Image Resolution for Print Calculator

Working backwards from the print

The useful direction is usually from the finished piece to the file: given this print size and this viewing distance, what pixel dimensions are needed.

Required pixels equals physical size times required resolution, and the required resolution comes from the viewing distance rather than from a default.

A 60 by 40 centimetre wall print viewed from a metre needs perhaps 150 DPI, which is 3543 by 2362 pixels. Insisting on 300 would demand 7087 by 4724, which is a 33 megapixel capture for no visible gain.

The viewing distance rule

A workable approximation is that required resolution in DPI is roughly 3438 divided by the viewing distance in inches, derived from the angular resolution of the eye.

At 12 inches that gives 286 DPI, which is why 300 became the standard for held material. At a metre, roughly 39 inches, it gives 87 DPI. At three metres, 29 DPI.

The rule breaks down at the extremes and it is a useful corrective against the habit of demanding 300 DPI for everything. Applied to a trade show banner it explains why files that look alarmingly low resolution on screen print perfectly well.

Megapixels, and how far they go

Converting a camera's megapixel figure to a maximum print size is a common question and the arithmetic is straightforward.

A 24 megapixel camera at 6000 by 4000 pixels prints 20 by 13.3 inches at 300 DPI, or 40 by 26.7 at 150. A 12 megapixel phone at 4000 by 3000 prints 13.3 by 10 inches at 300.

Which means almost any modern camera or phone produces files adequate for the prints most people order. The limiting factor is nearly always the capture quality rather than the pixel count: focus, motion blur, noise at high ISO and lens softness all reduce the effective detail well below what the pixel count implies.

Cropping, and what it removes

Cropping discards pixels, and the remaining file has to satisfy the resolution requirement on its own.

A 24 megapixel image cropped to a quarter of its area is a 6 megapixel image, which at 300 DPI prints 10 by 6.7 inches rather than 20 by 13.3.

This is where prints most often fail unexpectedly. The original file was ample, the crop was heavy, and nobody recalculated. Checking the pixel dimensions after cropping rather than before is the habit that prevents it.

The other things that matter more

Resolution is necessary and not sufficient. A perfectly sharp 300 DPI file prints badly if the colour profile is wrong, the black is a rich black where it should be flat, or the image was sharpened for screen rather than for paper.

Paper choice affects apparent sharpness considerably. Coated stock holds detail; uncoated absorbs ink and spreads it, which softens fine detail regardless of the file. Printing the same file on gloss and on uncoated produces visibly different sharpness.

Which is why a proof matters more than a specification for anything important. A printed proof on the actual stock answers every one of these questions at once, and it costs a fraction of a reprint.

It is also worth separating the resolution question from the sharpening question, because they produce similar-looking failures. An image with ample pixels that was sharpened for a screen looks harsh in print, with visible halos along high-contrast edges. An image with too few pixels looks soft no matter how it is sharpened. Both get described as a resolution problem and only one is, and viewing the file at 100% on screen distinguishes them immediately.