ExplainerPrinters and Scanners
Scanner resolution (DPI) and file formats explained
Understand what DPI means when you scan, which resolution suits documents and photos, and how to choose between PDF, JPEG, PNG and TIFF so files stay the right size.
- Any device
Two settings decide how a scan looks and how big the file is: the resolution (measured in DPI) and the file format. Here they are in plain words, with sensible starting choices for documents and photos.
What DPI means
DPI stands for dots per inch. A scanner turns a page into a grid of tiny dots, called pixels. The DPI number says how many dots it captures along each inch of the page. A higher number captures finer detail.
The effect on file size is bigger than many people expect. DPI applies both across and down the page, so doubling the DPI makes four times as many dots. For example, a 6 by 4 inch photo has 1,800 by 1,200 pixels at 300 DPI. At 600 DPI it has 3,600 by 2,400 pixels, four times as many, and the file is much larger.
More DPI is not always better. Once the scan is sharper than your eye, your screen or your printer can show, extra DPI only makes a larger file that is slower to open and send.
Text version of this diagram
- 150 DPI: Detail 2 out of 5, File size 1 out of 5, Scan speed 5 out of 5. Reading on screen.
- 300 DPI: Detail 3 out of 5, File size 3 out of 5, Scan speed 3 out of 5. Common starting point.
- 600 DPI: Detail 5 out of 5, File size 5 out of 5, Scan speed 1 out of 5. Photos you may enlarge.
Which DPI to choose
These are common rules of thumb. Your scanner may offer slightly different steps, and the app may call the setting Resolution.
| What you are scanning | Starting DPI | Why |
|---|---|---|
| A page to read on screen or email | 150 | Small file that is quick to send |
| Letters, forms and bills you may keep or print | 200 to 300 | Sharp enough to read and to run text recognition |
| A photo to view or print at the same size | 300 | Good detail at normal sizes |
| A photo you want to enlarge, or a small old print | 600 or more | Extra detail to scale up |
If you are unsure, scan one page at 300 DPI and check it. Zoom in on the screen. If the text or faces are clear, you do not need more.
Color, grayscale or black and white
Many scan apps let you choose how color is captured. On a Mac, the Kind setting offers Text, Black & White or Color in the scanner window under Show Details. Text gives high-contrast black and white, Black & White gives grayscale. Both make smaller files than color and suit text pages. Choose color for photos, stamps, highlighted notes and anything where color matters.
Common file formats
| Format | Suited to | Things to know |
|---|---|---|
| Documents, especially several pages | Opens on almost any device. Can hold many pages in one file, and with text recognition (OCR) can become searchable | |
| JPEG | Photos to share, email or post | Smaller files, but it loses a little detail every time it is saved again |
| PNG | Screenshots, graphics, line drawings | Keeps all the detail; files are usually larger than JPEG for photos |
| TIFF | Keeping a high-quality master copy | Files can be very large. Not every app can open them easily |
| Bitmap (BMP) | Rarely needed | Very large and uncompressed. Prefer another format |
Which formats you see depends on the app. The Windows Scan app offers JPEG, PNG, TIFF and Bitmap. On a Mac, the scanner window has a Format menu, and with PDF you can turn on text recognition and combine several items into one file. Many scanner makers also supply software that saves PDF.
Text version of this diagram
- A document: Pick PDF if your app offers it
- A photo to share: Pick JPEG for a smaller file
- A photo to keep: Pick TIFF or PNG and make JPEG copies later
What OCR is
OCR (optical character recognition) reads the shapes in a scan and turns them into real text. After OCR, you can search a PDF for a word, and copy text from it. OCR works well on clear scans of printed text at about 300 DPI, with the page flat and straight. Handwriting and faded pages are less reliable. On a Mac, OCR appears as an option when the format is PDF. If your scanning app does not offer it, check the scanner maker's own software.
What this means for you
- Start at 300 DPI for documents and photos, then adjust only if the result is too big or too soft.
- Use PDF for paperwork and JPEG for photos you send around.
- Keep a high-quality original of precious photos, such as a TIFF or high-DPI scan, and back it up. See how to back up your files.
- Check the first scan before doing a whole stack, because it is hard to repeat 50 pages.
- For the steps to scan, see how to scan a document to your computer. If the scanner is missing, see scanner not detected.
Frequently asked questions
Does a higher DPI always give a better scan?
No. It gives more detail only up to a point, and the file gets much larger. Past what you need to read or print, a higher DPI adds size, not value.
Why is my scan file so large?
High DPI, color and formats such as TIFF or Bitmap all make big files. Try a lower DPI, grayscale for text pages, or PDF or JPEG.
What is the difference between DPI and PPI?
DPI is the term printers and scanners use for dots. PPI (pixels per inch) is the word used for images on screens. For scanning, people use them loosely to mean the same thing.
Can I send a scan by email?
Yes. Check the size first. A few pages at 150 to 300 DPI as a PDF or JPEG is usually small enough, but many pages or color photos can be too big for email.
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