The pixels-to-inches formula
Inches = pixels ÷ PPI. For example, an image that is 1200 × 1800 pixels is 4 × 6 inches at 300 PPI. At 150 PPI, those same pixels cover 8 × 12 inches. Changing the intended print size does not create new image detail.
Inches to pixels
Pixels = inches × PPI. A 6 × 4 inch image at 300 PPI requires 1800 × 1200 pixels. Image dimensions must eventually be whole numbers, so round the calculated output when entering it into a resizing application.
Choosing a resolution for a PDF
Use the output requirements from your printer or publisher. A 300 PPI example is useful for planning, but it is not a universal requirement. Large prints viewed from a distance may use different resolutions. PPI describes image pixels per inch; printer DPI describes printer dots and should not be treated as the same measurement.
Screen inches are different
Browser CSS uses reference pixels, and a device’s physical pixel density varies. This calculator predicts image dimensions at the PPI you enter. It does not measure your monitor. See MDN’s CSS length reference for screen and print unit behavior.
Make a PDF from your image
After choosing the intended size, use JPG to PDF or Image to PDFJPEG to PNGJPEG to JPGPixels to inches to arrange the image on a page. Read pixels, inches and PDF print size for examples and common mistakes.