Notes on Guestimation

I can't tell you the number of times I've had to estimate a dimension when I haven't had access to a ruler. These are some of the tricks I've used over the years. It's surprising how accurate they can be.

Distance

Your wingspan (finger tip to finger tip) is usually the same as your height.
A size 10 shoe is just about one foot long.
A penny is almost exactly ¾" in diameter, and 17 pennies stacked up are an inch tall.
A penny is 19mm in diameter, and 1.5mm thick. Seven stacked are just over a centimeter.
Find two wrinkles on your fingers that are exactly one inch apart, and memorize which ones they are.

Mass and Weight

A pint of water weights about one pound. A gallon weighs just over eight pounds.
Eighty quarters weigh one pound.
A pre-1982 penny weighs 3.1 grams.
A post-1982 penny weighs 2.5 grams.
A nickel weighs 5.0 grams.

Time

An object will fall sixteen feet in very close one second.
A pendulum 3'-3" long will have a period (tick AND tock) of almost exactly two seconds. One second for the tick, one for the tock.
Measure your pulse at rest, and remember what it is. This is how Galileo timed pendulums and falling weights.

Pressure

As hard as you can blow is about 1.5 psi.
As hard as you can suck with your lungs, like you do when you siphon gas, is about 1 psi.
As hard as you can suck with your tongue, like you do when you suck a thick shake through straw, is about 4 psi.
The pressure inside a ballon is actually quite low - 0.3 psi. The smaller the balloon, the higher the pressure.

Angles/Slope

A 1-on-4 slope is 14°.
A 1-on-3 slope is 18°.
A 1-on-2 slope is 27°.
A 2-on-3 slope is 34°.
A 1-on-1 slope is 45°.
A 3-on-2 slope is 56°.
A 2-on-1 slope is 60°.
A 3-on-1 slope is 72°.
A 4-on-1 slope is 76°.
1% of slope is approximately equal to 1/8" per foot. Thus, a 4% slope is about 4/8" or 1/2" per foot.
There are three 60° angles in a semi-circle, and six in a full circle.
The corners of an equilateral triangle (where all the sides are equal) are 60°.
The width of your finger, held at arms length, is about 2°. Your thumb is about 2.5°.
The width of your palm, held at arms length, is about 10°.
The thumb-to-pinky span of your hand, fingers spread, is about 20°.

Metric/US Conversions in Your Head

Convert binary fractions (½ , ¼, etc) of inches into millimeters by expressing them in 256ths (double top and bottom until you get there) and then dividing the numerator (the one on top) by ten to get millimeters. For example, 17/32nds = 136/256ths, or 13.6 millimeters. (It's actually 13.5, but that's usually close enough.) Naturally, you can reverse this: 16 millimeters = 160/256ths, or 5/8ths.
Convert inches to millimeters by multiplying by 25 - or multiplying by 100 and dividing in half twice - and adding one for every 2½ inches.
Convert kilometers to miles by multiplying by ten and dividing in half four times, rounding down each time to make odd numbers even. 55 kilometers equals 550 ÷ 2 = 275, call it 274 ÷ 2 = 137, call it 136 ÷ 2 = 68 ÷ 2=34 miles. (It's actually 34.2 miles)
Convert miles to kilometers by dividing by ten and doubling it four times. For example, 88 miles = 8.8 x 2 = 17.6 x 2 = 35.2 x 2 = 70.4 x 2 = 140.8 kilometers. (It's actually 141.6 kilometers)

References

E-mail me at bill@gizmology.net if you find a mistake!

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