What is DPI and how is it different from PPI?

PPI (pixels per inch) describes screen pixel density — how many pixels fit in one linear inch of a display. DPI (dots per inch) is a print term describing how many ink dots a printer places in one inch. In everyday use the terms are often swapped, but the maths is identical. This calculator works for both by dividing a pixel count by a physical length in inches.

How is screen PPI calculated?

The standard formula is PPI = √(width_px² + height_px²) / diagonal_inches. For example, a 1920 × 1080 display on a 24-inch screen gives √(1920² + 1080²) / 24 = 2202 / 24 ≈ 92 PPI.

What DPI do I need for a good photo print?

A minimum of 300 DPI is the professional standard for photo-quality prints — brochures, greetings cards, and framed prints. 150 DPI is acceptable for posters viewed from a metre or more. Below 72 DPI the print will look noticeably soft or pixelated.

What is considered a Retina display?

Apple defines Retina as a display where individual pixels are indistinguishable at a normal viewing distance. In practice that means approximately 220 PPI or above for laptops and monitors, and 326 PPI for phones. Use the Screen PPI mode to check whether your display meets this threshold.

What is dot pitch and why does it matter?

Dot pitch is the physical distance (in millimetres) between the centres of adjacent pixels. It is the inverse of PPI scaled to millimetres — dot pitch (mm) = 25.4 / PPI. A smaller dot pitch means pixels are packed more tightly and the image appears sharper. High-end monitors often have a dot pitch below 0.25 mm.

My image is 6 megapixels — can I print it at A4 size?

An A4 sheet is 29.7 × 21 cm (11.69 × 8.27 inches). A typical 6 MP image is about 3000 × 2000 px. Using the "Required DPI" mode with those pixel dimensions and a 29.7 × 21 cm target gives roughly 257 DPI × 242 DPI — both above 150 DPI, so an A4 print will look sharp. For a 300 DPI museum-quality print you would need about 3507 × 2480 px minimum.

DPI / Dots Calculator

Understanding dots per inch is one of those invisible skills that separates a polished visual result from a blurry disappointment. Whether you are checking that a display qualifies as Retina, working out whether your photo will hold up at A3, or figuring out exactly how large you can print an image from your phone — this calculator gives you a single place to do all three.

How it works

The tool has three modes that each solve a different direction of the same core relationship:

DPI = pixels ÷ inches

Screen PPI mode uses the diagonal pixel count formula. A screen does not have a single horizontal or vertical pitch that tells the whole story — diagonal PPI captures both axes at once:

PPI = √(width² + height²) / diagonal_inches

A 1920 × 1080 monitor on a 24-inch panel produces a diagonal resolution of √(3,686,400 + 1,166,400) = 2,202 px, divided by 24 = 91.8 PPI. The calculator also derives the dot pitch (millimetres per pixel) and DPCM (dots per centimetre).

Print size from DPI mode inverts the formula: given an image’s pixel dimensions and a chosen DPI, it calculates the physical print size in both inches and centimetres. A 3000 × 2000 px image at 300 DPI will print at exactly 10 × 6.67 inches (25.4 × 16.9 cm).

Required DPI for size mode solves for the unknown on the other side: enter the print dimensions you want (in centimetres) and the pixel count of your image, and it tells you the effective DPI on each axis and flags the limiting axis (the lower of the two). This is the most practical mode when you have a fixed canvas size — such as an A4 poster or a banner — and need to know whether your image file is good enough.

Example: preparing a photo for a magazine cover

Suppose you have a 4500 × 6000 px photograph and a magazine page that is 21 × 28 cm (A4 portrait).

Using the Required DPI mode:

Width axis: 4500 px ÷ (21 ÷ 2.54) = 4500 ÷ 8.27 ≈ 544 DPI
Height axis: 6000 px ÷ (28 ÷ 2.54) = 6000 ÷ 11.02 ≈ 544 DPI

Both axes come out above 300 DPI, so the photo is well above professional print quality. The quality badge reads Photo-quality / fine art. If you only had a 1500 × 2000 px file, you would get ≈ 181 DPI — above the 150 DPI draft threshold but below the 300 DPI print threshold, flagged as Acceptable print (fine for a large-format display, borderline for a magazine).

Image size	Print at A4	Effective DPI	Rating
1000 × 1414 px	21 × 29.7 cm	~121 DPI	Draft
1754 × 2480 px	21 × 29.7 cm	~212 DPI	Acceptable
2480 × 3508 px	21 × 29.7 cm	~300 DPI	High-quality
4961 × 7016 px	21 × 29.7 cm	~600 DPI	Photo-quality

Formula note

All three modes use the single identity DPI = pixels / inches. The screen mode adds the Pythagorean step to reduce a two-dimensional resolution to a single diagonal metric, which is the industry-standard way manufacturers quote display sharpness. Dot pitch (mm) is then simply 25.4 ÷ PPI, and DPCM is PPI ÷ 2.54. No network requests are made — every calculation runs instantly in your browser.