We are often asked: what resolution is best for scanning?
There is not a simple one-size-fits-all answer to this question. Traditionally, document imaging systems were used for business documents (e.g., black and white documents, letter sized, letters, contracts, etc.). These documents were commonly faxed or mailed between businesses. The ITU (or CCITT) developed standards for the transmission of business documents via telephony, with the most important consideration being the transmission time. Using a combination of Huffman encoding (compression) and the minimum resolution for readability, 200 dots per inch (dpi) was the early standard for faxes, and became the de facto standard for document imaging and scanning.
As laser printers became popular, 300 dpi scanning became more prevalent because most early laser printers were 300 dpi, and scanning at the same resolution as printing saved processing time and thus yielded higher print speeds.
Geek alert! A 300 dpi image is more than twice the size of a 200 dpi image (3/2 squared = 9/4 or 2.25). With compression, the compressed image ratio is a little better for black and white images since the standard compression method mostly encodes transitions from white-to-black or black-to-white (which is independent of resolution) with one exception: the Huffman codes for the standard compression method (CCITT Group IV) were optimized for 200 dpi documents. The net: a compressed 300 dpi black-and-white image will typically be somewhere between 50% and 100% larger than the same image at 200 dpi.
So back to the original question: for black and white business documents, organizations such as ARMA and the Federal records administration (NARA) recommend scanning at a minimum resolution of 300 dpi. mindwrap also recommends this as a general standard for newly scanned documents.
Then, why not scan at even higher resolutions? Those images take up disk space, network bandwidth, and computer memory. Disk space is pretty cheap, but adds up over tens or hundreds of millions of documents. Network bandwidth and computer memory impact the operation of your system and its perceived speed. Therefore, the answer is still to scan at the lowest resolution that provides reasonable readability of the scanned documents.
Also keep in mind that although some scanners support high resolutions such as 1200 dpi, their actual resolution or optical resolution may be lower. Although the image may have more pixels, it cannot have more information than the optical resolution.
For color and grayscale images, you have two additional factors working against you. First a grayscale image is typically eight (8) times the size of the same black and white image, and a color image is typically 24 or 32 times the size of the black and white. Second, compression algorithms for color and grayscale are nowhere near as efficient as black and white, unless you are willing to throw away a lot of data through lossy compression (which sort of works against scanning at high resolution).
If you are just dealing with business images, consider scanning these documents as black-and-white. The color doesn't necessarily provide any information that you want to keep for the long term.
If you are merely trying to capture a representation of a piece of color art work, try a lower resolution scan such as 75, 100, or 150 dpi. If you need to capture high resolution color, then by all means you can increase the resolution, but remember that your system will be slower because of the amount of data you must move across your network and memory needed in your desktop or laptop computer. If high resolution is important, then you probably also care about color rendition or gamma correction so that the image captured has the same colors as the original - but that's a whole different topic.