# Format information

> Format information is the 15-bit block next to the finder patterns that records the error-correction level and mask pattern. It is stored twice and protected by its own BCH code, because a decoder cannot read anything else without it.

Source: https://useqr.app/glossary/format-information · Last reviewed 2026-08-21 · UseQR is free forever, MIT licensed, no signup.

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## Structure

5 data bits — 2 for the error-correction level, 3 for the mask — plus 10 BCH(15,5)
error-correction bits, then XORed with the fixed mask `101010000010010` so an all-zero
format never produces a blank region.

## Stored twice

One copy wraps the top-left finder pattern; the other is split between the bottom-left and
top-right. If the first copy is unreadable, the decoder uses the second. This redundancy is
why damage near one finder pattern is often survivable.

## Why it is the most critical region

Everything else depends on it. Get the format bits wrong and the decoder unmasks with the
wrong pattern and de-interleaves with the wrong block structure — the payload is
irrecoverable even though the data modules are perfect. When styling a code, the area
immediately around the finder patterns deserves the most caution.