“How big does a QR code need to be?” has a two-part answer: a hard floor set by printers and the format spec, and a practical size set by how far away the phone will be. Both are arithmetic, and both are cheap to get right before a print run.

The module is the unit of everything

A QR symbol is a square grid of modules — Version 1 is 21×21, and each higher version adds 4 modules per side up to Version 40’s 177×177. The version your content needs is decided by data length and error-correction level: a short URL lands around Version 2–4 (25–33 modules), a full vCard can climb past Version 10 (57 modules). The generator reports the version under the preview, so you don’t have to guess.

Two hard numbers come straight from DENSO WAVE’s documentation:

• Minimum module size ≈ 0.17 mm on a common laser printer — below that, print blur smears modules together. For a Version 1 symbol that’s a ~3.6 mm code, but that is a theoretical floor, not a target.
• The quiet zone is mandatory: a clear margin of 4 modules on all four sides. Borders, trim edges and background clutter that invade it are the classic cause of “it scans at home but not on the poster”.

The module size math is 25.4 ÷ dpi × dots-per-module: at 300 dpi, printing each module as a 3-dot block gives 0.254 mm per module; at 4 dots, 0.34 mm. DENSO WAVE’s own worked example — 50 alphanumeric characters at level M needs Version 3 (29 modules) → 29 × 0.254 mm + 8 margin modules × 0.254 mm ≈ 9.4 mm total symbol area at 400 dpi — is the template for any size calculation.

The practical floor: scan distance

Printer resolution is rarely the binding constraint — the phone camera is. The widely used rule of thumb across the QR industry:

Scan distance ≈ 10 × the code’s width.

A 20 mm code scans comfortably from about 20 cm; a poster code meant for across the room (2 m) needs roughly 20 cm of symbol. Treat 10:1 as the minimum for reliable reads — it’s a heuristic, not an ISO clause, but it tracks what autofocus phone cameras resolve in real lighting. When in doubt, bigger: dense high-version codes and low-contrast printing both erode the margin.

Working sizes by use case

These keep a version-appropriate code well inside the 10:1 envelope for a typical scan distance:

Where it lives	Typical scan distance	Sensible symbol size*
Business card, product label	~15 cm	15–20 mm
Flyer, table tent, menu	~30 cm	25–30 mm
A4 poster on a wall	~1 m	70–100 mm
Billboard, shop window	3–5 m	300–500 mm

*Symbol size excludes the mandatory quiet zone — add 4 modules of clear space on every side.

If space is tight, shrink the payload, not the module: a short URL (site.example/m) drops versions and buys you smaller, sparser, more forgiving codes. Every character of a shortened link is doing scanability work.

What actually kills printed codes

Size is the headline, but in the field these cause more failures:

• Quiet zone invaded — text or trim inside the 4-module margin. Keep the margin sacred.
• Low contrast — brand-coloured modules on coloured stock. Keep the code clearly darker than the ground; the generator warns when you dip below a safe ratio.
• Inverted colours — white-on-dark codes are rejected by many scanners.
• Logo too large — at ECC H the budget is ~30% of codewords; keep the mark under ~25% of area. See QR codes with logos.
• Pixelation — exporting a small PNG and stretching it in the layout. Use the SVG download for print: it’s vector, so the modules stay square at any size, and print shops prefer it for exactly that reason.
• Glossy lamination glare — a spotlight can white out a third of the code. Matte stock, or test under the real lighting.

The one-minute checklist

1. Generate the code; note the version and module count on the status line.
2. Pick the row in the table above for the real scan distance.
3. Confirm symbol size ÷ modules ≥ 0.17 mm so the printer can hold the detail.
4. Export SVG for print, PNG for screens; keep the 4-module quiet zone clear in the layout.
5. Print one proof and scan it with a phone from the intended distance — five dollars of test before five hundred of run.

Sizing figures and the worked example follow DENSO WAVE’s published guidance; the 10:1 distance rule is an industry heuristic. Full references are on the sources page.