DPI does not change a photo's size: what 300 DPI really means

DPI does not make a photo bigger or smaller on a screen. A photo's size is its pixel width and height. DPI, dots per inch, or PPI, pixels per inch, for digital images, is a number stored with the file that tells a printer or layout program how large to print those pixels. For a 10 × 15 cm print at 300 DPI you need about 1181 × 1772 pixels, found by multiplying the inches by 300. To change the size you get, change the pixels. Changing the DPI number alone leaves the picture as it was.

What DPI means for a photo

DPI describes ink on paper: how many dots a printer places in each inch. A digital photo has no dots, only pixels, so PPI is the more exact term. The DPI value in the file is metadata, a hint about the intended print size. Common values are 72 and 300, and the number often comes from the camera or the program that saved the file.

A worked example shows the effect. Take a photo that is 3000 × 2000 pixels.

• Tagged 72 DPI, it prints at about 41.7 × 27.8 inches (3000 ÷ 72 and 2000 ÷ 72).
• Tagged 300 DPI, the same pixels print at 10 × 6.7 inches (3000 ÷ 300 and 2000 ÷ 300).

The photo did not change between the two. The printing program divided the same pixels by a different number.

Why forms and screens ignore it

By default, web browsers show an image at its pixel size, scaled to the screen, and they do not use the DPI number to decide how large it appears. Upload forms look at pixel width, pixel height and file size. So a form that asks for 300 DPI almost always means enough pixels for a print of a stated size at 300 DPI. Read the wording. If the form asks for pixel dimensions, give it pixels. If it asks for a print size, multiply the inches by 300.

The pixel sums

Pixels equal inches multiplied by DPI. Convert centimetres to inches by dividing by 2.54 first.

For any size, the rule is centimetres divided by 2.54, then multiplied by the DPI. For 10 × 15 cm: 10 ÷ 2.54 is 3.937 inches, and 3.937 × 300 is about 1181 pixels. 15 ÷ 2.54 is 5.906 inches, and 5.906 × 300 is about 1772 pixels.

The fix

10 × 15 cm is a 2:3 shape, which is portrait. Work through these steps:

1. Match the shape. A landscape photo has to be turned or cropped. Open the crop tool, choose Free, and type X, Y, Width and Height in pixels. Rotate 90° turns the photo first if it is landscape. The crop tool's ratio options do not include 2:3, so type the size yourself.
2. Set the pixels. Open the resize tool, choose Exact pixels, type 1181 for width and 1772 for height, and keep proportions on. With both sides filled in, the photo fits inside that box without distortion. If the crop is slightly off, you get a thin border, not a stretched picture.
3. Save and check. Download the file and view it at 100 per cent zoom to confirm the detail is sharp. Change the DPI tag only if the print service asks for it. That is done in an image editor, and it does not change the pixels.

Read the pixel size of a file

Before you resize, confirm the pixel size of your file. On Windows, right-click the file, open Properties and look at the Details tab. On a Mac, select the file, press Command+I and open More Info. Both show width and height in pixels, which is the number a form or the resize tool needs. Do not rely on the size a print dialog shows, because that dialog converts pixels to inches using the DPI tag, and the result depends on the tag.

When the DPI tag does matter

Desktop publishing and layout programs use the DPI tag to report how large a placed picture is. A photo tagged 72 DPI can look enormous on a layout page, while the same photo tagged 300 DPI shows the size you expect. The picture itself is identical. If you work in such a program, check the placed size there and set the tag to match the print. Many print services, however, check the pixel dimensions against the final print size, so the pixels are the number to get right first.

Edge cases

• Too few pixels. An 800-pixel-wide photo can print at 300 DPI only 2.67 inches (6.8 cm) wide, because 800 ÷ 300 is 2.67. Printing it larger means stretching the pixels, and no tool adds detail back, so the print looks soft. Print it smaller, or start from a larger original.
• Screens. On screen, a 72 DPI tag and a 300 DPI tag look identical. Only the pixel size affects what you see at 100 per cent.
• Enlarging. Enlarging a small photo to hit a pixel target only makes it softer. Use the largest original you have.
• Bleed. Some print services ask for bleed, an extra margin beyond the trimmed size. Check their template, because the pixels must cover the bleed too. Resize for forms covers the pixel side of online uploads.