vCard
The standard contact-card format a QR code can encode directly.
vCard (RFC 6350) is the standard format for electronic business cards, and the format phones use internally for contacts. A QR code can encode a complete vCard, so scanning it offers to create a contact with the fields pre-filled, with no internet connection needed.
Structure. Plain text lines between BEGIN:VCARD and
END:VCARD, each a property and value: FN for formatted name,
TEL, EMAIL, ORG, ADR.
Escaping, which generators get wrong. Backslash, comma, semicolon and newline are structural and must be escaped with a backslash. A job title of "Director, Operations" will split into two fields in a generator that does not escape the comma.
Length discipline. Every field adds data, and a business card is small. Name, one phone, one email and a website is usually the right amount; a full postal address plus a note can push a code past what scans reliably at business-card size.
In practice
A minimal vCard — name, one mobile number, one email — runs about 120 characters and encodes to roughly version 6. Add a company name, job title, website and full postal address and it passes 300 characters, reaching version 11 or higher.
On a business card where the code can be at most 20mm wide, version 6 gives modules of about 0.49mm, and version 11 gives about 0.33mm. The first is marginal; the second will disappoint people at a trade show. Cutting the postal address is usually the single most effective fix.
Questions
Which vCard version should I use?
3.0 has the broadest support across phones. 4.0 is newer and better specified but handled inconsistently by older devices.
Can a vCard code include a photo?
Technically yes, as base64. In practice never — even a small image pushes the code past what can be printed and scanned at any reasonable size.