DPI vs PPI: what actually matters when you print
DPI is a printing term that spent thirty years being used wrongly about screens. Here is the version that works.
Very few settings cause as much confusion as DPI. People change it hoping an image will look sharper on a website, printers reject files over it, and phone apps expose it as a mysterious number with no explanation of what it does.
The confusion has one root: a file contains pixels, and nothing else. DPI is a note attached to those pixels saying how big to make them on paper. Once that clicks, every DPI question answers itself.
What you need to know first
Before the step-by-step, these are the points that decide the outcome. If you are in a hurry, the first one covers most cases.
Only the pixel count is real
A 1200 × 1800 image is exactly the same file whether its DPI field reads 72 or 300. Changing that number without resampling changes nothing but the printing instruction.
DPI only matters on paper
It tells the printer how many of your pixels to squeeze into each inch. High DPI means small, dense, sharp. Low DPI means the same pixels spread larger and coarser.
Screens ignore DPI entirely
A browser renders pixels at the screen density, not at whatever the file claims. Setting a web image to 300 DPI achieves precisely nothing except confusing whoever opens it next.
The formula is inches x DPI
Physical size in inches multiplied by the target DPI gives the pixels you need. Work it out before you shoot or buy an image, not after the printer rejects the file.
The two terms, properly defined
PPI (pixels per inch) describes how many pixels of your image occupy one inch of a display or a print. It is a property of how the image is presented, not of the file.
DPI (dots per inch) describes how many ink dots a printer lays down per inch of paper. Printers build each of your pixels from several tiny dots of ink, which is why a 300 PPI image is often printed by a device rated at 1440 DPI.
In everyday use the two words are swapped constantly, including by printers themselves. Treat them as the same thing when someone asks you for “300 DPI”. What they want is enough pixels to print at 300 per inch.
The only calculation you need
Pixels = printed size in inches × target DPI.
That is it. Everything else follows from it. Since most of the world specifies print in millimetres, divide by 25.4 to get inches first.
| Printed item | Size | At 300 DPI | At 150 DPI |
|---|---|---|---|
| Business card | 3.5 × 2 in | 1050 × 600 px | 525 × 300 px |
| Photo print | 4 × 6 in | 1200 × 1800 px | 600 × 900 px |
| A5 flyer | 5.8 × 8.3 in | 1748 × 2480 px | 874 × 1240 px |
| A4 page | 8.3 × 11.7 in | 2480 × 3508 px | 1240 × 1754 px |
| A3 poster | 11.7 × 16.5 in | 3508 × 4961 px | 1754 × 2481 px |
| Roll-up banner | 33 × 79 in | 9900 × 23700 px | 4950 × 11850 px |
Look at that last row. Nobody prints a roll-up banner at 300 DPI. The file would be unusable and the extra detail invisible from three metres away. This is why “always use 300” is bad advice as often as it is good.
How to check what you actually have
Step 1: Read the pixel dimensions
Right-click the file and open its properties (Windows) or press Command-I in Finder (macOS). You want the number in pixels, not the file size in megabytes. Megabytes tell you nothing about resolution.
Step 2: Divide by the printed size
Take the pixel width and divide it by the printed width in inches. A 2400 px image printed 8 inches wide is 300 DPI. The same image printed 24 inches wide is 100 DPI. The file never changed; only the plan for it did.
Step 3: Compare against the viewing distance
Match the number to how close people will be. Held in the hand, aim for 300. On a wall at two metres, 150 is plenty. On a banner seen from across a room, 100 is fine. Printers quote 300 because it is safe for everything, not because everything needs it.
Step 4: Set the DPI field without resampling
If the printer insists on the metadata saying 300, open the image size dialog, turn resampling off, and type 300. The pixel count stays identical and the print size shown will update to match. This satisfies the requirement without touching a single pixel.
Turning resampling on is how files get destroyed. With it enabled, changing 72 to 300 tells the software to invent four times as many pixels. The file balloons, the image goes soft, and the printer sees a large blurry file rather than a small sharp one. Check that box every time.
Why “set it to 300 DPI” does not rescue a small image
This is the single most common misunderstanding in print. A 600 × 400 pixel image tagged at 300 DPI will print at two inches by one and a third, correctly and sharply. Tag the same file at 72 DPI and it prints at eight by five, and looks soft.
The pixels never changed. All you did was change how large they are told to be on paper. If you need an eight-inch print and you only have 600 pixels, no metadata edit creates the missing 1800.
The real options, in order of preference: find the original file, reshoot or rescan, print it smaller, or, if the piece is viewed from a distance, accept the lower DPI deliberately rather than pretending.
Scanning at the right resolution
| What you are scanning | Scan at | Why |
|---|---|---|
| Text document for reading | 200–300 DPI | Legible, reasonable file size |
| Document for OCR | 300 DPI | Below this, recognition accuracy drops |
| Photo print to reprint same size | 300 DPI | Matches the print you want back |
| Photo print to enlarge 2× | 600 DPI | Gives you the pixels for the bigger print |
| Film negative or slide | 2400–4800 DPI | The original is tiny; you need the density |
| Artwork for archival | 600 DPI | Captures paper texture and brush detail |
The rule that covers most of it
Count pixels, not DPI. The pixel dimensions are the only real quantity in the file; the DPI field is a printing instruction that you can set freely as long as resampling is off.
Work out what you need with inches × DPI before you commit to a size, and you will never be surprised by a rejection. For the resizing side of the job, see resizing without distortion; the image size cheat sheet has the print dimensions already converted.
Frequently asked questions
Is 72 DPI bad for a website?
The number is irrelevant on a website. What matters is the pixel dimensions: a 1400 px wide image looks sharp in a 700 px column whether its metadata says 72, 300 or nothing at all. The "72 DPI for web" advice is a leftover from 1980s Macintosh screens and has been meaningless for decades.
How do I change DPI without ruining the image?
In any editor, find the image size dialog and untick "resample" (Photoshop) or "scale image" (GIMP) before changing the resolution figure. With resampling off you are only editing the printing instruction and the pixels stay untouched. With it on, the software adds or removes pixels, which is where the damage happens.
My printer says the file is low resolution. What now?
Divide your pixel width by the printed width in inches to see your real DPI. If it comes out under 150, you need a bigger original. No setting fixes it. Options in order: find the source file, reshoot, print smaller, or accept a lower DPI if the piece is viewed from a distance, as with a banner.
What DPI do I need for a poster?
Less than you think, because viewing distance matters more than density. A poster read from two metres away is fine at 100–150 DPI. A brochure held at reading distance wants 300. Billboards are routinely printed at around 15 DPI and look perfect from a car.
Keep reading
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How to compress images without losing quality
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