Alphanumeric mode

The 45-character set

Each character maps to a value 0–44: digits to 0–9, A–Z to 10–35, then space and $ % * + - . / : filling 36–44. A pair encodes as 45 × first + second in 11 bits; a trailing single character takes 6. The full value table is in the character set reference, and the mode's ceiling (4,296 characters at version 40-L), in the capacity table.

The set was chosen in 1994 with industrial part numbers in mind, which is why it contains / and : (enough for a URL's scheme) but no lowercase, no =, &, ?, # or _.

The uppercase-URL trick

Because scheme and hostname are case-insensitive, HTTPS://EXAMPLE.COM/MENU is a valid URL that stays entirely inside the set, encoding roughly 31% smaller than its lowercase form, often one whole version smaller, meaning larger modules at the same printed size. The limits:

• Paths are frequently case-sensitive; uppercase only the scheme and host unless you control the server.
• Any query string (?a=b) breaks eligibility, ?, = and & are not in the set.
• Encoders switch modes automatically; you cannot request alphanumeric, only supply text that qualifies. Decode a code to see which mode it used.

FAQ

Why doesn't a QR code's alphanumeric mode include lowercase letters?

The 45-value set had to fit two characters into 11 bits (45² = 2,025 < 2,048). Adding 26 lowercase letters would need 71 values, and 71² overflows 12 bits, so 1994's designers kept uppercase only.

Does capitalising a URL really make the QR code smaller?

Yes, an all-uppercase URL without a query string encodes at 5.5 bits per character instead of 8, often dropping the code a full version. The win disappears the moment any character falls outside the 45-character set.