Email Signatures: The Optional Scan
QR codes in email signatures — typically linking to a vCard, booking page, or portfolio — are scanned from another screen or a printout at 30 to 50 cm. But email is a hostile medium for images: many clients block them by default, signatures get mangled by replies and forwards, and mobile clients resize unpredictably. An email-signature code is a convenience for the fraction of recipients who see images — never the primary call to action.
The 10:1 Rule of Thumb
At 40 cm the rule suggests 40 mm on screen — roughly 150 px at standard display scaling. In practice, 100–150 px square is the workable range; larger codes bloat the signature, get stripped by some clients, and look disproportionate next to your name. 120 px displayed (supplied at 240 px for retina sharpness) is the balanced choice most signature designers land on.
Recommended Size
Render at 120 × 120 px (about 30 mm on a standard display). Supply the image at 2× resolution (240 px) so it stays sharp on retina displays without layout bloat. In HTML signatures, set explicit width and height attributes so clients that resize aggressively still render it correctly.
Substrate and Printing (Digital)
The "substrate" is the recipient's screen and email client. PNG with a solid white background is the safe format — transparent backgrounds inherit whatever the client renders behind them, and dark mode can invert a transparent-background code into unscannability. Keep the file under ~20 KB so it doesn't trip attachment filters or get stripped as a suspicious payload.
Placement and Practical Tips
- Never make the code the only call to action — always pair it with a plain text link, because image blocking is the default in many corporate clients and the recipient may never see the code at all.
- Link to something with lasting value (booking page, vCard download, portfolio), not a campaign that expires.
- Test the signature by sending to Gmail, Outlook, and Apple Mail — rendering, image proxying, and dark-mode handling differ wildly between them.
- Remember dark mode: clients that invert or restyle emails can destroy a code without a white background. White background is cheap insurance.
- Changing roles? Use a redirect you control, because your code lives in thousands of sent emails you'll never update.
Common Mistakes
- A code with no text fallback in a client that blocks images — the recipient sees a broken-image icon where your call to action should be.
- Encoding a 400-character vCard directly; link to a hosted contact page instead and keep the code simple.
- Oversized signature images that get clipped by mobile clients or flagged by corporate filters.
- Forgetting that every reply and forward carries the signature — keep the destination permanent.