A print shop asks for “300 DPI”. Your photo editor says the image is 72 DPI. The camera wrote 240 into the file, and your monitor apparently has 109 of something. These look like four versions of the same setting. They are really two different measurements, and only one of them is in your image.
Two different things
PPI (pixels per inch) describes an image or a screen. It says how many of the image’s pixels will be laid side by side in one inch when the image is printed, or how tightly a display packs its pixels.
DPI (dots per inch) describes a printer. It counts the tiny ink or toner dots the machine can place in an inch. An inkjet rated at 4800 × 1200 DPI is not printing 4,800 of your pixels per inch; it uses many dots to build up the color of each pixel.
In everyday use the two are mixed up constantly, and that’s mostly harmless: when a print shop or a stock site asks for “300 DPI”, they mean 300 pixels per inch in your file. The important part is knowing which number is the real constraint.
The only formula you need
An image file is a grid of pixels. The resolution number tells software how large to print that grid:
- Print width in inches = pixel width ÷ PPI
- Print width in centimeters = pixel width ÷ PPI × 2.54
So a photo 3000 pixels wide, printed at 300 PPI, comes out 10 inches (25.4 cm) wide. The same file at 150 PPI prints 20 inches wide, with each pixel twice as large.
A typical 12-megapixel phone photo is 4000 × 3000 pixels. At 300 PPI that’s 13.33 × 10 inches, a little larger than a sheet of Letter paper. At 150 PPI it’s 26.67 × 20 inches, poster territory. The pixels are the same in both cases. Only the instruction about how far apart to spread them has changed.
The image print size calculator does this for any pixel size and resolution, and the DPI calculator works the other way: from pixels and a target size to the resolution you’ll get.
Why changing 72 to 300 does nothing on screen
Open the image size dialog in Photoshop, Affinity Photo or GIMP, change the resolution from 72 to 300 with resampling turned off, and nothing visible happens. The pixel dimensions stay the same and so does the file on screen.
That’s correct behavior. A screen shows one image pixel per screen pixel (or per group of pixels, on a high-density display), whatever the resolution field says. The PPI value is a note stored in the file’s metadata for printing. Changing it only changes the printed size.
Turn resampling on and the software instead invents or discards pixels: changing a 3000-pixel-wide image from 72 to 300 PPI while keeping the print size fixed multiplies the pixel count by about 4.2 in each direction. That makes the file larger but not sharper; no detail is added that the camera didn’t capture.
The “72 DPI” default dates back to early Macintosh screens, which showed about 72 pixels per inch so that one point of type matched one pixel. Windows chose 96 pixels per inch, and CSS later adopted 96 as its reference: one CSS pixel is defined as 1/96 of an inch. Neither number says anything about modern displays, and neither affects how an image looks on the web.
How many pixels a sharp print needs
300 PPI is the usual target for prints viewed in the hand: photos, brochures, books. It comes from human eyesight. A person with normal vision can resolve detail down to about one arcminute (1/60 of a degree). At a distance d in inches, that works out to roughly 3438 ÷ d pixels per inch before a viewer can no longer separate neighboring pixels:
- At 12 inches (a photo in your hand): about 286 PPI
- At 24 inches (a print on a desk or a monitor): about 143 PPI
- At 36 inches (a poster across a small room): about 95 PPI
- At 120 inches (a banner across a hall): about 29 PPI
That’s why 300 PPI is the standard for close reading, 150 PPI is usually plenty for a poster, and billboards get away with far less. A useful rule: work out how far away people will stand, and aim for the PPI at that distance.
For common sheets at 300 PPI:
- US Letter (8.5 × 11 in) needs 2550 × 3300 pixels.
- A4 (210 × 297 mm, about 8.27 × 11.69 in) needs 2480 × 3508 pixels.
- An 8 × 10 in photo needs 2400 × 3000 pixels.
The paper size calculator lists pixel dimensions for every ISO and North American sheet at any resolution, and the megapixels to print size calculator starts from a camera’s megapixel rating instead.
What the printer’s DPI is for
A printer can only lay down a few ink colors, each either present or absent at every dot. To produce a smooth tone from that, it clusters or scatters dots in patterns (halftoning or dithering). Many printer dots go into each image pixel, which is why printer DPI figures run into the thousands while image PPI figures sit in the hundreds.
Higher printer DPI gives smoother gradients and finer text edges, but it can’t add detail to an image that lacks the pixels. A 600-pixel-wide image printed 8 inches wide is 75 PPI, and it will look soft on any printer.
Screens have PPI too
Displays are rated in pixels per inch, calculated from the resolution and the diagonal:
- A 24-inch 1920 × 1080 monitor is about 91.79 PPI.
- A 27-inch 2560 × 1440 monitor is about 108.79 PPI.
- A 27-inch 3840 × 2160 (4K) monitor is about 163.18 PPI.
- A 15.6-inch 1920 × 1080 laptop is about 141.21 PPI.
Screen PPI doesn’t change your files, but it explains why an image looks crisper on a phone than on a desktop monitor, and why operating systems scale the interface on dense displays. The screen PPI calculator works it out for any display.
Converting pixels for print and screen
When a designer says an element is “200 px”, the physical size depends on the resolution used to print or display it. At 96 PPI (the CSS reference), 96 px is one inch. At 300 PPI, it takes 300 px to fill that inch. The pixels to inches and pixels to centimeters converters let you choose the resolution, so the answer matches the output you’re preparing.
A checklist before sending a file to print
- Check the pixel dimensions first. Divide by the print size in inches. If the result is 300 or more for a hand-held print (150 or more for a poster), you’re fine.
- Don’t trust the resolution field alone. A file marked 300 PPI may be too small for the size you want, and a file marked 72 PPI may be huge.
- Avoid upscaling to hit a number. It satisfies an upload form but adds no detail. If the image is too small, print it smaller or find the original.
- Allow for bleed. Print shops often want an extra 0.125 in (3 mm) on each side, which adds pixels to every dimension.
- Ask what they mean by DPI. If a shop asks for “300 DPI”, they almost always mean 300 pixels per inch at the final printed size.