Scan Resolution Calculator: DPI to Pixels, File Size and Drive Space

Every scanning job starts with three questions: how many pixels a setting gives, how big the masters will be, and how much disk space the project needs with more than one copy. The calculator above answers all three, for a snapshot, a letter, a 35 mm slide or a ledger page alike.

How to use it

1. Enter the width and height of the original, in inches or centimeters, and the scan resolution in dpi.
2. Pick a bit depth: 24-bit color, 48-bit color, 8-bit grayscale, 16-bit grayscale or 1-bit black and white.
3. Read off the pixels, megapixels and uncompressed TIFF size.
4. For an enlargement, enter the long edge of the original, the long edge of the print and the print resolution (300 ppi by default). You get the minimum scan dpi, rounded up.
5. For a project, enter the number of items and the size of one master, or pick a preset (prints, slides, letter pages, ledger pages). Then set access copies as a percentage of the masters, the number of copies and the free space to leave on each drive.

How it works

Pixels: each side in inches times the dpi, rounded (centimeters are divided by 2.54 first). Megapixels: width times height, divided by one million.

File size: pixels times bytes per pixel (3 for 24-bit color, 6 for 48-bit color, 1 for 8-bit grayscale, 2 for 16-bit grayscale, 1/8 for 1-bit), the size of an uncompressed TIFF without its few kilobytes of header.

Enlargement: print long edge divided by original long edge, times the print ppi. Backwards, pixels divided by ppi gives the largest print a scan supports.

Storage: masters (items times MB per item) plus access copies make one set; one set times the copies is the total; one set plus the headroom percentage is what each drive must hold, matched to the smallest common drive size (500 GB, then 1, 2, 4 TB and up). Sizes are decimal, as drive makers count: 1 MB is 1,000,000 bytes.

On the labels: DPI (dots per inch) strictly describes how densely a printer lays down ink; PPI (pixels per inch) counts the image pixels on each inch of the original or the print. Scanner software calls its setting dpi, but it really sets pixels per inch of the original, so here the two mean the same thing.

Worked example

A 6 x 4 inch print, scanned at 600 dpi in 24-bit color:

• 6 in x 600 = 3,600 pixels across; 4 in x 600 = 2,400 pixels down.
• 3,600 x 2,400 = 8,640,000 pixels, or 8.64 megapixels.
• 8,640,000 pixels x 3 bytes = 25,920,000 bytes: 25.9 MB as an uncompressed TIFF.
• To print the long edge at 12 inches and 300 ppi: 12 / 6 x 300 = 600 dpi. The scan you already have is enough.

Now the project: 1,000 such scans, access copies at 10 %, 3 copies, 20 % free space per drive.

A 500 GB drive is far more than 34 GB needs; the spare room absorbs the next shoebox relatives hand over.

Choosing resolution and bit depth

The Federal Agencies Digital Guidelines Initiative (FADGI) grades image quality in star levels by type of material, and most cultural heritage guidance follows the same logic: the smaller the original and the finer its detail, the more pixels per inch it needs. That usually means 300 to 400 ppi for typed and handwritten pages, 600 ppi for prints and several thousand ppi for 35 mm film. FADGI also checks what a scanner really delivers with test targets; a setting above the optical resolution only interpolates.

Choose 48-bit color or 16-bit grayscale to rescue faded color or dark shadows, since the extra levels survive heavy editing. Twenty-four bit suits most prints and documents at half the size. Save 1-bit for clean text where only legibility counts; it discards pencil notes and stamps. See imaging and microfilm as a preservation copy for the full plan, and scanning bound volumes without breaking them for ledgers.

Limits

• Compression changes real sizes. LZW or ZIP in a TIFF is lossless, but pages with white margins shrink a lot, grainy photos much less, and LZW can even enlarge 16-bit files. JPEG access copies often run a tenth of the master or less, hence the 10 % default.
• Systems that count in binary (1 MB = 1,048,576 bytes) show the 25.9 MB example as about 24.7 MB.
• Edit files, versions, thumbnails and checksum files are not counted; raise the headroom if you keep them.
• A higher setting is not always worth it: a faded snapshot at 1200 dpi gains grain, not detail.

Why three copies

The default of three copies follows the usual digital preservation rule: three copies, on at least two kinds of storage, one in another building. The NDSA Levels of Digital Preservation build the same way, from two copies in separate places to three with one in a different region. An unchecked drive is a guess, not a backup, so compare checksums now and then and replace drives on a schedule. Our piece on file formats for long-term preservation covers fixity checks and the formats worth keeping as masters.

Frequently asked questions

How do I convert dpi to pixels?

Multiply each side in inches by the dpi. A 6 x 4 inch photo at 600 dpi is 3,600 x 2,400 pixels.

What is the best dpi for printing a scanned photo?

Aim for about 300 pixels per inch at print size: divide the print size by the original size, multiply by 300, and scan at that dpi or higher.

What resolution should I scan old photos at?

600 dpi in 24-bit color is a sound baseline for prints; 1200 dpi for wallet photos, contact prints or big enlargements; 48-bit if the color needs heavy correction.

How big a drive do I need for a scanning project?

Enter your item count and master size in the storage planner. It adds access copies and headroom and suggests a size for each drive; plan one drive per copy.