300 dpi is one number applied to everything

Where the number came from

300 dpi is the conventional figure for printed material held in the hand, and it comes from offset printing: twice the standard 150-line halftone screen. It happens to match what the eye can resolve at about 30 cm, roughly 290 points per inch, which is why it suits a magazine page.

It is a terrible number for a billboard, and applying it everywhere is how people end up trying to produce a 30,000-pixel image for something a viewer sees from across a car park.

The arithmetic

The eye resolves around one arc-minute. Converted to a practical rule:

required dpi ≈ 3438 ÷ viewing distance in inches

Which gives, rounding sensibly:

• Held in the hand, 30 cm: 290 dpi. Use 300.
• A poster at 1 metre: 87 dpi. Use 150 for safety.
• A roll-up banner at 2 metres: 44 dpi. Use 100 to 150.
• An exhibition graphic at 3 metres: 29 dpi. Use 72 to 100.
• A billboard at 30 metres: 3 dpi. Printers commonly use 10 to 15.

Worked example. An A1 poster, 594 × 841 mm, viewed at a metre:

at 300 dpi  =  7016 x 9933 px   (70 megapixels, ~200 MB TIFF)
at 150 dpi  =  3508 x 4967 px   (17 megapixels, ~50 MB)

The 150 dpi version is indistinguishable at the real viewing distance and is a quarter of the work. The 300 dpi version needs a generative upscale that will invent detail nobody will see.

Ask the printer, not the internet

Commercial printers have a preferred figure for each process and will tell you in one email. The screen used for large-format printing is coarser than for litho, and supplying more resolution than the screen can carry does nothing at all.

The question to ask: “What resolution do you want artwork at, at final size, for this process?” The answer is frequently lower than you expected, and it is authoritative in a way no general advice is.

For screen

Different problem, same discipline.

• Web hero: 2× the layout size for high-density displays, so a 1280-pixel slot takes a 2560-pixel image. Beyond 2× there is no visible gain and a real cost in page weight.
• Social: the platform’s stated size. Uploading larger means their compression does the resizing, usually worse than you would.
• Email: 600 to 700 pixels wide, because that is the width of most email clients.
• Presentation slides: 1920 × 1080 is enough for anything projected.

The web case deserves emphasis because it is where oversupply actually hurts. A 6000-pixel hero image served to a phone is several megabytes of download for a 400-pixel display, and page weight is a real cost paid by the people least able to afford it.

How this changes the workflow

If the deliverable is a roll-up banner at 100 dpi, your 1024 render upscaled conventionally to 3500 pixels is finished. No tiled diffusion, no generative upscale, no forty minutes.

So calculate first. The sequence is: final physical size, viewing distance, required dpi, required pixels, and only then decide what enlargement method the gap needs. People do it in the opposite order and upscale as far as their machine allows.

The limitation

The arithmetic gives a floor, not a ceiling, and two situations justify going above it.

The client will zoom. Not the audience, the client. They will open it at 400% on a large monitor to check the product. Supplying only what the medium needs is correct and will still produce an awkward conversation, and it is worth delivering a generous file for that reason alone.

The image will be re-used. A poster image that later becomes a brochure cover needs the brochure’s resolution. If re-use is plausible, produce the higher-resolution version once while the project is live rather than reconstructing it in eight months from a file you have archived.

There is also an honest caveat about the arithmetic itself: it assumes normal vision, typical contrast and a viewer who is not looking for faults. Fine repeating detail, high-contrast type and thin lines all demand more than the formula suggests. Use it for photographic content, and give type and line work the resolution they need regardless, which is another argument for setting type separately, since vector type has no resolution at all.