# Metric, fractions or decimals. This is about decimals.

 Blog entry by GaryK posted 02-03-2011 07:51 AM 25056 reads 2 times favorited 25 comments

This post is not to start any arguments or debates about what is best, but to explain the decimal system to those not familiar with it. Please don’t let this blog devolve into an argument about which is best. There are other blogs where that is discussed.

First a quick overview on the different measuring systems.

Here is a good example of the 3 main measuring systems in the world side by side on this Incra Pro scale.

Metric is used by most all the world. You can see it at the bottom of the scale.
1mm and .5mm

Above that is the inch decimal system in 100ths (.010) of an inch.

Above those are the fractions down to 1/64”
Fractions came about because in the old days it was real easy to divide things in half.
Then in half again and so on. 1/2 – 1/4 – 1/16 – 1/32… Anyone could make a fractional scale.

The area I am going to talk about is decimal inches. They are a commonly used in lieu of fractions.

This is basically 1 inch divided by 1000.

1/1000 of an inch. Represented like this it’s a fraction.
You want a foolproof way to convert a fraction to decimal? Divide the top number by the bottom number

Take ½ for example 1 divided by 2 is .5 Just add zeros to get 3 decimal places.
.500 or expressed as 500 thousands. 1/8” would be .125 (thousands) Foolproof.

This is probably the best conversion chart I have found.

Since a lot of people are using CAD programs like Sketchup where you enter dimentions, using decimal makes it real simple. Math is a lot easier also. Try adding 7/16 and 1/8. Adding .437 and .125 is a lot easier.

Once you start using decimals you will see 1/4 written down and automatically think .250 (250 thousands)
When I am writing dimensions on a drawing by hand I will still write 3/4 and .750 on different parts of the same drawing. Old habits die hard. In my mind they are the exact same thing, but when I add 3/4 to something I will always use .750.

So using decimal in a CAD program is easy, but what about measuring something in the real world? Well, there are scales that use decimals. The Incra Pro scale pictured above for example. For things under 6” you can use calipers. I use a dial version since it will never run out of batteries. You can pick them up for less that \$20 at some places.

Here’s what they look like.

Here’s a little video on how to use one. Video

They are very handy for measuring and marking wood. Set the calipers for what ever dimension you want, press both the sharp ends into your wood and get exact marks.

For things longer than 6” I use a 24” flexible scale. It’s called a 5R scale which means that one side has 1/16 and 1/32 and the other has 10ths (.100) and 100ths (.010).
Here are both faces of the scale:

Most of the time you don’t need things super accurate so using the 10ths part is real easy to see and estimate between the lines.

Look at this picture.

Suppose you want to mark 3/8” (.375) or 3/4” (.750). Just imagine there were 100 lines in between each of those numbers and mark either 375 or 750. You can usually mark them within a 1/32”. Close enough for most things.

But what if you wanted it marked exactly? Well, look at the scale marked in 100ths (actually they are 10 thousands of an inch apart.) Now you wanted to mark .375. Just put your mark between the .370 and .380 lines. That will be twice as precise as a 1/64”.

Have something longer that 24”. Then use a decimal tape measure. Here is one I did a review on.

http://lumberjocks.com/reviews/1156

This is just a quick overview to show you about using decimals in woodworking. If you have any specific questions just ask.

-- Gary - Never pass up the opportunity to make a mistake look like you planned it that way - Tyler, TX