DPI and PPI are numerically identical. Both count marks per inch, both convert to metric with the same 2.54, and a converter treats them the same way. The difference is not arithmetic — it is what the marks are, and which machine you are talking to.
Two different machines
| PPI — pixels per inch | DPI — dots per inch | |
|---|---|---|
| Counts | Image pixels, or display pixels | Ink or toner dots a printer lays down |
| Each mark can be | Any colour, full range | Usually one ink, on or off |
| Belongs to | A file, or a screen | A printer |
| Typical values | 72–110 (desktop), 300–500 (phone), 300 (print prep) | 600–4,800 |
The reason a printer needs thousands of dots per inch while an image needs only 300 is that a printer dot is nearly binary — a spot of cyan ink is either there or not. To reproduce one full-colour pixel, the printer lays down a cluster of dots in a halftone pattern. Several dots per pixel per ink, hence the order-of-magnitude difference in the numbers.
This is why "print at 300 DPI" is loose language. What is meant is 300 PPI in the file. The printer's own DPI is a property of the hardware and is not something your image controls.
PPI is metadata, not a property of the pixels
This is the part that trips people up most. An image file is a grid of pixels — say 3,000 × 2,000. That grid is the image. The PPI value stored in the file is just a note saying "print me at this density", and changing it does not add or remove a single pixel.
So the same 3,000 × 2,000 file prints as:
- 10 × 6.7 inches at 300 PPI
- 20 × 13.3 inches at 150 PPI
- 41.7 × 27.8 inches at 72 PPI
Same file, same pixels, three different physical sizes and three different print qualities. When image software offers "resample" alongside a PPI change, that checkbox is the difference between relabelling the file and actually inventing or discarding pixels.
Sizing an image for print
The arithmetic is: pixels = inches × PPI. Work backwards from the physical size you want.
| Print size | At 300 PPI | At 150 PPI | Megapixels needed (300 PPI) |
|---|---|---|---|
| 4 × 6 in | 1,200 × 1,800 | 600 × 900 | 2.2 |
| 8 × 10 in | 2,400 × 3,000 | 1,200 × 1,500 | 7.2 |
| A4 (8.27 × 11.7 in) | 2,480 × 3,508 | 1,240 × 1,754 | 8.7 |
| A3 (11.7 × 16.5 in) | 3,508 × 4,961 | 1,754 × 2,481 | 17.4 |
| Poster, 24 × 36 in | 7,200 × 10,800 | 3,600 × 5,400 | 77.8 |
The last row explains why large posters are not printed at 300 PPI. Viewing distance scales with size: nobody presses their nose to a billboard, so 100 PPI or less is entirely adequate at that scale. 300 PPI is a rule for things held in the hand.
UnitFlip's paper size converter does this calculation for every standard sheet at whatever DPI you choose.
Screens: where the CSS inch is not an inch
On a display, PPI describes physical pixel density: a 27-inch 4K monitor is about 163 PPI, a modern phone 400–500 PPI. But web and design software work in a reference pixel, not a physical one.
The CSS specification pins this down: 1 inch is defined as exactly 96 CSS pixels. So a CSS inch is a unit of layout, not of physical length — on a high-density screen it covers two or three physical pixels per CSS pixel, which is what a device pixel ratio of 2 or 3 means.
The practical consequence: an image that is 1,000 CSS pixels wide needs to
be 2,000 actual pixels to look sharp on a 2× display. That is the entire
reason for srcset and @2x assets. It has nothing to do with the
PPI tag inside the file, which browsers ignore completely.
The 72 DPI myth
"Web images should be 72 DPI" was true in the 1980s, when the original Macintosh display happened to be about 72 physical pixels per inch and one screen pixel matched one PostScript point. It has been meaningless for decades.
A browser renders an image at its pixel dimensions and ignores the PPI metadata entirely. A 1,200-pixel-wide image displays identically whether it is tagged 72, 96 or 300 PPI. What matters for the web is pixel dimensions and file size; nothing else.
Quick reference
- Preparing a print file? Think in PPI. Aim for 300 PPI at final size for handheld pieces, less for large-format.
- Buying or comparing a printer? That is DPI, and it is a hardware capability.
- Publishing on the web? Neither. Think in pixel dimensions, and supply 2× assets for high-density screens.
- Scanning? Scanner "DPI" is really samples per inch — scan at the PPI you need for the output size, not at the scanner's maximum.