PPI Calculator
Work out the pixel density of a screen, the size an image prints at, or how many pixels a print needs. PPI describes pixels in an image or on a screen; DPI is a printer term, explained below.
PPI and DPI are different things
- PPI (pixels per inch) is the number of pixels per inch in a digital image or on a screen. It describes the pixel density, how fine the image is.
- DPI (dots per inch) is the number of ink dots a printer puts down per inch. It describes the printer, and it is a print term.
People often write "dpi" for the resolution of an image, so the two words get mixed up, but the relation between the number of pixels and the size is about PPI. A printer's DPI does not have to equal the PPI of the image, because the printer's capability is separate from how many pixels the image has.
This calculator does not compute a printer's DPI. It uses the plain relation pixels ÷ length in inches. Values such as "72 dpi" or "96 dpi" are often just information stored in the image file or a display setting; they do not add or remove pixels. Once the pixels and the print size are fixed, the PPI is fixed too.
How it is calculated
One inch is exactly 25.4 mm.
- Screen PPI = diagonal in pixels ÷ diagonal in inches, where the diagonal in pixels is √(width² + height²). The size of one pixel (mm) = 25.4 ÷ PPI.
- Print size = pixels ÷ PPI (inches). It is also shown in centimeters and millimeters.
- Pixels needed = print length (inches) × PPI. The pixel count is rounded up, so that the image is never short of pixels.
The square root is computed with whole-number arithmetic to 12 decimal places, so there is no floating-point error.
Examples
- A 1,920 × 1,080 screen with a 27-inch diagonal gives about 81.59 PPI (one pixel is about 0.3113 mm).
- A 3,000 × 2,000 image printed at 300 ppi gives 10 × 6.67 inches (25.4 × 16.93 cm).
- An 8 × 10 inch print at 300 ppi needs 2,400 × 3,000 pixels.
Notes
- Pixels must be whole numbers from 1 to 999,999. The diagonal, the print size and the PPI must be greater than 0 (up to 6 decimal places). Commas are accepted only as thousands separators.
- There is no paper-size menu (such as Letter or A4). Enter the paper size as a length.
- How many PPI a print needs depends on the viewing distance and the purpose. This tool only calculates numbers; it does not say what PPI is "enough".
- The screen PPI uses the diagonal of the visible area. The Aspect Ratio Calculator helps with the proportions of an image or a screen.
Frequently asked questions
Are DPI and PPI the same?
No. PPI is pixels per inch in an image or on a screen, and DPI is dots of ink per inch from a printer. They are often confused because many people call the resolution of an image "dpi".
How many PPI do I need to print?
It depends on the purpose and the viewing distance, so the tool does not judge it. You can enter a PPI you want, such as 300, and see the pixels that a print size needs.
What decides the PPI of a screen?
The number of pixels (width × height) and the diagonal in inches. With the same pixels, a larger screen has a lower PPI.