"Send me the logo at 300 dpi." "Photos for the website should be saved at 72 dpi." Anyone who works with images has heard these phrases many times, and they often cause confusion. Resolution is one of the most misunderstood concepts in digital graphics, because the same word means different things depending on the context. Let's sort it out.
Pixels, PPI and DPI: three different things
- Pixel dimensions: the only measure that truly describes how much information an image contains. A 6000 × 4000 pixel photo has 24 million samples, full stop.
- PPI (pixels per inch): an indication, stored in the metadata, of how large to print those pixels. It does not change the image's content.
- DPI (dots per inch): strictly speaking it concerns the printer, that is, how many ink dots it can lay down per inch. An inkjet printer uses many tiny dots to reproduce the color of a single pixel.
In everyday language "DPI" is used instead of "PPI," and programs themselves often mix them up. When someone asks for "300 dpi," they almost always mean "enough pixels to print at 300 pixels per inch at the intended size."
The fundamental calculation
One inch is 2.54 cm. The relationship between pixels, resolution and print size is:
print size (inches) = pixels / PPI
pixels needed = print size (inches) × PPI
Practical examples:
| Print format | At 300 PPI | At 150 PPI |
|---|---|---|
| 10 × 15 cm | about 1181 × 1772 px | about 591 × 886 px |
| A4 (21 × 29.7 cm) | about 2480 × 3508 px | about 1240 × 1754 px |
| A3 (29.7 × 42 cm) | about 3508 × 4961 px | about 1754 × 2480 px |
| 50 × 70 cm | about 5906 × 8268 px | about 2953 × 4134 px |
A 6000 × 4000 pixel photo can therefore be printed at 300 PPI up to about 50 × 34 cm, and at 150 PPI up to twice that in each direction.
The 72 dpi web myth
For images meant for screens, the PPI value written in the metadata is simply ignored. A browser shows a 1200-pixel-wide image at 1200 CSS pixels (or their equivalent depending on screen density), whether the file declares 72, 96 or 300 PPI.
Two files identical in pixels but with different PPI are displayed identically on the web and weigh almost the same. What matters for the web is pixel dimensions. The 72 dpi convention goes back to the early monitors of some computers, but today it has no practical effect.
Changing PPI without resampling
In retouching programs the "Image Size" dialog offers an option, often called resample. What it means is crucial:
- Resampling off: changing the PPI changes the print size, but the pixels stay the same. No loss of quality. Taking an image from 72 to 300 PPI this way only reduces the intended print size.
- Resampling on: the program creates or removes pixels. Reducing is fine; enlarging invents pixels by interpolation and adds no real detail.
A frequent case in my work: a client sends a logo "at 300 dpi" that actually measures 200 × 80 pixels, obtained by raising the PPI with resampling on from a tiny image. The metadata value is correct, but the image is still pixelated. The right question to ask is always "how many pixels?"
Viewing distance changes everything
300 PPI is a good reference for prints viewed up close, such as album photos, magazines and flyers. But the resolution needed depends on the distance from which the image will be viewed:
- prints held in the hand: 240–300 PPI;
- wall posters, viewed from a metre or more: 150 PPI is often enough;
- billboards and banners seen from several metres: much lower values suffice, sometimes a few dozen PPI.
A six-metre billboard at 300 PPI would require an enormous image, and nobody will ever look at it from 30 centimetres. Before requesting or supplying a file, it is worth clarifying the final size and viewing distance with the printer.
Resolution and scanners
For scanners, resolution does have a physical meaning: how many samples are read per inch of the original. Scanning a 10 × 15 photo at 600 PPI produces about 2362 × 3543 pixels, enough to reprint it at twice the size at 300 PPI. For text documents to archive, 300 PPI is usually adequate; for slides and negatives, which are very small, much higher resolutions are needed.
In short
- Pixel dimensions are the true measure of an image's information.
- PPI only says how large to print those pixels; DPI concerns the printer's dots.
- Pixels needed = print size in inches × desired PPI.
- On the web the PPI value in the metadata is ignored: only pixels count.
- The right resolution depends on viewing distance: 300 PPI up close, much less for posters and billboards.
