What is the DPI of my scans?

The DPI (dots per inches) is a measure of the number of pixels the scanner will associate to one inch of document. The more, the higher the quality.

The number of points depends on the phone's camera resolution and on how much you crop the scan.

The inches depend on the physical size of the document (eg. is the document a Letter page, a receipt or a business card?)

If you maximize your document in the camera preview, your scan won't be cropped and will have about the number of pixels of the camera. For instance, on a iPhone 6 with an 8 MP pixel camera (3264 dots), if you scan a Letter page and it's full screen on the camera and then print it on a letter page (11 inches) you will get about 3264 dots/11 inches = 296 dpi.

How to maximize the DPI?

Make sure the document takes as much of the camera preview as possible.

Bad

The document is too far away from the camera and will have a low resolution after cropping

Good

The document fills the camera frame.

Use Actual settings for the document Resolution.

What DPI can I achieve with my iPhone?

iPhones with a 12 MP camera (iPhone 6S Plus, iPhone 6S, iPhone SE)

Type of scan	Letter document	A4	Receipt
Maximized on the preview	~ 360 dpi 4000 pixels/11 inches	~ 340 dpi 4000 pixels/11.69 inches	~ 2000 dpi 4000 pixels/2 inches
Cropped (taking ¾ of the preview)	~ 270 dpi 3000 pixels/11 inches	~ 250 dpi 3000 pixels/11.69 inches	~ 1500 dpi 3000 pixels/2 inches
Cropped too much (taking ½ of the preview, not recommended)	~ 180 dpi 2000 pixels/11 inches	~ 170 dpi 2000 pixels/11.69 inches	~ 1000 dpi 2000 pixels/2 inches

iPhones with a 8 MP camera (iPhone 6 Plus, iPhone 6, iPhone 5S, iPhone 5, iPhone 4S)

Type of scan	Letter document	A4	Receipt
Maximized on the preview	~ 300 dpi 3296 pixels/11 inches	~ 280 dpi 3296 pixels/11.69 inches	~ 1500 dpi 3296 pixels/2 inches
Cropped (taking ¾ of the preview)	~ 220 dpi 2472 pixels/11 inches	~ 210 dpi 2472 pixels/11.69 inches	~ 1200 dpi 2472 pixels/2 inches
Cropped too much (taking ½ of the preview, not recommended)	~ 150 dpi 1648 pixels/11 inches	~ 140 dpi 1648 pixels/11.69 inches	~ 824 dpi 1648 pixels/2 inches

I have followed the recommendations to get high DPI. Why are my document still not sharp?

While having a high resolution document is important for it to be readable when printed or faxed, other factors such as light and a steady hand will also have a major impact on the quality.

Why do my PDF scans always show up as 72 DPI?

PDF files are a bit weird in that they are vector documents. However, if you have scans with higher DPI in a PDF document, they will be printed at their full resolution even though the PDF file is "only" 72 DPI.