DPI and print size: how many pixels you need

DPI and PPI: what they mean

PPI means pixels per inch: how many of the image's pixels are placed along each inch of the print. Photo-editing help pages usually measure resolution in PPI. DPI, dots per inch, is the term print shops and forms usually use for the same requirement, as in 'images should be at least 300 DPI'. This guide uses DPI, as the forms do.

A digital image has no physical size until you decide how densely to print it. A photo of 1800 × 1200 pixels can be a small sharp print or a large soft one: same pixels, different spacing.

The formula

Everything follows from one relationship: pixels = inches × DPI. Going the other way, inches = pixels ÷ DPI. For centimetres, divide by 2.54 first, because one inch is 2.54 cm; for millimetres, divide by 25.4.

• A 6 × 4 in print at 300 DPI: 6 × 300 = 1800 and 4 × 300 = 1200, so 1800 × 1200 pixels.
• A 10 × 15 cm print at 300 DPI: 10 ÷ 2.54 × 300 ≈ 1181 and 15 ÷ 2.54 × 300 ≈ 1772, so 1181 × 1772 pixels.
• An A4 page, 210 × 297 mm, at 300 DPI: 210 ÷ 25.4 × 300 ≈ 2480 and 297 ÷ 25.4 × 300 ≈ 3508, so 2480 × 3508 pixels.
• The same A4 page at 150 DPI needs half as many pixels each way: about 1240 × 1754.

The DPI tag doesn't change the pixels

Many image files carry a DPI value in their details. It is only a suggestion for printing. Changing it without resampling changes the print size the file suggests, but the total number of pixels stays exactly the same, and so does the picture.

So a form that asks for '300 DPI' really needs enough pixels for the requested print size at 300 DPI. A small image whose tag has been changed from 72 to 300 still has the same few pixels and will print small or soft.

Screens don't use DPI

On a screen, an image is displayed by its pixel dimensions. How big it looks depends on those pixels, the size of the monitor and the monitor's resolution setting, not on the DPI value in the file. Two copies of a photo with identical pixels look the same on screen whether they are tagged 72 or 300.

The old idea that web images should be '72 DPI' is why many images online carry that value. For screen use, pay attention to the width and height in pixels instead.

When 150 is enough and when to aim for 300

300 DPI is the commonly quoted standard for printing, and printing services such as Newcastle University's ask for at least 300 DPI, with 150 DPI accepted for large banners. Carleton University's print shop gives 150 to 300 DPI for a standard sheet and says 150 is sometimes acceptable, but never lower.

The reason larger prints can use fewer pixels per inch is viewing distance. People hold a photo close but stand back from a poster. Carleton's guide lists minimums that fall with distance, from 300 DPI at about 0.6 m to 180 DPI at 1 m and 90 DPI at 2 m. Treat these as guidelines, and if a printer gives its own requirement, follow that.

• Photos and documents held in the hand: aim for 300 DPI.
• Large posters and banners seen from a distance: 150 DPI is often accepted; check with the printer.
• Official photo prints, such as 35 × 45 mm passport photos: follow the size the authority sets and print at the stated scale.

Getting the exact pixel size

In the resize tool, choose Print size, set the Unit to cm, mm or in, enter the width and height, and set DPI (300 unless you change it). The tool shows the exact pixel size it will make. If the photo's shape is different from the print shape, the middle of the photo is kept. Allow making images bigger is off by default, for the reason above.