Film Scan Resolution Calculator (DPI for Print Size, Pixel Dimensions & File Size)
Find the scanning resolution a negative or slide needs for a given print size at 300 ppi, the pixel dimensions and file size at 8 or 16 bits, the largest quality print your scanner's real resolution allows, and where film grain, not pixels, becomes the limit.
How it works
scan dpi = print long side (in) × print ppi ÷ frame long side (in) · pixels = frame (in) × dpi · MB = pixels × channels × bytes per channel ÷ 1,048,576 · largest print = frame (in) × scanner real dpi ÷ print ppi
A scan has to carry enough pixels for the print: multiply the print size by the printer's pixels per inch and divide by the size of the film frame to get the dots per inch the scanner must resolve. Small frames need very high dpi for big prints, which is where the film's own grain and the scanner's real (not rated) resolution take over — a 35 mm negative has roughly 4,000 dpi of information at best, and a consumer flatbed delivers about half that however it is set. The calculator shows both limits alongside the pixel dimensions and file sizes that result.
Worked example
A 16-inch print at 300 ppi from a 35 mm frame (36 mm ≈ 1.42 in long) needs 4,800 pixels along the long side, so a 3,387 dpi scan — 4,800 × 3,200 px (15.4 MP), about 88 MB as a 16-bit RGB TIFF. A 3,200 dpi film scanner just falls short (its largest 300-ppi print is about 15.1 × 10.1 in); a 6 × 7 negative would need only 1,742 dpi for the same print.
Frequently asked questions
How many megapixels is a 35 mm negative?
Practically 12–24 MP of real detail depending on film speed, lens and focus — around 4,000 dpi of useful resolution for slow films. Scanning at 8,000 dpi does not extract more, it only enlarges grain into bigger files.
Is 300 ppi always necessary?
No. 300 ppi suits prints viewed at arm's length; a 30 × 40 in print seen from two metres looks sharp at 180–200 ppi. Halve the ppi and the required scan dpi halves too.
Why is my flatbed scan soft at 6,400 dpi?
Because the optics and film-flatness of consumer flatbeds limit real resolution to about 2,000–2,400 dpi regardless of the setting. Scan at 2,400–3,200, keep the film flat (wet mounting or glass holders help), and sharpen in post.
Related calculators
Darkroom figures are starting points. Film, developer batch, agitation, water and thermometer accuracy all shift results — run a test roll or strip, keep notes, and adjust from your own results.