Upscaling an image for print: the numbers that matter

Printing is where a shortage of pixels finally becomes visible. A picture that looks perfect on a phone is being drawn at roughly 460 pixels per inch on a screen you hold close to your face. Paper spreads the same pixels over a much larger area and hides nothing.

This guide is the arithmetic, the honest limits, and the handful of practical points that cause most reprints.

The one calculation

Pixels divided by dots per inch equals inches. Rearranged for the thing you actually want:

pixels needed = inches on paper times dpi

Everything else is choosing the dpi.

If your file already meets the number in the right column, you have nothing to do. Image resolution explained covers where these numbers come from and why the dpi stored in a file is only a note.

Choosing the dpi by viewing distance

300 dpi is not a law. It is a figure calibrated for a print held about 30 cm from your eye, which is roughly where the human eye stops resolving more detail. Increase the distance and the requirement falls away quickly.

Large format printers work with these numbers every day and will happily tell you theirs. Asking is faster than guessing and free.

There is a second reason the requirement falls: large prints are usually seen as a whole rather than inspected. Nobody puts their nose against a two metre banner, and if they do, they are looking at the printing dots rather than your pixels.

How far you can enlarge

The comfortable ranges, assuming a reasonably sharp original:

• Two times. Essentially free on a photograph with a good model. Almost nobody will be able to tell.
• Three to four times. Very usable for print. Fine detail is reconstructed rather than recorded, but it reads as detail on paper.
• Six to eight times. For prints seen from a distance. Up close it looks like a painting of the photograph.
• Beyond eight times. Possible, rarely wise, and only for backdrops.

Enlarge by whole factors. A model is trained at two, three and four times, and six and eight are better done as two passes with the right model each time than as one stretch. How to upscale an image covers the method.

Choose the engine by the content, not by the destination. A photograph enlarges best with a model. A logo, a chart or anything with text should go through a classical filter, or better, be replaced with a vector before printing at all.

Things that cause reprints

Sharpening twice. Many print workflows apply their own output sharpening for the paper and the press. If you sharpen heavily as well, every edge gets a halo. Send it clean and say you have not sharpened, or ask whether they want you to.

The wrong colour space. Screens are sRGB. Commercial presses are CMYK, and some labs want Adobe RGB. Converting at the last moment, or letting the printer do it, usually beats converting badly yourself. Ask which they want.

Transparency that should have been flattened. A PNG with an alpha channel going into a print workflow may be composited onto white, onto black, or onto nothing depending on the software. If the design has a background, put it there yourself. PNG shows a black or white background covers why this happens.

JPEG artefacts enlarged. If the original is a heavily compressed JPG, the blocks and the ringing around edges get enlarged along with everything else, and a model may interpret them as texture. Start from the least compressed copy you can find.

No bleed. If the picture runs to the edge of the page, the printer needs 3 mm more on every side. Enlarge to the bleed size, not the trim size, or you will be cropping after the fact.

Paper and ink decide how much detail survives

Pixels are only half of the story. What reaches the eye is ink on a surface, and the surface has opinions.

Uncoated stock, the ordinary matt paper used for most flyers and books, absorbs ink and lets it spread slightly. That spread, called dot gain, softens fine detail and gently darkens midtones. Coated and gloss stock holds ink on the surface, prints noticeably sharper, and is why photographic work is rarely put on uncoated paper.

Newsprint is the extreme case: it spreads so much that magazines and newspapers have historically printed photographs at effective resolutions that would look unacceptable anywhere else, and nobody notices because the paper is doing the blurring.

The practical consequence is that an enlargement which looks slightly soft on screen may print perfectly well on uncoated stock, and an enlargement which looks acceptable on screen may look obviously reconstructed on gloss. If the job matters and the run is large, ask the printer for a proof on the actual stock. It costs a little and it answers the question completely, which no amount of zooming on a monitor can.

A worked example

A supplier sends a product photograph at 1200 by 900 pixels. It has to fill an A2 poster for a trade stand, seen from about three metres.

A2 is 420 by 594 mm, which is 16.5 by 23.4 inches. At three metres, 150 dpi is ample, so the target is about 2480 by 3508 pixels for the full sheet. The photograph is landscape and the poster is portrait, so it will not fill the sheet: assume it occupies the top two thirds, roughly 16.5 by 15 inches, which at 150 dpi is 2480 by 2250 pixels.

2480 divided by 1200 is 2.07. So two times is very nearly enough and three times is comfortably enough. Choose three, enlarge with the model because it is a photograph, add bleed, and the job is done with something in hand.

Had the same photograph been destined for an A2 print to hang in an office and be looked at closely, the dpi would have been 300 rather than 150, the target 4961 pixels, and the factor four times or more. Same file, same paper size, twice the demand, because of where somebody stands.

Scanning instead of enlarging

If the source is a physical print, a negative or a document, rescanning beats every upscaler on earth and takes two minutes. A flatbed at 600 dpi turns a 6 by 4 inch print into 3600 by 2400 pixels, which is enough for A4 at photographic quality with room to spare. Dpi explained has the scanning numbers.

The same logic applies to anything that exists in a better form somewhere: a logo that exists as a vector, a document that exists as a PDF, a photograph that exists on the photographer’s card. Enlarging is what you do when the better source genuinely is not available.

What to send

Export at exactly the pixel size you calculated, at the finished dimensions, in the format the printer asked for. Put the intended print size in the file name or the email: “hero-A3-300dpi.jpg” removes an entire category of misunderstanding. If you enlarged it, say so and say by how much, and mention which engine you used. A good printer would far rather know in advance than discover it on a proof, and will often suggest a different dpi or a different stock once they do.

The steps, in short

1. Get the finished size in inches. Measure or ask for the width and height the picture will occupy on paper, in inches. Divide millimetres by 25.4 if that is what you have.
2. Choose the dpi by viewing distance. 300 for something held in the hand, 150 for a poster on a wall, 100 or less for a large banner. Ask the printer if you are unsure; they will tell you their minimum.
3. Multiply to get the pixels you need. Inches times dpi, for both width and height. That is the target.
4. Enlarge by the next whole factor. Divide the target by what you have and round up to 2, 3, 4, 6 or 8. Use a model for photographs and a classical filter for anything graphic.
5. Send a flat file at the finished size. Export at the target pixel size, without extra sharpening, in the colour profile and format the printer asks for, and say what size it is meant to print at.

Questions

How many pixels do I need to print A4?

About 2480 by 3508 at 300 dots per inch, which is the standard for photographic quality. At 200 dpi, which is acceptable for most everyday printing, 1654 by 2339 is enough.

Is 300 dpi always necessary?

No. 300 dpi is calibrated for something held about 30 cm from your eye. The further away the print will be viewed, the less you need: roughly 150 dpi for a poster and 50 to 100 for a large banner. Printers for large format work routinely accept much lower numbers than photographic labs do.

How much can I enlarge a photo before printing it?

Two to four times is comfortable for something held in the hand. Six to eight times is fine for a print seen from a distance. Beyond that you are mostly printing the tool's opinion, which may still be the right call for a backdrop.

Should I sharpen before sending to a printer?

Only lightly, and ask first. Many print workflows apply their own output sharpening, and two rounds of it produce halos along every edge. If in doubt, send it unsharpened and say so.

Does changing dpi in Photoshop improve the print?

Only if you resample at the same time. Changing the dpi field without resampling just relabels the same pixels, which changes the size the file claims to be and nothing else.

What format should I send to a printer?

Ask, because it varies. A high quality JPG or a TIFF is usual for photographs, PDF for anything with text or vector artwork. PNG is fine for artwork with transparency but is rarely what a commercial printer prefers.