Scan resolution calculator

Why enlargement changes the scan resolution

A scan records pixels across a physical original. If the later print is twice as wide and twice as high, those pixels are spread over edges twice as long. To keep the same image PPI in that larger print, the original must be sampled at twice that PPI.

Let the physical enlargement factor be s and the intended print resolution be P. Then original-size scan samples per inch = s × P. Scanner controls commonly call this setting dpi. Here it describes image sampling over the original; it is not the printer’s ink-dot resolution.

Epson’s scan-resolution guidance distinguishes enlarging after an original-size scan from choosing a final size inside the scanning software. That distinction matters: applying enlargement in both places can produce a much larger scan than intended.

Example: a 4 × 6 original for an 8 × 12 print

The width ratio is 8 ÷ 4 = 2 and the height ratio is 12 ÷ 6 = 2. At a target of 300 print PPI, original-size sampling is 2 × 300 = 600 dpi. The full original corresponds to 4 × 600 = 2,400 pixels across and 6 × 600 = 3,600 pixels down, or 8.64 megapixels.

A 300 dpi original-size scan would instead have 1,200 × 1,800 pixels. Spread across the same 8 × 12 print, it provides 150 PPI. This is a sampling comparison, not a prediction that either print will look good or bad.

Fit and Fill give different answers when shapes differ

For a 4 × 6 original and an 8 × 10 target, the edge ratios are 2 and 1.667. Fit uses the smaller ratio so the whole original remains visible. At 300 print PPI, the original-size target is 500 dpi; the image occupies about 6.667 × 10 in, leaving centered side gaps of about 0.667 in.

Fill uses the larger ratio so the target rectangle is covered. It needs 600 dpi at original size. A 4 × 5 in portion of the original supplies the 8 × 10 print, so about 16.667% of the original area is outside the crop. The result does not choose which part to discard. If you crop before scanning, you can enter the selected area’s actual dimensions instead.

Fit scale = min(print width ÷ original width, print height ÷ original height)
Fill scale = max(the same two ratios)
Effective print PPI = original-size scanner setting ÷ scale

Choose a supported setting, then inspect the file

The calculator rounds the sampling target upward to the next whole dpi and the full-scan pixel edges upward to whole pixels. Your scanner may offer a different list of settings. Choose a supported setting at or above the target if available, and use the optional field to see its mathematical print PPI. A calculated requirement above your device’s capability is not a promise that the device can meet it.

The scan estimate assumes square pixels and the physical dimensions you entered. Actual capture, selected scan boundaries and driver rounding can change the saved dimensions. Check the resulting file with the image PPI calculator, or use the pixels-to-inches converter to see its physical size at another PPI.

Adobe’s printed-resolution explanation connects pixel dimensions, physical size and PPI. Enlarging a saved file by resampling changes its pixel count; it does not establish that the scanner captured additional original detail. Original sharpness, capture quality and print conditions still matter.

A worksheet for the capture decision

This page helps plan sampling before capture. It does not scan, upload, restore or enlarge an image. Calculations stay in your browser and entries are not stored. If the file already exists and only the print-dialog size needs changing, use the print scaling calculator.

Updated September 20, 2026. Corrections are welcome at contact@pixelstoinches.org.