Approximate answers are often good enough, even if technically wrong.Don Norman, The Design of Everyday Things

"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.*