# 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 palm, held at arms length, is about 10°.