How many pixels you need to print a photo at 4×6, 8×10, Letter or A4

The print shop asked for 300 DPI and your file says 72

A print shop, a photo contest or a journal asks for the image "at 300 DPI." You open the file's properties and read 72 dpi, or 96. Before you go looking for a converter, read the pixel dimensions in the same window. Together with the size of the paper, they decide how the print looks. The DPI field does not.

PPI means pixels per inch: how many image pixels land on each inch of paper. So pixels = inches × PPI and, going the other way, inches = pixels ÷ PPI. For metric sizes, divide the centimeters by 2.54 first; the inch is defined as exactly 25.4 mm.

A 5 × 7 at 300 PPI needs 5 × 300 = 1500 by 7 × 300 = 2100 pixels. A 12-megapixel photo, 4000 × 3000 pixels, covers 13.3 × 10 inches at 300 PPI.

The State Department's digital image rules for visa photos use the same arithmetic: a 2 × 2 inch print scanned at 300 pixels per inch gives the 600 × 600 pixels they set as the minimum.

DPI or PPI?

Strictly speaking, DPI (dots per inch) describes the printer, not the picture: Adobe notes that most inkjet printers operate between 720 and 2880 dpi, and the GIMP manual explains that several dots are used to print one pixel. In everyday requests, "300 DPI" means 300 pixels per inch.

Pixel dimensions for 4×6, 5×7, 8×10, 11×14, Letter and A4

Values are in pixels (width × height). Megapixels (MP) are in parentheses so you can compare with your camera. The last column is the aspect ratio to crop to.

Letter and A4 are not the same sheet

US Letter is 8.5 × 11 inches. A4 comes from ISO 216, the international standard for paper sizes, and measures 210 × 297 mm, about 8.27 × 11.69 inches: slightly narrower and taller. Every ISO A size has the same 1:1.414 ratio (the square root of 2), each being the previous one cut in half, so one crop fits A4 and A3 alike. It will not fit Letter without a margin or a trimmed edge.

Why 300 PPI became the reference

Adobe calls 300 pixels per inch the industry standard for high-quality prints, especially those viewed up close. Epson recommends 300 to 360 and offers a rule of thumb: 200 or more gives a satisfactory print, 300 or more a high-resolution one. For inkjet photo prints, Adobe says at least 220 is usually sufficient.

The number tracks what the eye can do. A person with normal 20/20 vision resolves detail about 1 arcminute wide, one sixtieth of a degree. At 12 inches, roughly where you hold a print, that angle spans 0.0035 inch, which works out to about 286 pixels per inch. Beyond that, normal eyesight no longer separates one pixel from the next. Treat it as an estimate: lighting, contrast and individual eyesight all move the limit.

Posters and banners: viewing distance lowers the requirement

The same arithmetic works at any distance: double the distance and the pixel density you need is cut in half.

What manufacturers recommend

Manufacturers leave a margin above those limits. Adobe says lower resolutions work well for large-format prints viewed from a distance, such as billboards; Epson, that a slight loss of detail is not a big problem from far away; HP suggests 150 to 300 for posters.

A 16 × 20 at 150 PPI needs 2400 × 3000 pixels, exactly what an 8 × 10 needs at 300, so a 12-megapixel photo covers it.

The 72 or 96 DPI stored in the file means nothing on a screen

The number lives in a header field: the density values of a JPEG (JFIF or Exif), the pHYs chunk of a PNG. The Exif specification, the metadata standard digital cameras follow, says to write 72 when the image resolution is unknown. It is a placeholder, not a measurement.

96 is a display convention: Microsoft explains that for many years Windows treated one logical inch as 96 pixels, and CSS defines one inch as 96 px.

On a screen, Adobe notes, the size at which an image appears depends on its pixel dimensions and on the monitor. On web pages the CSS default is one image pixel per CSS pixel: a 1200 × 800 image looks the same tagged 72 or 300.

Technical exception: under the HTML Standard, a browser uses Exif resolution only when the dimensions recorded in Exif agree with it, which an ordinary photo tagged 300 does not satisfy.

What software does when it changes the DPI

Image editors offer two operations that sound alike. In the first, with resampling off, you replace 72 with 300 and the program only recalculates the printed size. According to Adobe, the image data stays constant unless the image is resampled; the GIMP manual says the same of its Print Size command. Preview on the Mac has the same switch, Resample image.

Example: 3000 pixels across tagged 72 DPI nominally measure 41.7 inches; tagged 300, they measure 10 inches. The pixels are the same and the print comes out identical.

In the second, resampling, the program discards or creates pixels. Enlarging means calculating in-between pixels from their neighbors (interpolation): the file grows, the detail does not. Mozilla's documentation warns that images can turn blurry when scaled up.

So "convert to 300 DPI" can only end two ways: the label changes and the photo stays the same, or the photo is enlarged by interpolation and gets bigger, not sharper. If a printer insists on a file tagged 300, set the field in an editor with resampling off (in Photoshop, Image > Image Size with Resample deselected).

Getting exact pixel dimensions in LivreImage

LivreImage works in pixels and has no DPI field. There are two steps, in this order.

1. Find your target in the chart: 8 × 10 in at 300 PPI, for example, is 2400 × 3000 pixels, a 4:5 ratio.
2. In Crop image, set the shape. The ratio buttons cover 1:1, 3:4, 4:3, 16:9 and 9:16; for 4:5, 2:3 or 5:7, choose Free and watch the box size, shown in pixels. In 4:5 the height is 1.25 times the width, such as 2000 × 2500.
3. Open the cropped file in Resize image. Under Exact output, type the width and the height. Fill in only one and the other is calculated from the ratio.
4. Click Generate images, check the dimensions on the result card and download.

Limits worth knowing

• There is no AI upscaling. If the cropped photo has fewer pixels than the target, Exact output enlarges by interpolation: it reaches the size but gains no detail. Printing smaller or finding the original works better.
• When the width and height have a different ratio from the image, the page crops the center to fit instead of stretching. Cropping first is how you choose what stays in the frame.
• Max width only shrinks, and the longest side of any output is capped at 10,000 pixels: a 24 × 36 at 300 PPI is out of range.
• The exported file does not carry the original's DPI value: give the print size in inches when you print or order.