The rule that governs all large-format work

A poster is not read at arm’s length, so it does not need the resolution an A4 page needs. Required resolution falls as viewing distance grows, and it falls predictably, because the limit is set by the resolving power of the human eye rather than by the printer.

Human acuity is roughly one arc minute — about 1/60th of a degree. Working that through geometry gives a useful approximation:

required dpi ≈ 3438 ÷ viewing distance (inches)

What that works out to in practice

Viewing distance	Formula result	Practical target	Typical product
30 cm / 12 in	287 dpi	300 dpi	Handheld print, brochure
1 m / 39 in	88 dpi	150 dpi	Poster on a wall
2 m / 79 in	44 dpi	75–100 dpi	Retail banner
5 m / 197 in	17 dpi	30–50 dpi	Trade-show backdrop
10 m / 394 in	9 dpi	15–25 dpi	Large outdoor banner
50 m / 1969 in	2 dpi	10–15 dpi	Billboard

The “practical” column is deliberately higher than the formula, and that is not a contradiction. The formula gives the point below which a viewer with normal acuity can start to resolve individual pixels. Real work needs headroom because:

• people stand closer than the intended distance, especially at events;
• the substrate itself limits how finely a press can print, regardless of the file;
• viewing angles are often oblique, which effectively shortens the distance;
• large-format printers are typically driven at 720–1440 dpi, and giving them an overly low-resolution file wastes that.

A worked example

A 3 × 1 metre banner, viewed from about 2 metres:

1. Distance in inches: 2 × 39.37 = 78.7 in.
2. Formula: 3438 ÷ 78.7 = 44 dpi minimum.
3. Practical target with headroom: about 75 dpi.
4. Pixels needed at 75 dpi: 118 × 39.4 in → 8850 × 2950 pixels at full size.

That is a large file, which is why large-format jobs are frequently set up at scale — design at 1:10 and print at 10×, so a 885 × 295 pixel file at 750 dpi describes the same output. Ask your printer which they prefer; the answer affects how you set up the document from the start.

Common questions

Can I just send a 300 dpi file to be safe?

You can, but a 3 × 1 metre banner at 300 dpi is 35,400 × 11,800 pixels — around 1.2 gigapixels, which no practical workflow handles, and which contains detail nobody at 2 metres can see. Oversupplying resolution is not free: it slows every step and often forces lossy compression that hurts more than the resolution helps.

Does upscaling help?

No. Interpolation cannot create detail that was never captured. If a file is genuinely below the required resolution, the options are to print smaller, view from further away, or accept a softer result.

Why do my 72 dpi images print fine on a banner?

Because at banner viewing distance, 72 dpi is often sufficient — the formula says so. The “72 dpi is unusable” rule is about close viewing, and it does not apply at four metres.