Best DPI to scan old photos

What the official guidance says

The Library of Congress personal-archiving guide says to scan 4x6 and 5x7 prints at 300 dpi, and photographs you intend to enlarge to 8x10 or greater at 400 or 600 dpi, adding that anything above 600 may be unnecessary. The Minnesota Historical Society says 300 ppi is fine in general for prints, and that for especially detailed or precious photos you may wish to scan at 600 ppi, which doubles the size of the original so that the details hidden in the print are captured.

DPI by original size and what you want to print

These figures are from the Minnesota Historical Society brochure. They are the scan resolution needed so that, at 300 ppi, the file is large enough for the print size shown (cropped where stated).

For slides, the Library of Congress guide says to scan at a higher rate than prints, around 1900 dpi, because slides are smaller than prints.

Why more DPI stops helping

In a 2013 Library of Congress post, a digitization-guidelines researcher wrote that he doubted you would see significant improvement for reflective materials such as prints beyond 400 ppi, and that consumer scanners gain nothing above 1200 ppi unless you scan 35mm film on a specialized film scanner. A second photographer in the same post noted that scanning a 4x6 print at 600 or 1200 ppi mostly gives you more dust to spot out and enlargements that do not look as good as the original. The same post points out that scanners often advertise an interpolated rate and that you will often get better results at the scanner's maximum optical resolution.

Higher DPI does not create detail that is not in the print. It only captures what is there.

Why other guides say 600 to 1200 DPI

Scanning-service and scanner-maker guides often recommend more: for example Photomyne lists 300 to 600 DPI for family archives and 600 to 1200 DPI for professional archival preservation, and CZUR lists 600 DPI for small prints and 1200 DPI or more for slides and negatives. Both are commercial sources. The Library of Congress and the Minnesota Historical Society figures above are lower for prints, and the practical middle is 600 ppi when a print is small, precious, or likely to be enlarged or cropped.

File size at each setting

This is arithmetic, not a measurement: a 4x6 print scanned in 24-bit color and saved without compression is about 6.5 MB at 300 ppi (1200 x 1800 pixels), about 26 MB at 600 ppi (2400 x 3600), and about 104 MB at 1200 ppi (4800 x 7200). Each doubling of ppi quadruples the data. Reviva accepts photos up to 12 MB each, so for a 600 ppi scan save a high-quality JPEG copy to upload and keep the larger file as your master.

A practical rule

Scan prints at 300 ppi if you only want to view or reprint at the same size. Scan at 600 ppi if the print is small, precious, damaged, or you may crop or enlarge it. Do not go above 600 for a print. Scan slides and negatives with a scanner built for film.