Pixel Converter

A pixel has no physical size until you decide how many of them fit into an inch. That number is the density, written as DPI (dots per inch) or PPI (pixels per inch); for converting sizes the two mean the same thing. The formula is short: pixels = inches × DPI, and inches = pixels ÷ DPI. Centimeters follow from the exact definition of the inch as 2.54 cm, published by NIST, and millimeters are ten times that.

The chart uses 300 DPI because it is the usual starting point for photo and document printing. At that density a print 6 inches wide needs 1,800 pixels across, and every inch more adds another 300. If a picture does not have enough pixels for the size you want, it can still be printed, but each pixel becomes larger and edges start to look soft. Converting the other way tells you the largest size a picture can print at before that happens: divide its width in pixels by 300 to get inches.

Camera resolution in megapixels is the same idea seen from the other end. Multiply the width by the height in pixels and divide by a million to get megapixels; to see how large a photo prints, convert each side separately at your DPI. A camera's megapixel number says nothing about the shape of the frame, so always work from the two sides rather than the total.

Print shops add one more detail: bleed. Artwork that runs to the edge of the paper is usually made a few millimeters larger on every side and trimmed after printing. Ask for the shop's bleed value, add it to the trimmed size in millimeters, and convert the total to pixels so the extra margin is covered at full resolution.

Screens work differently. A web page measures everything in CSS pixels, at 96 to the inch by definition, and phones and high-resolution monitors then draw each CSS pixel with two, three or more physical pixels. That is why an image for a sharp display is often made at twice the size it is shown at. For screen work, leave Pixel density at 96; for print, set it to the printer's DPI.

Official photo and document rules are often written in millimeters, while upload forms check pixels. Converting the stated size at the DPI the rule gives, or at 300 when it gives none, tells you the pixel dimensions to aim for before you crop. The passport photo maker applies those sizes for many countries directly, so the conversion here is most useful for forms and print jobs that it does not cover.

Points are the unit of type and page layout. Word processors, design apps and PDF files measure font sizes and margins in them, and CSS defines one point as exactly 1/72 of an inch. Converting points to pixels therefore depends on DPI as well: at 96 DPI, 12 pt text is 16 pixels tall, the same as the default font size in most browsers.

File sizes are a separate problem with the same kind of confusion. In the metric system a kilobyte is 1000 bytes and a megabyte a million. Computers work in powers of two, so 1024 bytes became a common meaning of a kilobyte as well. The binary prefixes kibi, mebi and gibi, listed by NIST, were introduced to tell the two apart, but most software still writes KB and MB for both. Windows File Explorer reports sizes in steps of 1024, while macOS and the capacity printed on drives and phones use steps of 1000, as Apple explains.

The gap is small at first and grows with each step: a kilobyte differs by 2.4 percent, a gigabyte by a little over 7 percent, and a terabyte by almost 10 percent. That is the whole reason a new drive seems to have less space than its label promises. When an upload form says a file must be under a limit in KB or MB, it may count either way and rarely says which, so leave a small margin below the limit.

A picture file often carries a DPI value in its metadata, such as 72 or 300, but it is only a label. Changing it does not add or remove pixels; it only changes how large some programs, such as word processors, place the picture by default. A form that asks for a photo at 300 DPI really wants enough pixels for the stated print size at that density, which is what this converter helps you work out.

Keep aspect ratio adds the second side for you. Choose a ratio such as 16:9 for video and presentations, 9:16 for vertical clips, 4:5 for a portrait social post, 3:2 for camera photos or 1:1 for a square, then enter one side; the other is worked out from the ratio. Custom takes any pair of numbers, for example the width and height of an existing picture, so you can scale it to a new size without distorting it. Decimal places controls rounding of every result; pixel counts are whole numbers in practice, so round those at the end, while centimeters and inches usually need one or two decimals.

Once you know the pixel size you need, the rest is a quick job with other tools. Resize Image scales a picture to exact pixel dimensions, the Compress Image tool brings the file under a KB or MB limit, and the QR code generator asks for an image size in pixels that you can work out here from the print size of the code. For plans and drawings, the CAD converter sets the resolution of PNG and JPG output in DPI as well.