interleaved](/docs/spec/error-correction-block-interleaving) — data codewords
round-robin across blocks, then check codewords the same way.

A final oddity: some versions' module counts are not a multiple of 8, so **remainder
bits** — always `0` — top up the stream: 7 of them for versions 2–6, 3 or 4 for various
versions from 14 to 34, none elsewhere.

## Stage 3: placement — the zig-zag

Function patterns claim their territory first; data fills what is left, in a pattern best
described as reading a book backwards:

- Placement works in **vertical column pairs, two modules wide**, starting at the
  **bottom-right corner**.
- The first pair fills **upward** — within each two-module row of the pair, right cell
  then left cell — until it hits the top, then the next pair to the left fills
  **downward**, alternating all the way across.
- Any cell claimed by a function pattern or reserved field is **skipped**, the sequence
  flowing around it; and the entire single column of the vertical
  [timing pattern](/docs/spec/timing-patterns) (column 6) is stepped over, keeping pairs
  aligned either side of it.

The most significant bit of the first codeword lands in the bottom-right corner cell.

## Stage 4: mask, then metadata

The placed data region is XORed with the winning
[mask pattern](/docs/spec/mask-patterns-0-to-7). Finally the 15-bit
[format information](/docs/spec/format-information) (level + mask, BCH-protected, XORed
with `101010000010010`) is written twice, and from version 7 the 18-bit
[version information](/docs/spec/version-information) twice more. The symbol is complete.

To watch every stage with real numbers, follow the worked example in
[encode HELLO WORLD by hand](/docs/spec/encode-hello-world-by-hand) — or generate any
payload and [decode it back](/validate) to confirm the pipeline round-trips.

## FAQ

### What are the pad bytes 0xEC and 0x11 in a QR code?
Filler appended after the terminator to bring the data stream exactly to the version's capacity. They alternate — 11101100, 00010001 — and their balanced bit patterns avoid creating large uniform areas that would scan poorly.

### How is data physically placed in a QR code?
In two-module-wide column pairs, starting at the bottom-right corner, filling upward then downward alternately across the symbol, skipping function patterns and stepping over the vertical timing column entirely.

### What are remainder bits?
Zero bits appended after the final interleaved codeword because some versions have a few more data modules than a whole number of codewords fills — 7 bits for versions 2–6, 3 or 4 for several mid-range versions, none for the rest.

### What order do the encoding stages run in?
Segments with mode and count headers, terminator, byte padding, pad bytes, block split, Reed–Solomon check codewords, interleaving, remainder bits, zig-zag placement, masking, and finally the format and version information fields.

## Try it

- https://useqr.app/url
- https://useqr.app/validate
- https://useqr.app/scan