300 DPI, and what it really changes.
Set an image's DPI to 300 in your browser, and know what it does: JPEG and PNG store it, WebP and AVIF cannot, and pixels decide the print size.
By Suraj Kumar4 min read
A print shop or a publisher asks for images at 300 DPI. It sounds like a setting for quality, and it is not quite. This guide explains what the number is, when changing it helps, and how to set it with the KeepSimp image resizer in your browser.
What DPI is.
A picture is a grid of pixels. DPI, dots per inch, is a number stored beside the grid that says how many of those pixels to print in each inch. It does not change the pixels. The same 3,000 pixels wide prints 10 inches wide at 300 DPI and over 41 inches wide at 72 DPI, with exactly the same detail.
So what a printer really needs is enough pixels for the print size. DPI tells its software how large to place them.
pixels = inches × DPI
4 × 6 in at 300 DPI = 1,200 × 1,800 px
10 × 15 cm at 300 DPI = 1,181 × 1,772 pxChange the number, keep the pixels.
Open the image resizer and add your picture. Under More options, set Size to Percent at 100, so no pixels change. Then, under Names and print resolution, turn on Save print resolution and choose 300. Download the JPEG or PNG.
The resizer writes the number into the file's own field: the JFIF header of a JPEG and the pHYs part of a PNG. The file is written anew, so its other details, such as camera data and location, are left out, and a JPEG is saved once more; choose High under quality for print.
Resize for a print size.
When you know the paper size, let the resizer do the sum. Under Size, choose Dimensions, set the unit to centimetres, millimetres or inches, type the size and pick 300 DPI. Or choose Presets, then Print: 4 × 6, 5 × 7 and 8 × 10 inches, A4, A5, US Letter and a 2 × 2 inch photo, all at 300 DPI.
The resolution is written for you whenever the size is given in a unit of length or by a print preset.
Not every format can hold it.
JPEG and PNG have a place for the print resolution. WebP and AVIF do not, and the page says so when one of them is chosen. For print, save JPEG or PNG. The image converter can also write a print resolution into JPEG, PNG, BMP and TIFF while it converts.
When there are too few pixels.
Setting 300 DPI on a small picture makes it print small and sharp, not large and sharp. Enlarging adds pixels but not detail, and the resizer says so whenever the result is larger than the source; it does not invent detail with AI. For a large print, start from the largest original you have.
Further reading
Topics: change dpi · change image dpi · 300 dpi · convert image to 300 dpi · dpi to pixels · image resolution for print · set dpi of jpeg · png dpi
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