Convert fractional inches to decimals in your head

[squishy36]

KeepItSimpleStupid made a comment in another thread about having to look these things up in a table. I wrote up a response describing how I do it; then I thought it would be of more general interest in its own thread rather than buried in another thread.

I didn't mind mills and thousandths when machining things, but I kept having to look at the pesky table for fractions to decimal equivalents.

Memorize that 1/16" = 0.062, 1/32" = 0.031, 1/64" = 0.016. Memorize the decimal equivalents of all the eighths. Then start from a number you know and make corrections.

Example: you want the decimal equivalent of 27/64". That's 1/64th below 28/64 = 14/32 = 7/16. That's 1/16 below 1/2. Thus, doing the problem in mils (0.001"), the answer is 500 - 62 - 16 = 500 - 78 = 422. Thus, you get 0.422".

After a while you come to know the 16ths too; thus, since this is 1/64th below 7/16, you just say "438", "428", "422" in your head. This method will usually get you within a thousandth or so of the true value.

Businesses like Cleveland Twist Drill used to sell handy little circular "slide rules" that would do fraction and number drill conversions along with cutting speed calculations (see attached photo) -- see if anything similar is sold at a local industrial supplier (I've had mine since the 60's). This is handy to keep at a machine. If you can program in e.g. Postscript, such things are not hard to make. Or, find one of the old Gilson circular slide rules, as they had nice fraction to decimal conversions on them. Or print out scale.pdf from **broken link removed** (has other useful stuff on it too).

 

[Reloadron]

Hi Squishy

Years ago companies like Ohmite made little scales that would show resistor colors. Pretty cool stuff. While not a machinist my department does have a small but well equip machine shop where we do prototype work. I have been known to be found there very early morning doing machining work on rifle barrels.

Ron

 

[Ian Rogers]

I found the easiest way is to use our good old friend " hexadecimal " 1" = 25.4 mm if you just add .2 you have 25.6, Most imperial sizes are divisions of 8,16,32..etc..

256 / 16 = 16 (most programmers know this ) so 5/8' = 256 / 8 = 32 (easy) 5 * 32 = 160 (approximately 16mm, actually 15.87mm ) but close enough for drilling and such. For a tiny bit more precision remember the .2 and factor it back in ie 1/2' ends up at 12.8 half of .2 is .1 therfore 12.7 ( exact answer )

 

[squishy36]

Hi, Ian: just to make things clear, you're talking about converting fractional inches to mm; the method I gave was just to convert a fractional inch to a decimal inch. I love the idea, as the 256 part had never occurred to me before, but it's obvious after you mention it.

To convert approximately back and forth between mm and inches, I divide or multiply by 2 twice and shift the decimal point (from 1 mm being 0.03937 inches). The error is 1.6% if you want to make corrections. Of course, these rules are for when all you have is a pencil and piece of paper, as the easiest thing to do is to multiply or divide by 25.4 if you have a calculator -- and it's exact.

Example: 19 mm is nearly identical to 3/4" (which I discovered long ago when I didn't have a metric wrench handy). Double 19 twice to get 76 and divide by 100 to get 0.76. Correct down by 1.6% for more precision; I use 1.5% because it's easier: the 1% gives 0.0076 and the 0.5% is half of that, so we subtract 0.0076 + 0.0038 = 0.0114 to get 0.749. Close enough.

As Ian mentions, this is all you need for drilling-type work.