How many pixels does an image need to print sharp as a full A4 page?
For a full A4 page at the usual close up print sharpness of 300 DPI you need about 2480 × 3508 px; for a document read at arm's length 150 DPI, about 1240 × 1754 px, is usually enough. The tool will stretch any image across the page, but an 800 × 600 image on a full A4 page works out to only about 73 pixels per inch and looks soft.
Where those numbers come from
A4 is 210 × 297 mm, roughly 8.27 × 11.69 inches. Printed sharpness is measured in pixels per inch. At 300 pixels per inch, a common figure for print viewed up close, 210 mm needs about 2480 px and 297 mm about 3508 px. At 150 you need half in each direction. These are not limits in the tool, just simple arithmetic so you know what to expect before you print.
What happens to a small image on the page
Take an 800 × 600 px image on A4 landscape with no margin and Fit inside. The layout function in Images to PDF draws it 793.71 pt wide, which is 11.02 inches. Divide 800 px by 11.02 inches and you get about 72.6 pixels per inch. That is a quarter of the usual density for close up print, so edges look soft and small text looks fuzzy on paper, even though it seemed fine on your phone screen.
The tool neither stops you nor warns you, because it cannot know whether you will print the file or view it on screen. On screen it is usually fine, since the viewer shows the page at screen size, not paper size.
Quick guide by use
- A full page to be printed and viewed up close, such as a product shot in a printed catalog: about 2480 × 3508 px or more.
- A photographed paper document to be read in print, such as a contract or invoice: about 1240 × 1754 px is usually enough. Any recent phone camera exceeds that easily.
- A file for on screen viewing only: large dimensions add almost nothing and make the file heavier. Shrink them with the Image resizer.
- An image to print small, such as a label: work from its real printed size, not A4, and use Original size with the DPI you need.
Does enlarging the image first help?
No. Upscaling an 800 × 600 image to 2400 × 1800 gives you a bigger number, but the new pixels are guessed from the old ones and add no detail. The print will look about as soft, from a heavier file. The real fix is a bigger source: the original camera file instead of the copy from a chat, the supplier's original instead of the website image, or simply photographing the document again.
How to check your image's dimensions
When you add images to the converter, each row shows its pixel dimensions and file size. Before printing, scan the list: any image well under 1240 px on its short side that will fill a whole A4 page will print soft. Replace it before you spend ink and paper on it.
Common questions
Does the margin change the calculation?
Slightly. A margin shrinks the area the image is drawn in, so the pixel density there rises a little.
Why does an image look sharp on screen but soft on paper?
A screen shows the page at the screen's own size, while paper forces 8.27 inches of width however small the image is.
Does the tool reduce the dimensions of large images?
No, every pixel stays in the file. That keeps sharpness but makes the file large.