DPI, PPI and print size: a practical guide

An image file contains a grid of pixels. It has no intrinsic physical size.

A 3000 × 2000 photograph consists of three thousand pixels across and two thousand pixels down. It acquires physical dimensions only when an output device maps a specific number of those pixels to each inch of paper or screen.

DPI is not a property the file carries. It is the ratio between the pixels you have and the space you want them to occupy, which is why the same file prints at three different sizes without a single byte changing.

DPI (dots per inch) and PPI (pixels per inch) express that conversion ratio:

physical size (inches) = pixels ÷ DPI
pixels needed = physical size (inches) × DPI

Every calculation in print resolution follows this formula:

The pixel buffer and disk storage remain identical across all three outputs. Only the physical spacing of the pixels changes.

DPI vs. PPI

Strictly speaking:

• PPI (pixels per inch) measures pixel density in digital files and display panels. Pixels are the discrete colour units of the digital raster.
• DPI (dots per inch) measures physical printer resolution. Dots are individual droplets of ink applied to paper, and printers combine multiple ink droplets to render each digital pixel.

Commercial print shops use "300 DPI" interchangeably with "300 PPI in your submitted file." The technical distinction matters only when reviewing printer hardware specifications: a printer quoting "1440 DPI" requires a 300 PPI image, not a 1440 PPI source file.

What metadata DPI tags do

Most raster formats (JPEG, TIFF, PNG) can include an optional DPI tag in their header metadata. This tag does not modify pixel data:

• Changing the tag from 72 to 300 without resampling leaves the pixel count identical. It only changes the default physical dimensions suggested when opening the file in layout software.
• Web browsers ignore metadata DPI tags completely. Digital displays render pixels according to screen CSS rules and display scaling.
• The tag matters primarily when layout programs or automated print upload systems enforce a metadata check before accepting a submission. Passport photo portals, for example, validate both pixel dimensions and the 300 DPI header tag.

Selecting required print resolution by viewing distance

The standard requirement of 300 DPI corresponds to the resolving limit of normal human vision at an arm's-length reading distance (approximately 30 cm). Above 300 DPI at that range, the human eye cannot distinguish individual pixel boundaries.

As viewing distance increases, angular resolution limits allow lower DPI values without perceived loss of sharpness:

Supplying 300 DPI for distant viewing generates unnecessarily large files. A 2 × 1 metre banner at 300 DPI requires 23,600 × 11,800 pixels (roughly 278 megapixels). At 100 DPI, the same banner requires 7,900 × 3,900 pixels, producing an identical visual appearance at a two-metre viewing distance.

Practical resolution calculations

Determining maximum print size from known pixels:

Divide each pixel dimension by your target DPI. A 4000 × 3000 pixel image, which is what a 12-megapixel phone camera produces, yields:

• At 300 DPI: 13.3 × 10 inches (suitable for A4 prints).
• At 150 DPI: 26.7 × 20 inches (suitable for wall posters).
• At 100 DPI: 40 × 30 inches (suitable for large exhibition signage).

The same three lines answer the question people usually ask instead, which is whether a phone photograph is good enough for print. Up to roughly A3 at full 300 DPI, it is. Past that, lowering the DPI in step with the viewing distance keeps it usable well beyond poster size.

Determining required pixels from target print dimensions:

Multiply target inches by target DPI. An A4 document (8.27 × 11.69 inches) at 300 DPI requires 2481 × 3507 pixels. A standard 6 × 4 inch print requires 1800 × 1200 pixels.

Handling oversized files:

Resize the file down to the computed target dimensions. Downsampling discards unneeded pixels cleanly and reduces file transfer times.

Handling undersized files:

When pixel count falls short of your target:

1. Reduce print dimensions to match the available pixel density.
2. Target a lower DPI if the intended viewing distance allows it.
3. Apply neural upscaling. Standard bicubic interpolation blurs missing data. An AI upscaler generates plausible fine texture for photographic scenes, though it should not be used on fine typography or official documentation (see our breakdown on AI upscaling capabilities).

Modifying the file's metadata header from 72 to 300 without resampling does not add pixels.

Accounting for bleed and margins

Commercial trimming requires bleed, typically 3 mm added to every outer edge to prevent unprinted white borders if cutting paper shifts slightly.

An A4 document trimmed to 210 × 297 mm requires an image canvas of 216 × 303 mm. At 300 DPI, the image dimensions must measure 2551 × 3579 pixels rather than 2480 × 3508 pixels.

Place vital content (such as text, faces, and brand marks) at least 5 mm inside the final cut line. Between bleed and that safety margin, roughly 10 mm of each edge is unreliable: the outer 3 mm will be trimmed away, and the next few millimetres may sit close enough to the cut to look like an error. Compose with that in mind rather than discovering it on the proof.

Scanner resolution settings

Unlike digital camera files where DPI is metadata, scanner DPI settings physically govern how finely optical sensors record the original document:

For document OCR, 300 DPI provides the minimum reliable threshold for text extraction. Below 300 DPI, character segmentation breaks down and word error rates increase.

Technical FAQ

Does 72 DPI harm web performance? The metadata DPI tag has no effect on web browsers. Display size depends on CSS pixel declarations and physical display pixel ratios.

When should you change a file's DPI tag to 300? Only when submitting files to an automated print queue or validation portal that checks the EXIF header, and only after resampling the image to the corresponding pixel count.

Why does a 300 DPI print look soft? The source file was likely stretched or resampled up from a low-resolution source. The metadata tag records a target density, but it cannot restore unrecorded source detail.

Is 600 DPI necessary for photo printing? Standard photographic paper viewed at normal distance shows no visible difference between 300 and 600 DPI. Line drawings, fine typography, and vector engravings benefit from 600 to 1200 DPI because sharp edges expose pixel stepping.

Is a 12-megapixel phone photo enough for print? For anything up to about A3 at 300 DPI, yes. Larger than that, set the DPI from the viewing distance in the table above rather than defaulting to 300, and the same file covers poster sizes without any upscaling.

Two of the three numbers are yours to choose

Pixels, physical size, and DPI are bound by one division, so fixing any two determines the third. That is the whole subject.

A print shop that specifies both a 50 × 70 cm finished size and 300 DPI has already decided the pixel count: about 5,900 × 8,270. Either the file has those pixels or it does not, and editing a metadata header will not produce them. The productive move at that point is to negotiate one of the other two numbers, usually the DPI, against the distance the print will actually be seen from.