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 https://www.languagesystems.com/