DPI Print Size Calculator (pixels ⇄ cm)

Understanding the DPI print size calculator — concept and principle

This calculator turns an image's pixel dimensions and a chosen print resolution into a physical print size in centimetres and inches, and tells you the largest ISO A paper size it can fill. Use it before ordering photo prints, posters or printed documents to check whether your file has enough pixels for a sharp result.

How it is calculated

• Print width (in) = width in pixels ÷ DPI; print height (in) = height in pixels ÷ DPI.
• Size in cm = size in inches × 2.54 (1 inch = 2.54 cm exactly).
• Megapixels = width × height ÷ 1,000,000.

The headline size in cm is rounded to 1 mm, while the table shows three decimals. For the paper check, the long and short sides of the print are compared with A0 to A6 (A4 = 210 × 297 mm, A5 = 148 × 210 mm), with 0.5 mm tolerance.

The principle

A digital image has no physical size of its own, only a grid of pixels. The resolution setting decides how many pixels are placed in each inch of paper. Spread the same pixels more thinly and the print gets bigger but each pixel becomes larger and easier to see. Strictly, PPI (pixels per inch) describes the image and DPI (dots per inch) describes printer ink dots, but print services use "DPI" for both. ISO A sizes all share the 1 : √2 aspect ratio, and each size is half the area of the one before.

Worked example

Defaults: 3,000 × 2,000 pixels at 300 DPI.

1. Width: 3,000 ÷ 300 = 10 in; 10 × 2.54 = 25.4 cm.
2. Height: 2,000 ÷ 300 = 6.667 in; 6.667 × 2.54 = 16.933 cm, shown as 16.9 cm.
3. Megapixels: 3,000 × 2,000 ÷ 1,000,000 = 6.
4. The print is 254 × 169.3 mm. A4 needs 297 × 210 mm and does not fit; A5 (210 × 148 mm) does, so the result is A5.

The calculator shows 25.4 × 16.9 cm (10 × 6.67 in), matching these steps.

How to read the result

300 DPI is the usual target for photo prints and magazines viewed at reading distance. Around 150–200 DPI is often acceptable for posters viewed from a metre or more, and large banners can go lower. If the calculated size is smaller than you need, lower the DPI only if the print will be viewed from further away; otherwise use a higher-resolution file. A4 at 300 DPI requires about 2,480 × 3,508 pixels.

Common mistakes

• Thinking that changing the DPI value in a file adds detail. Upscaling only interpolates existing pixels.
• Forgetting that cropping reduces pixel count and therefore print size.
• Ignoring aspect ratio: a 3 : 2 photo on A-size paper (about 1 : 1.41) needs cropping or borders.

Frequently asked questions

How big can I print 3,000 × 2,000 pixels at 300 DPI?

25.4 × 16.9 cm (10 × 6.67 in).

How many pixels do I need for A4 at 300 DPI?

About 2,480 × 3,508 pixels.

Is DPI the same as PPI?

Technically no, but for print size calculations they are used the same way: pixels per inch of paper.

Sources: ISO 216, Writing paper and certain classes of printed matter — Trimmed sizes — A and B series · NIST Special Publication 811, Appendix B (inch = 2.54 cm exactly)