DPI — dots per inch — is one of those technical terms that appears constantly on scan settings, print job dialogs, and form instructions, and is almost never explained properly. Most explanations either assume you already understand it or get so deep into color theory and printer mechanics that you lose the practical point. Here's the version that skips all that.
What DPI is measuring
DPI is a density measurement: how many individual dots (or pixels, in the case of screens) fit inside one inch. Higher DPI means more dots per inch, which means more detail per inch of the final output.
Think of it as the resolution of a specific size. A 300 DPI image printed at 4 inches wide is 1200 pixels wide (300 dots × 4 inches). The same 1200-pixel image printed at 8 inches wide is only 150 DPI — the same pixels spread over twice the physical space, so it's half as dense, and looks less sharp.
DPI for screens vs. DPI for printing
These are two genuinely different things that share the same acronym, which causes a lot of confusion.
Screen DPI (also called PPI — pixels per inch) describes how tightly packed a monitor's pixels are. A standard 1080p laptop screen might be 96 PPI. A modern Retina display might be 220+ PPI. This affects how sharp things look on the display, but it doesn't change the file.
Print DPI is what a scanner or printer uses. When you scan at 300 DPI, you're telling the scanner to capture 300 samples per inch of the physical document. When you print at 300 DPI, you're telling the printer to lay down 300 dots per inch of paper. This determines how sharp the physical output looks.
The numbers that actually matter
- 72 DPI: The old web standard — one pixel per screen dot on an older monitor. An image saved at "72 DPI" is fine for screens but will print at roughly letter-size at poor quality.
- 150 DPI: Minimum for acceptable printed photos. Fine for documents that will be read at arm's length.
- 300 DPI: The standard for quality print — business cards, brochures, photos you'd frame. Use this as the default when printing anything you care about.
- 600 DPI: Used for fine line art, engineering drawings, anything with very thin lines that needs to reproduce crisply. Most photos don't benefit from going this high.
When a form asks for a specific DPI
This is where people get confused. A form that says "scan at 300 DPI" isn't asking you to check or change a metadata tag — it's asking you to set your scanner to capture 300 samples per inch, which produces a physically larger pixel grid. A 300 DPI scan of an A4 page produces an image that's roughly 2480 × 3508 pixels.
If a form asks for "at least 200 DPI" and you have a photo from your phone camera, the phone doesn't report DPI in a meaningful way — just check the pixel count. A photo that's 3000 pixels wide will print at 300 DPI at 10 inches, or 200 DPI at 15 inches. That's what matters, not whatever DPI number the metadata says.
Why the DPI number in the file metadata doesn't always matter
JPEG and PNG files can store a DPI tag in their metadata. This tag tells a printer how large to print the image by default — it's a suggestion, not a limit. The same 3000-pixel-wide image might have its DPI tag set to 72 (making the "default" print size very large) or 300 (making it 10 inches wide by default), but the actual pixel data is identical. What matters for quality is the pixel count, not the metadata tag.
So if a form says your image needs to be 300 DPI: either scan at 300 DPI to create a high-resolution image, or make sure your existing image has enough pixels to cover the required print size at that density. A metadata-only DPI change (which some tools offer) doesn't add pixels — it just changes the label.