DPI, PPI and resolution — what really matters

The one idea that clears up the confusion

Almost all DPI/PPI confusion comes from a single misunderstanding: pixels are a count, DPI is a density. An image has a fixed number of pixels (say 3000 × 2000). How densely you spread those pixels when printing is what DPI describes. The same image can be printed small and razor-sharp (high DPI) or huge and coarse (low DPI) — the pixels stay the same.

PPI: pixels per inch

PPI (pixels per inch) describes how many pixels fit into one inch — on a screen or in a digital image. It's a property of the digital image and its display. A phone screen might pack 400+ PPI; that's why modern displays look so crisp.

DPI: dots per inch

DPI (dots per inch) is the print equivalent: how many ink dots a printer places per inch of paper. In everyday use DPI and PPI are used almost interchangeably, and for practical purposes that's fine — both describe density. The key point stays the same: density only becomes meaningful once you fix a physical size.

Why "300 DPI" only matters for print

You'll hear "images need 300 DPI" everywhere. That's true — for print. On screen, DPI is irrelevant: a screen shows pixels, and only the pixel count and the display's own density matter. A web image "at 72 DPI" and the same image "at 300 DPI" look identical on screen, because the DPI tag is just metadata the screen ignores. DPI only comes alive on paper.

The formula you actually need

To find the pixels required for a print, there's one simple formula:

centimeters ÷ 2.54 × DPI = pixels

(Or: inches × DPI = pixels.) Examples at 300 DPI:

• 10 × 15 cm photo → about 1181 × 1772 pixels
• A4 (21 × 29.7 cm) → about 2480 × 3508 pixels
• A business card (85 × 55 mm) → about 1004 × 650 pixels

When less than 300 DPI is fine

Viewing distance changes everything. A business card is read from 30 cm — it needs the full 300 DPI. A large poster is seen from several meters — there, fine detail blurs anyway, so 100–150 DPI is plenty. That's why big posters can be printed at far lower DPI than a photo print. The larger the format and the greater the distance, the fewer DPI you need.

Megapixels vs DPI

One more distinction: megapixels count the total pixel amount of an image; DPI describes how densely you spread them when printing. A 12-megapixel photo has enough pixels for almost any everyday print — the megapixels don't change, only the print size changes the resulting DPI.

Conclusion

Pixels are the count, DPI is the density, and density only matters once you fix a physical size. For the web, ignore DPI and think in pixels. For print, use the formula — and remember that bigger, farther-viewed prints forgive lower DPI.