We've known the basic guidelines for writing for the Web since our earliest studies in 1997. A key finding is that most website users don't read all your words. Instead, they scan the text and pick out headlines, highlighted words, bulleted lists, and links. Scanning is even more prevalent for readers of email newsletters.
(There are a few exceptions to this rule: lower-literacy users can't scan, while higher-literacy users read the entire page if they're really interested or in desperate need of the information. But it's the height of arrogance to assume that all of your customers are extraordinarily interested in everything you write — more likely, they'll read a few pages and scan the rest.)
One of eyetracking's greatest benefits is that it lets us follow users' reading behaviors in great detail, especially when we watch slow-motion gaze replays after test sessions. Our recent eyetracking studies have given us new insights into how users read various website elements, including bulleted lists, the table-of-content-style list of links at the top of many FAQs, and those rare advertisements that actually attract fixations.
Among our discoveries was that numerals often stop the wandering eye and attract fixations, even when they're embedded within a mass of words that users otherwise ignore.
Why do users fixate on numerals? Because numbers represent facts, which is something users typically relish. Sometimes people are looking for specific facts, such as a product's weight or size, so product pages are certainly one place where you should write numbers as numerals. But even when a number doesn't represent a product attribute, it's a more compact (and thus attractive) representation of hard information than flowery verbiage.
How do users' eyes locate numerals while skipping past words? The shape of a group of digits is sufficiently different from that of a group of letters to stand out to users' peripheral vision before their foveal vision fixates on them. 2415 looks different than four even though both consist of 4 characters. (As the previous sentence shows, stating the number of characters as a numeral makes it stand out, even without the bold highlighting.)
Digits enhance the scannability of web content. It's that simple.
Departing from Traditional Writing Style
Traditional copywriting style guides for print publications dictate that you spell out many numbers. For example, The Chicago Manual of Style says to spell out:
"whole numbers from one through ninety-nine;"
any of these numbers "followed by hundred, thousand, hundred thousand, million, and so on;"
very big numbers (millions, billions, etc.); and
numbers that appear as the first word in a sentence.
I've frequently said that the guidelines for online writing differ from those for writing for print. I can now add that the guidelines for presenting numbers are different for websites than for print publications.
When writing for the Web:
Write numbers with digits, not letters (23, not twenty-three).
Use numerals even when the number is the first word in a sentence or bullet point.
Use numerals for big numbers up to one billon:
2,000,000 is better than two million.
Two trillion is better than 2,000,000,000,000 because most people can't interpret that many zeros.
As a compromise, you can often use numerals for the significant digits and write out the magnitude as a word. For example, write 24 billion (not twenty-four billion or 24,000,000,000).
Spell out numbers that don't represent specific facts.
As an example of the latter, if I say something like "in recent years, we have tested thousands of users and seen their use of breadcrumbs increase," it's better to write "thousands" as a word than to write "1,000s" or something like that. "Thousands" is not really data in this context, it's intended to give an idea of the scope of the research. On the other hand, it's better to use numerals when stating the exact number (e.g., "we have tested 2,692 users"). Disclosing the exact number also increases the statement's credibility.
In addition to spelling out extremely big numbers, you might also need to explain them if you write for a non-scientific audience. You might, for example, say that a trillion is a thousand billions.
Most people don't understand numbers above a billion, and many people don't even know what a billion represents. There's also an internationalization issue here: "billion" represents a thousand millions in American English, but a million millions in many European languages. Thus, the guideline is:
Explain numbers starting with a trillion for most audiences. Explain numbers starting with a billion if you write for less numerate audiences or international users (you might say, for example, that "a trillion is a thousand billions.")
Also, in testing e-commerce sites over the last few months, we found that most users couldn't understand phrases like "1 TB," which some sites use to state hard-drive capacity. Few users had previously encountered storage media in the terabyte range, so they had no clue what "1 TB" meant. Thus, the guideline is:
Explain unusual abbreviations and measurement units, particularly those representing very big numbers.
For now, sites that sell computer equipment should spell out and explain TB when they use it. In a few years, people will be accustomed to the unit and won't need such explanations. Thus, the detailed design guideline to explain TB might remain in force for only a few years. In contrast, the general usability guideline to explain unusual units will probably stay valid forever.
Digits Boost Usability and Credibility
For specific facts, representing numbers using digits rather than letters increases usability for people who are looking for either a particular piece of information or the gist of a page. This often includes e-commerce shoppers and very often includes customers visiting B2B sites, whether they're shopping or just doing research.
Even when users aren't scanning for data, having your facts stand out visually by presenting them as numerals is an easy way to enhance credibility by making your page seem more useful.
Full eyetracking report on how users read on the web is available for download.