Idiom: rule of thumb; used as a noun
Karla: How many pizzas do you think I should order for the party?
Dan: How many people are coming? And are you going to have anything else?
Karla: I'm expecting about 20 people. And no, just the pizza. I want to keep it simple.
Dan: Well, I think a good rule of thumb is to get one large pizza for every three people, so I'd go with 7 pizzas.
Karla: One pizza per three people; isn't that a bit too much?
Dan: Most people only take a couple slices, so you might end up with a bit left over, but it's better to have too much than not enough. And make sure you get a good variety, including a couple vegetarian pizzas.
Karla: Yeah, everyone always eats the veggie pizzas, even when they're not vegetarian!
Meaning: A rule of thumb is a general rule established by a person based on personal experience. It's not an exact rule, but it's considered a good estimate. In the example above, Karla is unsure of the number of pizzas to get, so Dan suggests a number based on his personal rule of thumb: one pizza per person. It might be too many pizzas, but it's an estimate based on his own experience. Here is another example:
In general, gardens need about an inch of water per week. Of course, this is only a rule of thumb, and some plants need less water while others need more. Watch the leaves. If they turn yellow (a sign of too much water), water less; if they start to wilt, water more.
Meaning: In this case, the rule of thumb is about how much water plants in gardens need.
Note: There are a couple theories about the origin of the phrase "rule of thumb," but most people think it comes from builders using the width of their thumbs to estimate an inch. Try it. Put your thumb against a ruler. Is it about an inch?
This idiom is from LSI's new edition of "Reading Horizons," which will be used in the Level 6 Reading classes. For more information, please visit http://www.languagesystems.com/