How do you tell someone what size something is?
What diameter is that hole?
How long is that part?
What we are really doing is comparing the size of the item in question to some known or agreed upon standard.
What is the standard? Is there more than one kind?
In the old days a foot was the length of the Kings foot or a meter was a fraction of the distance from the north pole to the equator..
In the United States we use two basic systems commonly referred to as the English system or the Metric System.
The English system commonly uses the inch and foot for measures of length.
The Metric system uses the meter and millimeter.
How long is a millimeter? 1/1000 of a meter. Mil refers to one thousandths of something.
For more precise measurements we divide a known standard length into smaller divisions.
Ever hear a mechanic or a machinist say “it’s bored 30 thou over” or “ bored 30 over”.
What does that mean?
Bored refers to a machining process called “boring”, but let’s concentrate on the “30 thou” part for now.
Thou is short for thousandths of something.
30 is short for the decimal equivalent of 30/1000 or .030
Most precision measurements in the USA are expressed in thousandths of an inch.
So when the guy says 30 thou in the USA he usually means .030 of an inch.
How big is that? Roughly 10 times the thickness of your hair.
One thou is 1/1000 or .001, 2 thou is 2/1000 or .002 and 500 thou is 500/1000 or .500 of something, could be an inch, meter or a potato.
Pick a unit of measure and tell me how many of those units it is.
Some of the time it is some number of units plus some fraction of a unit.
You could say the part is 2.500 inches or 2 1/2 inches long.
You could also express that same measurement in the Metric system.
How many millimeters is 2.5 inches?
Since there is 25.4 millimeters in an inch, the answer is 2.5 multiplied by 25.4, or 63.5 millimeters.
In the USA we are stuck using both systems for now.
When you express a dimension use the unit of measure that fits the situation.
If you talking to a farmer in Oklahoma about a cotton picker he is probably talking inches.
If you are talking to Europe or Asia they are probably talking Metric.
For clarity you can always say 2.500 inches or 2.50 mm or millimeters to be sure the recipient knows what unit of measure you mean.
Get your micrometer and calipers out and practice measuring something that has a known dimension, like a drill bit.
See if your measurement is accurate.
The accompanying chart is a shortcut method of comparing the same length, expressed 3 different ways.
This chart has fractions of an inch, decimal expressions of an inch and decimal expressions of a millimeter.
Let’s talk about 1/2 inch. Find that in the fractions and notice it’s also expressed as .500 of an inch in decimals and 12.7 mm in Metric.
All 3 of those are the same length just described different ways.
You need to drill a hole the size of a linkage that goes in that hole.
I measure the linkage and find that it is .187 inch in diameter.
I only have a fractional drill bit set. What size drill will produce a .187 diameter hole? 3/16 of an inch is the correct answer.
This chart saves me time figuring out the correct drill.
There are dozens of kinds of charts like this and we will have them available on our website.