The idiom rule of thumb, meaning a principle that’s widely useful but not strictly accurate in all circumstances, has origins in the practice of making measurements with one’s thumb.1 In this idiom, rule originally carried one of its lesser used definitions—a straight-edged device used for measuring or drawing lines—but it’s now taken on the more common definition. Now, when you hear the phrase rule of thumb, it’s more or less synonymous with general rule.
Contrary to the web-borne myth, rule of thumb‘s origins have nothing to do with wife-beating, so it’s not against decorum to use this idiom.2
Examples
Here are a few examples of correct rule of thumb usage:
A good rule of thumb is to use less than 10% of the credit that’s available to you. [The Takeaway]
A key rule of thumb to follow when the snowstorm of the century bears down upon you is to not drink too much red wine the first night of the blizzard. [Market Watch]
Good rule of thumb: If they offer you a role that requires messed-up teeth, the role’s probably not that good, anyway. [San Francisco Chronicle]
Writers sometimes use rule of thumb as synonymous with indicator, sign, or clue, but this stretches the idiom into sloppy territory—for example:
One general rule of thumb on the difference between pairs and ice dancing: if the costumes are really wild, it’s ice dancing. [USA Today]
In this sentence, almost any alternative would make more sense. Also, the inclusion of general makes the phrase redundant, as rules of thumb are already general.
References
1. http://www.oed.com/view/Entry/168726 ↩
2. http://www.straightdope.com/columns/read/2550/does-rule-of-thumb-refer-to-an-old-law-permitting-wife-beating ↩
Other resources
“Rule of thumb” at Phrase Finder
“Rule of thumb” at World Wide Words
