The two units get used interchangeably so often that even professional software gives up and mislabels them. But they measure different physical things, and the difference isn’t pedantic — it’s the reason a “4800 dpi” printer needs only a 300-ppi file, and why “the image is 72 dpi” is nearly a meaningless sentence.

PPI: pixels per inch — a property of images and displays

Pixels per inch is a sampling density: how many discrete image samples fit in one linear inch.

• For a display, PPI is fixed hardware: √(w² + h²) ÷ diagonal. A 27-inch panel at 3840×2160 is 163.2 ppi — every pixel is physically there, at a fixed pitch of 25.4 ÷ 163.2 ≈ 0.156 mm. (The calculator does this math exactly.)
• For an image file, PPI is intent: “place these pixels this densely on paper.” A 3000×2000 file tagged 300 ppi is asking to be printed 10 × 6.67 inches. The same file tagged 150 ppi would print twice as large — same pixels, different instruction.

Notice what that means: a digital file doesn’t intrinsically have a PPI at all until someone decides how big it will print. “What’s the DPI of this image?” is only answerable with another question — “printed at what size?” The megapixels tab shows why: the same 24 MP file is a 20-inch print at 300 ppi or a 40-inch print at 150 ppi.

DPI: dots per inch — a property of printers

Dots per inch counts the individual marks a printing device can address in one inch — ink droplets for inkjets, toner dots for laser printers, halftone cells for offset presses.

The crucial asymmetry: one image pixel is never one ink dot. An inkjet reproduces each pixel’s color by spraying a cluster of cyan/magenta/yellow/black droplets and letting your eye blend them (dithering). So:

• A 1440×720 dpi inkjet has ~1 million possible dot positions per square inch (1440 × 720 = 1,036,800) — but uses them to simulate maybe 300 distinct pixel values.
• An offset press at 2400 dpi uses those dots to build halftone cells of ~150 lines per inch (lpi); the rule of thumb “2 pixels per line” is where the magic 300 ppi number comes from.
• Printer DPI specs are also partly marketing: “4800×1200 optimized dpi” describes droplet placement addressability, not resolving power.

Where the mess came from

The confusion is historical. The 1984 Macintosh screen was almost exactly 72 pixels per inch, and early print dialogs used “dpi” loosely for both screen and paper. Windows settled on 96 “DPI” as its logical display density — still printer vocabulary, applied to a device with no dots. Photoshop’s “Image Size” dialog calls the field “Resolution” (in pixels/inch), but everyone read it as “DPI” for decades — the habit stuck. Manufacturers happily let the bigger, impressive-sounding “dpi” number ride along.

So the vocabulary sorted itself into this truce: screens and image files get PPI; printers and scanners get DPI — and everyone agrees to wink at scanners, which also measure pixels but keep the DPI label by convention.

When the distinction actually matters

• Buying a display: compare PPI, not “resolution.” A 15.6″ 4K laptop (283 ppi) and a 27″ 4K monitor (163 ppi) have the same pixel count but very different sharpness.
• Sending files to print: the lab wants pixels sufficient for the size at ~240–300 ppi; obsessing over the file’s “dpi” metadata field misses the point entirely.
• Reading a printer spec: 1200+ dpi is table stakes for photo inkjets; image resolution of 300 ppi is the real input requirement.
• Scanning: “600 dpi” means 600 samples per inch of original — for a 4×6 photo that’s 2400×3600 px of real image data.

The short version: pixels are the thing you have; dots are the thing the printer makes. If the conversation is about an image file or a screen, say ppi and you’ll almost always be right.

Frequently asked questions

So which is correct for a monitor — PPI or DPI?

PPI. A display has a fixed grid of light-emitting pixels, so its density is literally pixels per inch. There are no 'dots' involved. 'DPI' stuck around for screens only because early operating systems (and Windows' display settings) borrowed printer vocabulary.

Why does Photoshop let me set 'DPI' on an image then?

It's a legacy label for what is really PPI metadata — a hint telling layout software how large the pixels should be placed on paper. Changing the number alone doesn't add or remove a single pixel; it only changes the suggested print size. Resampling is what actually changes pixel count.

If my printer is 4800 dpi, do I need 4800-ppi images?

No — that would be absurd (a 4×6 print would need 19,200×28,800 px). The printer's 4800 refers to ink-dot addressability; each image pixel is reproduced by a whole neighborhood of dots through halftoning/dithering. 240–360 ppi of image data is what those printers actually resolve in practice.

What about scanners — is their 'DPI' really DPI?

Same misnomer. A scanner samples pixels, so its number is sampling density — PPI in honest terms. But 'scan at 600 dpi' is universal industry language, so we keep the quoted phrase and just remember what's being measured.

Does a higher-PPI image always print better?

Only up to the resolving limit of the printing process and the viewing distance. Past ~300–360 ppi at reading distance, extra pixels add file size without visible gain — the press can't resolve them and neither can your eye. More in the print-resolution guide.