Compute pixels per inch, dot pitch, and total megapixels for any display from resolution plus diagonal size.
PPI Calculator is a free, browser-based pixels-per-inch calculator.
PPI stands for "pixels per inch" — how many physical display pixels fit into one inch of screen. It's computed from the resolution (horizontal × vertical pixels) and the diagonal size of the screen in inches. Two monitors with the same resolution but different physical sizes have different PPI: a 1080p 24-inch monitor sits at 92 PPI, while a 1080p 15-inch laptop comes in at 147 PPI. Higher PPI = sharper-looking text, smoother edges, and crisper images.
The formula is simple trigonometry. If the screen is w pixels wide and h pixels tall, the diagonal pixel count is √(w² + h²). Divide that by the diagonal size in inches to get PPI:
PPI = √(w² + h²) / diagonal_in_inches
For a 4K display (3840 × 2160) at 27 inches, the diagonal in pixels is about 4405, and 4405 / 27 ≈ 163 PPI. This tool runs the math live as you type and also outputs dot pitch (the millimeter distance between pixel centers) and total megapixels.
Apple popularized the term "Retina" for displays whose pixels are too small for the human eye to distinguish individually at normal viewing distance. For laptops and desktops that threshold is roughly 220 PPI, for phones about 300 PPI (because phones are held closer). Any modern MacBook Pro, iPhone, or iPad crosses that bar. Most Windows PCs don't — so you see pixel structure if you look for it — but 4K 27" is getting close.
PPI refers to display pixels, DPI refers to dots printed on paper. They're measured the same way but describe different outputs. When you read "300 DPI" on a print file, that's telling the printer to lay down 300 ink dots per inch. On-screen, 300 PPI means the display shows 300 distinct pixels per inch. Most "PPI calculator" searches are actually about displays, which is what this tool handles.
PPI Calculator FAQ
