Try this: Count the number of times a cricket chirps in 15 seconds, and add 37. No, you didn't just calculate your age in cricket years - you measured the temperature outdoors in degrees Fahrenheit. You just used a rule of thumb.

A rule of thumb is a homemade recipe for making a guess. It is an easy-to-remember guide that falls somewhere between a mathematical formula and a shot in the dark. A farmer, for in­stance, knows to plant his corn when oak leaves are the size of squirrels' ears. An economics profes­sor knows from sad experience that inviting more than 25 percent of the guests for a univer­sity dinner party from the economics depart­ment ruins the conversation. Rules of thumb are a kind of tool. They help you appraise a problem or situation. They make it easier to consider the subtleties of the topic at hand; they give you a feel for a subject.

A hundred years ago, people used rules of thumb to make up for a lack of facts. Modern­ day rule of thumbing is rooted in an overabun­dance of facts. The average person, confronted with the Internet’s oceans of data and multiple overlapping Ph.D. dissertations, often is as perplexed as a pioneer chemist trying to whip up a little gun­powder without a formula. A pilot in a tight spot doesn't ask questions about aeronautical en­gineering; a pilot in a tight spot asks "now what?" There are times when you don't need to know the best way to do something. These are times for ballpark figures, for knowing what you probably can get away with.

I like getting away with things as much as the next person. What if I had easy access to every rule of thumb on earth? You've seen movies where someone suddenly is dumped on a desert island or thrown back in time. How would you do in a situation like that? Could you impress your ancestors or do any flashy tricks? I think I'd take along a cheat-sheet; a few thousand rules of thumb could come in pretty handy.

So nearly thirty years ago, disguised as the Alpine Planetarium, I sent letters to everyone I could think of, asking for rules of thumb. I mailed letters to businesses and college newspapers. I made a list of maga­zines and wrote to everyone on the staff, from the mail clerk to the publisher, hoping to gener­ate office talk and some rules of thumb.  Sometime around 1980, a friend and I quietly entered a rule of thumb notice on a nationwide defense and research computer network.

Initially, old folks responded with the most enthusi­asm; good friends, the least. Experts and edi­tors fell somewhere in between. There were people who took days to come up with "The rain in Spain falls mainly on the plain." Others spouted four or five winners off the top of their heads. More than anything, the response was unpredictable. A local guitar maker's favorite rule of thumb had to do with stream morphol­ogy. A wind power expert from Cornell University sent one on digging graves.

Stewart Brand published the first batch of rules in CoEvolution Quarterly magazine with a call for further entries. CoEvolution readers sent post­cards and letters from as far away as Alaska, Japan, and Australia. This was all before the Internet. In those days, rules arrived by snail mail. Some were writ­ten on fancy letterheads. Others arrived on nap­kins, stolen office stationery, and airline barf bags. Many people included their ideas about the phrase "rules of thumb." Some suggested that "rule of thumb" refers to the relationship between the human thumb and the length of one inch. Others thought the term came from an old English law that said a man could legally beat his wife and children with a stick no thicker than his thumb.
 
Frankly, I've never been that inerested in where the term comes from. I do know that a rule of thumb is not a joke or a ditty. It is not a Murphy's Law. Murphy says that things will take longer than we think; a rule of thumb says how much longer. Neither do most old-time sayings and expres­sions qualify as rules of thumb. While a proverb says that a stitch in time saves nine, a rule of thumb says to allow one inch of yarn for every stitch on a knitting needle. A maxim cautions us not to risk more than we can afford to lose; a rule of thumb warns us not to lose more than twenty times the betting limit in a single poker game. Most of all, a rule of thumb is not always right. It is simply a personal tool for making things work most of the time, under most con­ditions.

If they were reading carefully, the people at my post of­fice would know how to avoid lunatics on city buses and how to win a duel with pistols (aim at the knees). Some­times I watched as the letter carrier pulled up to the mail­box in her rusty barge of a car. I wonder how she felt about delivering decorated postcards that said, "A rotten egg will float" or "A sex change operation will age you five years." I'll bet she was thrilled to learn that when Americans talk, they stand just far enough apart to reach out and stick their fingers into each other's ears.

With this website, you have at your fingertips all the best rules of thumb I've col­lected since book number one plus everything voted into the Main Collection by online contributors like you. Each rule is followed by a source, or rumored source, or the name of the first person who sent it to me. The people listed are not necessarily claiming a rule of thumb as their own invention. A lot of contributors have sent in rules they heard from someone else. Many sent clippings or paraphrased another source. Some rules arrived as hearsay. Don't feel cheated if your mother's rule of thumb was sent in by an Italian psychologist from Nashville - your mother's rule is getting famous.

Frankly, I can't say that all of these rules of thumb work. Some would be pretty hard to test. How would you check the rule that says one elephant will provide as much meat as one hundred antelopes?
There were lots of snail mail submissions I threw out. One said simply, "Monday is the windiest day of the week." Sure it is. Aaron "Airbag" Simpson, of the University of North Carolina, said, "Never kick a woman out of bed for eating crackers unless she wants to mess around on the floor." Come on, Airbag, you should be studying. Or how about this one? "My rule of thumb is that the leader of the rock band Stone Poneys catapulted ten feet into the audience when his electric guitar got a short in it." Huh? There were weird ones too, like the one written in crayon and signed "Ronald, age 38." Those aren't included here. They look best stuck on my refrigerator door.

But I’m not saying I tossed out everything I should have. In my younger days I was more flexible. There were only two rules of thumb from Nebraska, so I wasn’t about to throw them out. And sometimes the actual rule is less interesting than the person who sent it in. Why would Rob Shapiro, a charter pilot, have a rule of thumb for making tofu from soybeans?

A rule of thumb turns information you have into information you need. The goal of this website is to gather every rule of thumb on earth into one gargantuan, easily searchable online reference database that will be accessible from anywhere in the world and continue to grow forever.

thumb: “On rainy nights, 90 percent of the worms crossing a highway will be facing in the same direction.” It may be awhile before I need to use his rule of thumb, but I like to think of Emery out alone on a warm, wet Pennsylvania night, bent over a patch of flashlight, counting worms in the drizzle.

Then there’s Emmon Bodfish, of Oakland, California. Emmon figures that, as a rule of thumb, a relaxed person needs to inhale twelve times every minute. Emmon’s research also shows that it takes two minutes for the sun to drop out of sight after it first touches the horizon. These are not earthshaking rules, but it's nice to think of Emmon just sitting on his porch swing, counting breaths and watch­ing the sun go down. Emmon's rules stayed in.

Some rules depend on where you live. A rule of thumb for planting beans in Georgia might put you out of business in New York. If Emmon Bodfish was sitting on his porch swing in Nome, Alaska, instead of Oakland, he would have to wait months, not minutes, for the sun to go down.

So please join us and help make this online database the Google of guesswork, the bible for those who like to shoot from the hip. If a certain rule doesn't work for you, give it a low rating, leave a corrective comment, or better yet, in­vent your own version. If something really stinks, write and tell us about it. That's what Lola Marcel of Cleveland, Okla­homa, did. I know she's been doing her homework:

Dear Mr. Parker:

In regard to your book "Rules of Thumb," I would like to know where you got your information on the fact that your wedding ring size is the same as your hat size. My wedding ring size is 5 and my hat size is 6 and my husband's wedding ring size is 9 and his hat size is 7 1/4, so your fact is wrong. Don't you do research before you write a book? If you don't, I think you better start. Plus I think your publishers should check your facts before they publish a book for you. Needless to say, I would not pay any amount for this particular book.
An interested reader,

Mrs. Lola Marcel
Cleveland, Oklahoma

Do I need any more proof that a rule of thumb is a highly personal thing? So im­mortalize yourself: Sign up right now and become a rules of thumb rater and contributor.  The goal of this website is simple: gather every rule of thumb on earth into one gargantuan, easily searchable online reference database that will be accessible from anywhere in the world and continue to grow forever. 

And thanks for the letter, Lola. The rules of thumb ar­chive stands corrected. Heck, I probably shouldn't admit it, but I always thought Cleveland was in Ohio.