Rek Bell

"Good Enough Arithmetics," coined by Don Norman, permits us to estimate the answer to basic arithmetic equations in our heads with reasonable accuracy, without using a calculator or a pencil and paper.

Temperature conversions

Say you are a Canadian traveler in the United States and are unfamiliar with temperature readings in fahrenheit, how do you convert it back to celsius to know if you ought to wear a sweater?

The below equations to make the conversions from °F >°C and °C >°F, are difficult to do in one's head...

°C > °F   multiply by 9/5 and add 32.
°F > °C   subtract 32 and multiply by 5/9

Examples:

What is 25 celsius in farenheit?

25 x ( 9 / 5 ) + 32 = 77 °F

What is 56 fahrenheit in celsius?

56 - ( 32 x 5 ) / 9 = 13.3 °C

As stated before, the above formulas are difficult. For most purposes, anything within 5 °F of the real value works. To do a simple conversion without a calculator, use the following simplified formula:

(°F - 30) / 2 = °C
Subtract 30 and divide by 2

An example:

What is 65°F in Celsius?      .._     _..
                             / | \\_// | \
(65 - 30) / 2 = 17.5 °C     /*/|*.*.*.*|\*\
                            |_||x-x-x-x||_|
                            '''|_______|'''
                               '''''''''
                           Sweater? Yes? No?

For the above example, the accurate temperature conversion from fahrenheit is 18.3 °C. Close enough. I won't need to bring a sweater.

Multiplying Big Numbers

Most people cannot multiply big numbers in their heads quickly. Like when estimating temperature, estimating the answer by rounding a number up(or down) is good enough for most purposes.

Say you want to multiply 54 x 302, change 54 to 50 and 302 to 300.

50 x 300 = 15000
 5 x 3   = 15, then add back the 3 zeros

The accurate answer is 16,308. The above estimate is a bit too low, but as previously stated, that is often good enough. In the above equation, I changed 54 to 50, removing 4. To get a more precise result, take 15000 and add back 4 x 300 to the answer:

  
                          *         *      *  *
    *                                  *     *  /\
                *              *        * *    / /
4 x 300 becomes 4 x 3              * .___*___./_/
                                    |  _   _  |
The result is 12.         *        [| ^  |  ^ |]
Add back the 2 zeros to make 1200.   \___-___/
15000 + 1200 = 16,200               _(|| | ||)_ 
                                   /.|.......|.\  

The above is now way more precise, without being 100% accurate. If rounding the numbers up instead of down, substract instead.

In short, to get good enough answers:
+ Round numbers up, or down
+ Simplify equations using smaller numbers
+ Add back the zeros

When precision and accuracy are desired, use a calculator.

Good Enough Measurements

When you want to take quick measurements without the need for precision, consider measuring different parts of your hand and using that as a measuring device. Memorizing 5 numbers ought not to be too difficult.

Your hand goes with you wherever you go, while you may forget to take a ruler or measuring tape with you.

    3-finger span
    <----------->          
         ___                 Forefinger   _
     ___||_||___              length    /'|
    ||_||   ||_||           <---------->|  \   
    |   |   |   |             _________ /   `\
    |   | ~ |   |___        (__'__'__. |      |_ 
    | ~ |   | ~ ||_||           |____)  \     .
 __ |   |   |   |   |	         |___)   /    -
\_|\|   |   |   | ~ |             |__).______.-----
|   |   |   |   |   |                        
|   / "   "   "     \      Hand       Span   
/  |              "  |          _ _ _     
|  |<--------------->|     <---|-|-|-|--->
|  .     Palm span   |     _   | | | | /`)
|                   .|     \'\ | | | |/ /
 \      Wrist      ./       \ \|" " " "/
   \.    span    ../         |         |
    |<----------->|           \       /
    |            .)            \      | 

Measure the following:

• Hand Span. Outstretch your hand, measure the distance between the tip of your little finger to the tip of your thumb.
• Palm Span. Measure the width of your hand, from the base of the thumb(w/o the width of the thumb) to straight across, to the outer edge of the palm.
• Forefinger Length. Measure the length of your forefinger.
• Wrist Span. Measure the span of your wrist above the wrist bone, just before the hand begins.
• 3-Finger Span. Put your fingers together, measure the width of the 3 middle fingers together.

For more detailed instructions, visit Samina's Sew Everything website.

Large ASCCI hand modified from a design by David Berner. The two hands on the right, as well as the open head guy and the sweater were done by me.