Print sizes, aspect ratios and resolution: made clear

Three things are easy to mix up when preparing a print: its shape, its finished size and the number of pixels in the file. Separating them makes it much easier to decide whether an artwork is suitable.

Aspect ratio describes the shape

Aspect ratio is the relationship between width and height. A 30 × 40 cm portrait print has a 3:4 ratio. So does a 3,000 × 4,000 pixel image. The units differ, but the proportions are the same.

A-series paper uses approximately 1:1.414, often written 1:√2. A 50 × 70 cm image uses 5:7, approximately 1:1.4. Those shapes are close, but they are not identical. Check the selected product size rather than assuming every tall rectangle will fit.

• Square: 1:1
• 20 × 25 cm: 4:5
• 30 × 40 cm: 3:4
• 50 × 70 cm: 5:7
• A-series: approximately 1:√2

These examples explain ratios; they are not a promise that every example size is available in every product or region. Use the current product options for availability.

Check orientation and composition

A portrait 3:4 design becomes 4:3 in landscape. Rotating the canvas and rearranging the composition is different from stretching an image. If the proportions do not match, decide deliberately whether to recompose the design, add an intentional border or offer a different size.

Do not rely on the ordering system to crop the artwork for you. Include an intentional border in the supplied artwork unless the team has agreed another specification.

Resolution depends on the size you print

For a raster image, divide pixels by inches to estimate pixels per inch at the intended print size. A 3,000 × 4,000 pixel file printed at 12 × 16 inches provides 250 pixels per inch in both directions. The same file at 24 × 32 inches provides 125.

The DPI value saved in a file is not the whole story. Changing that number without adding meaningful image detail does not make the source sharper. Look at the pixel dimensions and the physical output size together.

PrintPost uses 150–300 DPI as guidance. Lower-resolution artwork can still be printed, so a low-resolution warning is a prompt to review the result, not an automatic rejection. Artwork style and viewing distance also affect how the finished print is perceived.

A PDF is a page, not just a picture

A PDF may contain vector shapes, text and raster images. Its page dimensions describe the output shape, but any images inside it still have their own resolution. Export the intended page size, check the page count and embed or outline fonts.

For a single print, prepare a single-page file. If you have several designs or pages, discuss the correct order setup rather than expecting the uploader to split them into separate products.

Before uploading

1. Check the chosen size and orientation.
2. Confirm that the artwork has matching proportions.
3. Inspect text and fine detail at a useful zoom level.
4. Export JPEG, PNG or PDF within the 200 MB limit.
5. Use RGB artwork.
6. Check the uploaded artwork preview and selected options before submitting.

A worked example: preparing an A3 file

A3 is 297 × 420 mm. Divide each measurement by 25.4 to convert it to inches, then multiply by your target pixels per inch. At 300 PPI, that gives approximately 3,508 × 4,961 pixels. At 150 PPI, it is approximately 1,754 × 2,480 pixels. These are calculations for the artwork area, not outside frame measurements or an instruction to add bleed.

Suppose your source is 2,000 × 3,000 pixels. It has a 2:3 shape, which differs from A3. Even if its resolution looks acceptable, the proportions still need a decision. You might recompose the design on an A3 canvas or include a deliberate border. Stretching it to fit changes the shapes inside the image.

Review the export, not only the working file

Open the file you intend to upload in an image viewer or PDF reader. Check that all text appears, the orientation is correct and nothing important touches an edge unexpectedly. A missing font, hidden layer or old export can be easy to miss when you only inspect the original design document.

For PNG files, make the intended background explicit. Transparent areas can behave differently between a screen preview and a production workflow; ask the team before relying on transparency for a particular result. For a PDF, check its page size and use one intended print page. A PDF container does not improve a low-resolution image placed inside it.

What the on-screen guidance can tell you

The ordering page can compare readable image dimensions with your selected size. It cannot judge whether an artistic texture is intentional, whether a photograph is in focus, or whether you own the rights to the artwork. PDF detail also needs a separate check. Review any warning, and use a physical sample when the result is uncertain.

Keep the full-resolution original separately. A product mockup, thumbnail or screenshot of your design is not a substitute for the production file.

Read the PrintPost artwork guidance