What's the difference between DPI and PPI?

Mechanically the same — dots-per-inch is the print term, pixels-per-inch is the display term. A 300 DPI print and a 300 PPI screen pack the same number of marks per inch; only the marks differ (ink dots vs subpixel triads).

Which DPI should I export at?

For print: 300 DPI for photo books and marketing, 150 for draft handouts. For web: don't export at a fixed DPI — export at the pixel dimensions your design tool calls for. Modern browsers ignore the DPI metadata in image files for display.

Why does horizontal DPI sometimes differ from vertical?

It shouldn't, assuming the physical aspect ratio matches the pixel aspect ratio. If they differ, it usually means the wrong physical dimension was entered. Modern displays use square pixels so the two should always agree.

How do I find a display's physical width?

Manufacturers quote the diagonal in inches. To get width, multiply diagonal × cos(arctan(height_pixels / width_pixels)). For 16:9 displays the conversion is diagonal × 0.872; for 4:3 it's diagonal × 0.8.

Apple's marketing term for any display dense enough that a typical viewer can't resolve individual pixels at normal viewing distance. For phones the threshold is ~326 PPI; for laptops it's ~220 PPI; for monitors ~150 PPI. The number drops as viewing distance grows.

Does DPI affect file size?

No — DPI is a metadata tag on raster images. Two PNGs at the same pixel dimensions are the same file size regardless of their stored DPI value. DPI only affects how big the image prints by default.

DPI / PPI Calculator

Pixel dimensions in, dots-per-inch out — the screen-density answer in one number.

DPI (dots per inch, print) and PPI (pixels per inch, display) are the same calculation seen from two industries. The tool below takes pixel dimensions and a physical width and returns the horizontal, vertical, and diagonal DPI. The diagonal figure is what manufacturers quote for displays — a 6.1″ iPhone 15 Pro is marketed as 460 PPI, which is the diagonal value.

Pixels — wide

Pixels — tall

Physical width

Unit

Resolution

81.7DPI / PPI

Pixels per inch along the diagonal — the figure manufacturers quote for displays.

Horizontal: 81.7 DPI
Vertical: 81.7 DPI
Diagonal: 81.7 DPI

DPI (dots per inch, print) and PPI (pixels per inch, display) are numerically identical — the difference is whether the dot is ink or a screen pixel. Common thresholds: 72 DPI legacy web, 96 DPI Windows default, 150 DPI draft print, 300 DPI quality print, 326 DPI Apple “Retina”.

How to use

Enter pixel dimensions
Wide × tall in pixels. For displays this is the native resolution; for prints it's the export size from your design tool.
Enter the physical width
Width in inches, centimeters, or millimeters. The calculator assumes the physical aspect ratio matches the pixel aspect ratio.
Read all three DPI values
Horizontal and vertical agree when the aspect ratios match (they should). Diagonal is the marketing number.

Common DPI thresholds

DPI / PPI	Used for
72	Legacy web (Mac OS Classic default — still cited but functionally meaningless)
96	Windows default, CSS reference DPI
150	Draft print quality (inkjet flyers, internal handouts)
300	Print-shop standard for books, photos, marketing collateral
326	Apple “Retina” threshold for phones held at arm’s length
400+	High-end phone displays (Pro / Ultra tier)
600+	VR headsets (per-eye, viewed centimeters from the face)

Frequently asked questions

What's the difference between DPI and PPI?: Mechanically the same — dots-per-inch is the print term, pixels-per-inch is the display term. A 300 DPI print and a 300 PPI screen pack the same number of marks per inch; only the marks differ (ink dots vs subpixel triads).
Which DPI should I export at?: For print: 300 DPI for photo books and marketing, 150 for draft handouts. For web: don't export at a fixed DPI — export at the pixel dimensions your design tool calls for. Modern browsers ignore the DPI metadata in image files for display.
Why does horizontal DPI sometimes differ from vertical?: It shouldn't, assuming the physical aspect ratio matches the pixel aspect ratio. If they differ, it usually means the wrong physical dimension was entered. Modern displays use square pixels so the two should always agree.
How do I find a display's physical width?: Manufacturers quote the diagonal in inches. To get width, multiply diagonal × cos(arctan(height_pixels / width_pixels)). For 16:9 displays the conversion is diagonal × 0.872; for 4:3 it's diagonal × 0.8.
What is 'Retina'?: Apple's marketing term for any display dense enough that a typical viewer can't resolve individual pixels at normal viewing distance. For phones the threshold is ~326 PPI; for laptops it's ~220 PPI; for monitors ~150 PPI. The number drops as viewing distance grows.
Does DPI affect file size?: No — DPI is a metadata tag on raster images. Two PNGs at the same pixel dimensions are the same file size regardless of their stored DPI value. DPI only affects how big the image prints by default.

About

Print vs screen

For print, DPI determines how 'crisp' a rasterised image looks. A 4×6 photo printed at 300 DPI needs at least 1200×1800 pixels of source. Below that you'll see jagged edges; above 600 DPI the human eye can't tell the difference at normal viewing distance.

Why DPI doesn't matter for the web

Browsers don't read the DPI tag in image files. They read the pixel dimensions and lay them out per CSS rules. The reason 'export at 72 DPI for web' persists as advice is purely habit; you can export at any DPI and the rendered page is identical. What actually matters is the pixel dimensions.