Pixels, print size or resolution: choose a calculation mode
Every calculation connects three values: pixels, physical dimensions and resolution. If you know two of them, the third follows. At the top, choose the direction that matches the values you already know:
- Pixels from print dimensions and PPI: You know how large the print should be, for example DIN A4, and at what resolution. The calculator gives you the required width and height in pixels.
- Print size from pixels and PPI: You know the pixel dimensions of your image or scan and a resolution. The calculator gives you the width and height in centimeters, millimeters and inches.
- Resolution from pixels and print dimensions: Both the image and the target format are fixed. The calculator shows the PPI for width and height separately and gives the lower value as the effective resolution.
For the dimensions, you can choose DIN A4, A5, A3 or US Letter, or enter your own values, and switch between portrait and landscape. Enter decimals with a period, for example 29.7. A decimal comma such as 29,7 is not accepted.
Most people search for a DPI calculator. Strictly speaking, though, the calculation here is about PPI, the number of image pixels per inch (Pixels per Inch). In everyday use, the two terms are often treated as the same. Further down, you'll learn why the distinction helps with printing. The calculator works only with the numbers you enter. It does not open any image file, does not read a stored DPI value, and does not write a new one into the file. You can find the pixel dimensions of your image in the file properties or in your image editor, for example.
Comparing print dimensions and pixels step by step
A pixel has no fixed size in centimeters. Only the resolution determines how many pixels fit into one inch, and an inch measures exactly 2.54 cm. Each axis is calculated on its own, so width and height are handled separately.
Example: the same image at 150 and 300 PPI
Suppose a photo has 3,000 × 2,000 pixels. At 300 PPI, the width is 3,000 ÷ 300 × 2.54 = 25.4 cm and the height is 2,000 ÷ 300 × 2.54 = 16.93 cm (exactly 16.9333…). At 150 PPI, the same pixels are spread over twice the distance, giving 50.8 × 33.87 cm.
If you halve the PPI, each side doubles in length and the area becomes four times as large. The pixel count does not change. No detail is added; each pixel is simply printed larger. In the other direction, a higher PPI value makes the print smaller, but it does not make the image itself any sharper.
Pixels for DIN A4 and other formats
In the opposite direction, you convert centimeters to pixels. The table shows the values at 300 PPI in portrait orientation, rounded to whole pixels the same way the calculator does. Here, 300 PPI is only a calculation example, not a required value. In landscape orientation, width and height simply swap places.
The formats use the bare trimmed size of the sheet. Bleed, margins and safety zones that a print shop may require are not included. If you need them, enter the dimensions your print shop gives you as custom values.
The calculation also helps with scanning. If you scan a full A4 page at 300 dpi, the resulting image is about 2,480 × 3,508 pixels. At 150 dpi, it would be about 1,240 × 1,754 pixels. That is half as many pixels per axis and only a quarter of the total pixels.
PPI for images, DPI for printer dots
PPI describes your image: how many image pixels fall on one inch of printed length. DPI (Dots per Inch) originally describes a device, for example how many ink dots a printer can place per inch. A printer can build a single image pixel from several printed dots. So a high DPI figure on a printer's data sheet says nothing about how many pixels your image has, and the printer does not add any image information.
On scanners, the setting is usually labeled dpi. In that context, it means how many image pixels are captured per inch of the original. That is exactly what this calculator treats as PPI.
300 PPI is often mentioned as a guideline for prints, and this page uses it as an example. However, there is no universal threshold above which every print becomes sharp. What counts is what the print shop, the photo service or your job requires. Enter that requirement as the target PPI.
PDFs need a closer look. A PDF file can contain text and graphics as vectors, which scale without a fixed pixel count. It can also contain embedded raster images such as photos or scans. The same calculation as above applies to these images. The PDF format alone therefore does not automatically make a print sharp or resolution-independent.
Is my image good enough for a poster?
Choose the Resolution mode and enter the pixel dimensions of your original image and the poster size. Here is a hypothetical example: an image with 4,000 × 6,000 pixels on a poster measuring 61 × 91.5 cm gives about 166.56 PPI on both axes (4,000 ÷ 61 × 2.54). Both values match because the image and the poster have the same aspect ratio of 2:3.
The number alone cannot tell you whether this value is enough for the poster. Other factors also matter: how sharp the source image is, how far away people will view the poster, the material and the printing process. So ask your print service provider what they require and compare it with your result. If your value is well below their requirement, entering a higher PPI value afterward won't help, because the image does not have the real pixels needed.
How to read rounding and aspect ratio
Pixels are whole units, but the formulas usually produce decimal numbers. The calculator rounds to the nearest whole number, up or down, depending on which is closer. DIN A4 at 300 PPI shows what this means:
For most planning, one pixel makes hardly any difference. But if a requirement explicitly calls for at least 300 PPI, 2,480 × 3,508 pixels fall just short in width. In that case, round the unrounded value up yourself and plan with 2,481 × 3,508 pixels. The calculator shows the rounded number, not this strict minimum. If in doubt, check the exact wording of the production requirement.
Why width and height can show different PPI
In Resolution mode, the calculator works out the PPI for each axis separately. If the image and the target format have different aspect ratios, you get two different values. For example, suppose the photo with 3,000 × 2,000 pixels, which has a 3:2 ratio, is to be printed in a square format of 30 × 30 cm.
- Width: 3,000 ÷ 30 × 2.54 = 254 PPI
- Height: 2,000 ÷ 30 × 2.54 = 169.33 PPI
The calculator gives the lower value, here 169.33 PPI, as the effective resolution, because the axis with the fewest pixels per inch limits the result. If the two values differ by more than 1 %, a note also appears telling you to check the aspect ratio. If you simply stretched the image to 30 × 30 cm, it would be distorted. The calculator does not crop or stretch anything, and it does not adjust the format on its own.
You have two clean options. One is to choose a target size with the same ratio: at 30 × 20 cm, both axes come to 254 PPI. The other is to crop the image to 1:1 in an image editor first and then calculate with the new pixel dimensions, for example 2,000 × 2,000 pixels. At 30 × 30 cm, both axes then come to 169.33 PPI.
From calculated value to the right PDF tool
The calculator gives you numbers for planning, but it does not change your files. Even if you change only the stored DPI or PPI value in an image file, the pixel count stays the same. Only the intended print size changes. To get more or fewer pixels, you have to recalculate the image (resampling) in an image editor. That is a separate step and has its own effects on image quality.
In the same way, a PPI value cannot tell you the exact size of a PDF. More pixels may mean more image data. But the final size of the file in bytes also depends on the image content, the compression and the rest of the content on the pages.
Depending on what you want to do with your result, here is where to go next:
- Combine photos or scans into one document: Save images as PDF
- Export the pages of a PDF as image files and choose the resolution: Export PDF pages as JPG or PNG
- Actually make a finished file smaller, for example for an email or an upload form: Reduce PDF file size