DPI, resolution and pixels: the complete guide for your PDFs and images
The terms DPI, resolution and pixels come up constantly whenever you scan, print or convert a document, yet they are often confused. This guide explains clearly what they mean, how to calculate them, and which value to choose for each use case.
DPI and PPI: what are we really talking about?
DPI stands for "dots per inch". It is a measure of density: it tells you how many dots are printed or displayed across one inch (2.54 cm). The higher the DPI, the closer the dots and the finer and more detailed the image looks.
PPI, "pixels per inch", is the equivalent notion for screens and digital files: it counts pixels rather than ink dots. In everyday language DPI and PPI are often used interchangeably. The key point to remember: DPI only means something when tied to a physical size. An image "at 300 DPI" means nothing on its own; "300 DPI for a print of a given size" is what matters.
A pixel is a single unit of color in a digital image. A photo 2480 pixels wide contains 2480 points along a line, regardless of the size at which you display it. DPI is precisely the bridge between this pixel count and the real dimension in centimetres or inches.
The formula: pixels = inches × DPI
The relationship between pixels, physical size and density comes down to one simple formula: pixels = inches × DPI. If you work in millimetres, first convert to inches by dividing by 25.4: pixels = (mm ÷ 25.4) × DPI. One inch equals exactly 25.4 mm.
Take a concrete example with an A4 sheet, which measures 210 × 297 mm. At 300 DPI, the width gives (210 ÷ 25.4) × 300 ≈ 2480 pixels, and the height (297 ÷ 25.4) × 300 ≈ 3508 pixels. So an A4 scanned or exported at 300 DPI is about 2480 × 3508 pixels.
The same A4 at 72 DPI is only about 595 × 842 pixels, which matches the classic "points" of the PDF format. Conversely, at 150 DPI it reaches roughly 1240 × 1754 pixels, and at 600 DPI about 4961 × 7016 pixels. Doubling the DPI doubles the pixels on each side, and therefore quadruples the total pixel count.
Which DPI should you choose?
For on-screen or web display, 72 to 96 DPI is more than enough: beyond that the screen shows no extra detail and the file grows needlessly. This is the ideal resolution for sharing a document by e-mail or publishing it online.
For everyday office documents that may occasionally be printed, 150 DPI offers a good balance between readability and file size. For quality printing (photos, brochures, official documents), aim for 300 DPI, the standard reference in the print industry.
For fine scans, archiving or optical character recognition (OCR) on small text, 600 DPI adds extra precision. Beyond that, the gain becomes marginal for most text documents. When you build a PDF from photos with the image-to-pdf tool, or transform a file with convert, keep these benchmarks in mind so you do not oversize your files.
Why high DPI makes files heavier
An image's size depends directly on its pixel count. Since doubling the DPI quadruples the total number of pixels, a scan at 600 DPI can weigh about four times more than at 300 DPI for identical content. Too high a DPI produces bulky PDFs that are slow to send and open, with no visible benefit.
This is where compression comes in. Shrinking a file can involve lowering the effective DPI of its images (resampling) and using more efficient encoding, such as JPEG for photos. The compress tool applies this kind of optimization to lighten a PDF while keeping a quality suited to its intended use.
The right approach is to choose the DPI based on the final destination, then compress if needed. Scanning at 600 DPI and then compressing heavily often wastes time and space: it is better to scan directly at the useful resolution.
Scanning well for OCR
OCR, optical character recognition, turns the image of text into selectable, searchable text. Its accuracy depends heavily on scan quality. For a standard printed document, scan at 300 DPI; for very small characters or fine handwriting, go up to 400 or 600 DPI.
Favour a well-contrasted, sharp scan, free of shadows and folds, ideally in greyscale or black and white for pure text. A slightly skewed document can lower the recognition rate: straighten the page before processing.
Once your pages are scanned, you can combine them into a single PDF with image-to-pdf, convert varied formats with convert, then reduce the whole thing with compress if the final file is too heavy. By choosing the right DPI from the start, you get documents that are readable, OCR-friendly and reasonable in size.
Frequently asked questions
How many pixels is an A4 at 300 DPI?
An A4 measures 210 × 297 mm. At 300 DPI that gives about 2480 × 3508 pixels, calculated with the formula (mm ÷ 25.4) × DPI. At 72 DPI the same A4 is about 595 × 842 pixels.
What DPI for printing?
For quality printing, 300 DPI is the standard reference. For everyday office documents, 150 DPI is often enough. Web and screen only need 72 to 96 DPI.
What DPI for OCR?
300 DPI suits most printed documents. For small characters or finer recognition, go up to 400 or 600 DPI, with a sharp, well-contrasted scan.
What is the difference between DPI and resolution?
Resolution refers to the total number of pixels in an image (for example 2480 × 3508). DPI is a density: the number of dots per inch, which links that pixel count to a given physical size.