Metric, fractions or decimals. This is about decimals.

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Blog entry by GaryK posted 02-03-2011 07:51 AM 39191 reads 3 times favorited 25 comments Add to Favorites Watch

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.

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

25 comments so far

View lilredweldingrod's profile


2495 posts in 2612 days

#1 posted 02-03-2011 07:56 AM

Thanks Gary, I appreciate your reasoning. No arguments form me. Rand

View a1Jim's profile


115348 posts in 3082 days

#2 posted 02-03-2011 08:00 AM

Good Blog Gary ,I convert to fractions too it seems easy after using them all these years. This should be very helpful for those who are not use to converting to fractions.

-- Custom furniture

View canadianchips's profile


2421 posts in 2502 days

#3 posted 02-03-2011 08:10 AM

Thanks Gary. I started converting fractions to using decimals in early 80’s, I was using a lotus spreadsheet to help me calculate how much solid material I needed to purchase on each job. I had to enter the numbers that my spreadsheet would recognize. I would simply enter the numbers down 2 columns, (say 4 pieces at 1.325 or 6 @1.5 or whatever, at the end of my column it automatically calculated what I needed.It was a very simple spreadsheet, it did save me a lot of time. After seeing some very detailed work on here I realize why micro-measurement can be important. Some projects are borderline ARTWORK, absolutley beautiful !

-- "My mission in life - make everyone smile !"

View Tony's profile


978 posts in 3535 days

#4 posted 02-03-2011 08:59 AM

Having been educated some 45 tears ago, when I was young and impressionable!, using both fractions and the new fangled metric system, I find merits in both and quite often in my drawings will be in both fraction and metric, which ever is easiest for the measurement (I do not advise doing this) I am trying to record, both systems have there merits, although I cannot say I have ever used decimal inches, except when we get down below 1/64th (machine settings an tolerances) – we are wood workers after all, not machinists.

What did surprise me last year when I was teaching a class of 18+ year old students (Finland, Europe) who only knew the metric system, they did not understand what a fraction was – They knew what a half and a quarter were, but that was as far as it went, they could not grasp the basic algebraic concept- lets hope that fractions are not lost purely to decimals – they are so easy to use and manipulate when you understand the basic principles.

Thanks for posting the chart Gary, I hope this comment does not start any debates on the subject.

-- Tony - All things are possible, just some things are more difficult than others! - SKYPE: Heron2005 (

View BertFlores58's profile


1684 posts in 2427 days

#5 posted 02-03-2011 09:01 AM

Thanks for this. It is very educational…

Just want to add…

Though we call this kind of caliper as micrometer… it comes into Inches or MM. It has also different ranges.
It works like a dial caliper. Just add sleeve reading and the timble reading (dial scale in a dial caliper) and you will get the actual reading.

how to read….

If anybody wants a vernier scale, it is very easy to make….
1. Make a scale on a piece of paper that is less 1 unit of the original scale (for example 1/16).
2. Divide to desired scale (8 points where 1 point = 1/128 or 10 if in decimal)

3. Use by aligning the 0 point to the last point in the main scale and look for the coinciding line in the vernier to get the actual reading.

Herebelow is my illustration:

-- Bert

View TopamaxSurvivor's profile


17731 posts in 3181 days

#6 posted 02-03-2011 10:42 AM

I love the vernier scales on my long range sights. Sure makes repeat setting easy when shooting 1,000 yard black powder competitions ;-)

For us 3/4” tape fans, here is one that has decimel on it:

-- Bob in WW ~ "some old things are lovely, warm still with life ... of the forgotten men who made them." - D.H. Lawrence

View spunwood's profile


1198 posts in 2341 days

#7 posted 02-03-2011 02:45 PM

Cool stuff guys. Thanks. To think math used to be my most dreaded subject, and now I find it exciting.

-- I came, I was conquered, I was born again. ἵνα ὦσιν ἓν

View brunob's profile


2277 posts in 3675 days

#8 posted 02-03-2011 03:00 PM

Thanks Gary,
Very informative and useful.

-- Bruce from Central New, if you'll pardon me, I have some sawdust to make.

View CharlieM1958's profile


16248 posts in 3724 days

#9 posted 02-03-2011 03:19 PM

The world would be a nicer place without fractions. :-)

-- Charlie M. "Woodworking - patience = firewood"

View Seer's profile


304 posts in 3148 days

#10 posted 02-03-2011 03:24 PM

I am not a math wiz but this is what I keep tacked on my wall to help with these:
To get decimal inches divde the Millimeters by 25.4 to get Decimal Inches (This cones in handy for penturning to get the right drill bit)
Fraction to Decimal is the first number Divided by the Second number ie: 15/32 is 15 diveded by 32
I know this is probably what we should know but this little bit has helped.


View BertFlores58's profile


1684 posts in 2427 days

#11 posted 02-03-2011 03:39 PM

Additional tip…. Almost exact equivalent are as follows:

19 mm = 3/4 inches
16 mm = 5/8 inches

1/4 = 6.3 mm
1/2 = 12.7 mm

With the above I easily convert by counting in the tape how many 3/4 I have then multiply it with 20 mm then minus 1 mm on how many I counted.

Example: 1 1/2 = 2×3/4 then it will be …. (2×20 mm) – (2×1mm) = 40 – 2 = 38 mm

On the other hand… I divide the mm by 12.7 mm to get 1/2
Example 30 mm / 12.7 = 2.36 That means 2×1/2 inch = 1 inch + 0.36 inch

-- Bert

View Peter Oxley's profile

Peter Oxley

1426 posts in 3380 days

#12 posted 02-03-2011 04:15 PM

Good info, Gary! I used to do drafting several hours a day for metalworking. I constantly went back and forth between 64ths and 1000ths. I had memorized all the decimal equivalents to 32nds and knew some of the 64ths off hand. I’ve forgotten most of them now, but still remember most of the 16ths … so I have a little chart like the one you posted that hangs on the wall by my desk, and another out in the shop.

-- -- --

View northwoodsman's profile


242 posts in 3252 days

#13 posted 02-03-2011 05:08 PM

Back in 1996 I was in Hungary working on a project for HfH. We were told to bring metric tape measures with us becuase the materials and blueprints were metric. It was very easy to learn the system. I cannot imagine how hard it would be for them to learn our system of fractions (Imperial) in a pinch. I remember back to my school days when we were taught the metric system becuase “any day now” we would be switching.

-- NorthWoodsMan

View lew's profile


11416 posts in 3261 days

#14 posted 02-03-2011 05:11 PM

Great info, Gary.

Still learning how to do this, mostly out of necessity because of the digital electronic measuring devices. Still have a little trouble navigating the micrometer, however.


-- Lew- Time traveler. Purveyor of the Universe's finest custom rolling pins.

View woodsmithshop's profile


1255 posts in 3051 days

#15 posted 02-03-2011 05:22 PM

I am very glad that I have a calculator that converts for me. lol

-- Smitty!!!

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