Two units, one question
PPI, pixels per inch, is a density. An image is a grid of pixels, say 3,000 by 2,000, and it has no physical size until you decide how many of those pixels to spread over each inch. Print the 3,000 pixels across 10 inches and the image is at 300 ppi; across 30 inches and it is at 100 ppi. A screen has a fixed PPI, because its pixels have a fixed size.
DPI, dots per inch, belongs to the printer. It counts the droplets of ink or toner the device can place per inch, a property of the machine, not of your file. Print shops nonetheless say "300 DPI" when they mean the pixels per inch of your image at its final size, and that is the sense in which the term appears on their help pages. For picking and preparing an image, read DPI as PPI and use this:
What print shops ask for
Two printers' own guidance, as written. PrintNinja recommends 300 dpi for all images and art files at final print size; images between 200 and 300 will still look pretty good, although not quite as sharp; under 200 is low resolution with notable pixelation or blurring. It also notes that submitting files above 300 dpi does not improve print quality, and that scaling an image in the layout lowers its effective resolution, so a 300 ppi image placed at 200 percent prints at 150 ppi. PrintingForLess asks for at least 300 DPI at the final size and 400 DPI for images that include text, and says web images at 72 to 96 DPI are not acceptable for printing.
Those figures turn into pixel counts with the formula. The table gives the pixels an image needs for common print sizes at 300 ppi, at the 200 ppi floor, and at 150 ppi, which is what a 300 ppi image becomes when placed at twice its size.
| Print size | Pixels at 300 ppi | Pixels at 200 ppi | Pixels at 150 ppi |
|---|---|---|---|
| 4 × 6 in photo | 1,200 × 1,800 | 800 × 1,200 | 600 × 900 |
| 5 × 7 in photo | 1,500 × 2,100 | 1,000 × 1,400 | 750 × 1,050 |
| 8 × 10 in photo | 2,400 × 3,000 | 1,600 × 2,000 | 1,200 × 1,500 |
| A4, 8.27 × 11.69 in | 2,481 × 3,507 | 1,654 × 2,338 | 1,241 × 1,754 |
| Letter, 8.5 × 11 in | 2,550 × 3,300 | 1,700 × 2,200 | 1,275 × 1,650 |
| 11 × 14 in | 3,300 × 4,200 | 2,200 × 2,800 | 1,650 × 2,100 |
| 16 × 20 in | 4,800 × 6,000 | 3,200 × 4,000 | 2,400 × 3,000 |
| 24 × 36 in poster | 7,200 × 10,800 | 4,800 × 7,200 | 3,600 × 5,400 |
The image resizer shows an image's pixel dimensions and resizes to an exact width and height, and the aspect ratio calculator tells you whether a 3:2 photo fits an 8 by 10 inch frame without cropping (it does not; 8 by 10 is 4:5).
How big your photo can print
Turn the formula around and a pixel count becomes a maximum print size. The table divides common image sizes by 300 and 200 ppi.
| Image | Pixels | At 300 ppi | At 200 ppi |
|---|---|---|---|
| Square social image | 1,080 × 1,080 | 3.6 × 3.6 in | 5.4 × 5.4 in |
| Full HD frame | 1,920 × 1,080 | 6.4 × 3.6 in | 9.6 × 5.4 in |
| 4K frame | 3,840 × 2,160 | 12.8 × 7.2 in | 19.2 × 10.8 in |
| 12-megapixel phone photo | 4,032 × 3,024 | 13.4 × 10.1 in | 20.2 × 15.1 in |
| 24-megapixel camera photo | 6,000 × 4,000 | 20.0 × 13.3 in | 30.0 × 20.0 in |
| 48-megapixel photo | 8,000 × 6,000 | 26.7 × 20.0 in | 40.0 × 30.0 in |
A 12-megapixel phone photo is good for a sharp print a little larger than Letter size, and a 24-megapixel camera frame for 20 by 13 inches. A Full HD screenshot, which looks enormous on a monitor, is a 6.4 by 3.6 inch print at 300 ppi. Cropping reduces the pixels and the print size with them, so crop first and read the remaining dimensions before you order.
The number in the file is a label
Image files can store a resolution value, often 72 or 96 ppi for screen images and 300 for scans. It does not change a single pixel. It tells layout and print software what physical size to suggest when you place the image: a 3,000 pixel wide image labelled 300 ppi opens at 10 inches wide, the same file labelled 72 ppi opens at 41.7 inches wide, and in both cases it is the same 3,000 pixels and the same quality at any given print size. Changing the label from 72 to 300 in an editor therefore costs nothing and gains nothing, unless the editor also resamples the image to add pixels, which adds softness rather than detail. When a print shop rejects a file as 72 dpi, the problem is almost always the pixel count, not the label.
The same applies to PDFs. The PDF compressor reduces file size by resampling the embedded images to a chosen resolution, 150 ppi for its medium setting; a page that will be printed at full quality should keep 300.
PPI on screens
A screen's PPI is its pixel diagonal divided by its physical diagonal: the square root of width² plus height² in pixels, over the inches. The PPI calculator does this for any resolution and size; a few examples computed the same way:
| Screen | PPI |
|---|---|
| 6.1 in phone, 2556 × 1179 | 461 |
| 10.9 in tablet, 2360 × 1640 | 264 |
| 14 in laptop, 1920 × 1200 | 162 |
| 27 in monitor, 2560 × 1440 | 109 |
| 27 in monitor, 3840 × 2160 | 163 |
| 55 in TV, 3840 × 2160 | 80 |
Screen PPI matters for a different reason than print PPI. An image is shown pixel for pixel, or scaled by the operating system, so what decides how sharp it looks is whether it has at least as many pixels as the area it fills on that screen. A 1,080 pixel wide image fills a quarter of a 4K monitor's width sharply and looks soft stretched across all of it. The image's own ppi label plays no part at all.
Checklist for a print
- Ask the shop for its minimum in pixels per inch at final size; 300 is the common answer, 200 the common floor.
- Multiply the print size in inches by that number. That is the pixel count you need, after cropping.
- Check the image's pixels, not its dpi label.
- Do not place an image larger than 100 percent in a layout, or divide its ppi by the scale factor.
- Keep the original. Compression for email or the web throws pixels away that print needs.
Frequently asked questions
What is the difference between PPI and DPI?
PPI is pixels per inch and describes digital images and screens: how many pixels are packed into an inch. DPI is dots per inch and describes printers: how many droplets of ink they put down per inch. Print shops often say DPI when they mean the PPI of your image at its final size, and for choosing an image that is the number that matters: pixels divided by the inches you print.
Do I need 300 DPI to print?
Print shops ask for 300 pixels per inch at the final printed size. PrintNinja recommends 300 dpi for all images, treats 200 to 300 as still looking good but not quite as sharp, and calls anything under 200 low resolution with visible pixelation; PrintingForLess asks for 300 and 400 for images containing text. What you need, then, is enough pixels: a 4 by 6 inch print at 300 ppi needs 1,200 by 1,800 pixels.
How big can I print a 12-megapixel photo?
A 4032 by 3024 pixel photo prints at 13.4 by 10.1 inches at 300 ppi and at 20.2 by 15.1 inches at 200 ppi. Larger prints are usually looked at from further away, which is why a poster can get away with less than a photo book page, but the shop's minimum is the number to ask for.
Can I increase the DPI of an image?
You can change the number stored in the file in any image editor, but that only relabels the same pixels; a 1,200 by 1,800 image is a 4 by 6 inch print at 300 ppi whatever the metadata says. Making the pixel count bigger means inventing pixels, and resampling upward adds softness, not detail. The honest fix is a larger original or a smaller print.
Why do people say web images are 72 DPI?
Because 72 pixels per inch is the resolution label many programs write into a screen image by default, and print software reads that label when it suggests a print size. The label is meaningless for a screen, which shows the pixels at its own density, and for print it only sets the suggested size. PrintingForLess warns against using web images for print not because of the 72 but because they usually have far too few pixels.
Sources
- PrintNinja: Recommended resolution (300 dpi recommended for all images at final size, 200 to 300 still looks good, under 200 shows pixelation, scaling an image in the layout lowers its effective resolution, above 300 does not improve print quality)
- PrintingForLess: Image resolution for printing (at least 300 DPI at final size, 400 DPI for images with text, web images at 72 to 96 DPI are not acceptable for print)