What DPI means — and when 300 DPI actually matters
DPI explained simply: why “send it at 300 DPI” is half an instruction, how many pixels a print needs, and what to do when you don't have enough.
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Tools for this job: Change Image DPI · Resize Image · Crop Image
“Please supply images at 300 DPI.” Print shops, publishers and application forms ask for it constantly, and it confuses almost everyone — because on its own, it does not say enough to act on.
What DPI actually is
DPI stands for dots per inch. For a digital image the more precise term is PPI, pixels per inch, but the two are used interchangeably. It describes how densely an image's pixels are laid down when it is printed: at 300 DPI, every inch of paper carries 300 pixels across and 300 down.
The important part is that DPI is not a property of the picture's quality. It is a number stored in the file that says how big to print it. The same 1,800-pixel-wide photo prints 6 inches wide at 300 DPI and 24 inches wide at 75 DPI — with exactly the same pixels.
The only formula you need
pixels = inches × DPI
Or the other way round: inches = pixels ÷ DPI. At 300 DPI, common prints need:
| Print size | Pixels needed |
|---|---|
| 6 × 4 in (15 × 10 cm) | 1800 × 1200 |
| 7 × 5 in | 2100 × 1500 |
| 10 × 8 in | 3000 × 2400 |
| A4 (210 × 297 mm) | 2480 × 3508 |
| A3 (297 × 420 mm) | 3508 × 4961 |
A typical 12-megapixel phone photo is 4,000 × 3,000 pixels, enough for about 13.3 × 10 inches at 300 DPI — comfortably more than most people ever print.
Why “300 DPI” on its own is half an instruction
A printer who asks for 300 DPI really means: “send enough pixels to print at this size at 300 DPI”. Changing the DPI number in a file without changing its pixels only changes the printed size it suggests. A small web image relabelled as 300 DPI simply becomes a tiny print. So when you are asked for 300 DPI, also ask — or check — the physical size it will be printed at, then work out the pixels.
Does 300 matter?
300 DPI is the standard for things held in the hand and looked at closely: photo prints, brochures, books, passport photos. Below about 200, fine detail and small text begin to look soft at reading distance.
Things seen from further away need far less. A poster read from across a room looks fine at 150 DPI, and large banners and billboards are printed at much lower densities still, because nobody stands close enough to see individual pixels. That same 12-megapixel photo makes a perfectly good 26.7 × 20 inch poster at 150 DPI.
Screens ignore DPI completely
On a website, a phone or in a presentation, an image is shown by its pixels. The DPI number stored in the file has no effect at all. A 1,200-pixel-wide image looks the same whether it says 72 DPI or 300 DPI. The old idea that screen images must be “72 DPI” is a leftover from early Macs and has no practical meaning today.
Setting the DPI for a print
Change Image DPI offers the two honest options:
- Keep pixels — change only the DPI number, leaving the picture exactly as it is. Use this when the image already has enough pixels for the print size and the printer simply wants the file to say 300.
- Resize pixels — enter the physical width you need, and the image is recalculated so it measures exactly that at the chosen DPI.
When you do not have enough pixels
Enlarging an image cannot invent detail that was never captured. Stretching a 600-pixel picture to the 2,400 pixels a print needs makes it softer, whatever the DPI label says. Your options, best first:
- Find a larger original — the camera file rather than a copy from social media or a messaging app.
- Print smaller, at the size the pixels you have can support.
- Accept a lower DPI if the print will be seen from a distance.
- Enlarge modestly; up to about 150% usually still looks acceptable.
Cropping costs pixels
Every crop removes pixels, so a tightly cropped detail from a photo may no longer have enough for a large print. Crop with Crop Image first, look at the remaining pixel dimensions, and work out the print size they support before ordering. To reduce an over-large image for the web instead, use Resize Image, which works in pixels.
How to check an image's DPI and size
On Windows, right-click the file, choose Properties and open the Details tab: it lists the dimensions in pixels and the horizontal and vertical resolution. On a Mac, open the image in Preview and choose Show Inspector from the Tools menu. Remember that the pixel dimensions are the figure that really matters.