What PPI measures
PPI is a density figure for pixels. Divide an image's pixel width by the inches it covers and you have its PPI, which tells you how tightly the pixel data is packed into physical space.
A phone screen packs far more pixels into each inch than a desktop monitor, so the same image file looks smaller on the phone and sharper to the eye because the pixels are tinier.
PPI compared with DPI
The two are cousins. PPI counts pixels that a screen shows or a file holds, while DPI counts dots a printer places on paper. People use them interchangeably in conversation, but the domains differ.
Confusing them leads to odd results. Changing DPI does not add or remove pixels, and a screen setting for PPI does not affect what a printer does with the same file.
- PPI describes pixel density in a file or on a display
- DPI describes physical dots laid down by a printer
- Both are used to convert between pixels and inches
- Neither value is stored inside the raw pixel grid itself
Why PPI matters for print layouts
Document layouts use PPI to decide how large a picture appears on a page. Dropping a 600-pixel-wide image into a layout at 300 PPI places it two inches wide, and increasing the PPI shrinks it.
On the web, PPI is mostly informational. What matters is pixel dimensions, which drive both rendering and file size, so reducing them is the edit that changes what users download.