How can QR code store so much data?

问题:

Quick googling result that QR code can hold nearly 3kb (8 bit) data. But isn't it just using black/white dots to represent bits? If so there's no way that there's over 20,000 dots on a code.

So I'm obviously misunderstanding. Can someone explain how it works?

回答1:

Denso Wave says that the QR bar code can be as large as 177 by 177, allowing for 31,329 dots. That is enough space for about 3kB, even allowing for position, alignment, timing, version, and formatting information.

回答2:

QR Code data capacity

Numeric only:      Max. 7,089 characters 
Alphanumeric:      Max. 4,296 characters 
Binary (8 bits):   Max. 2,953 bytes 
Kanji/Kana:        Max. 1,817 characters 

Version 40 is 177 x 177 and can store up to 7,089 numerics or 4,296 alpha-numerics (at lowest error correction)