Woodturners in the US often have to deal with Millimetres. Bowl gouges made in Europe are often 12mm
rather than 1/2″. Etc.

I often see Woodturners get all twisted up by 1″ = 25.4mm.   My advice is to forget about the 0.4mm!

Metal people do things in “thousands” of an inch (0.001″).  In this world the 0.4 mm at the end of 25.4mm is a big deal!   0.4mm is 0.015748″   That is 15 thousands of an inch.  You don’t want to drive a car with bearings off by 15 thousands of an inch!

In the woodworking world some fanatics do things to an accuracy of  1/32″ of an inch.  1/32″ = 0.03125″.  This is a lot more than 0.4mm = 0.015748″.  Can you really plane off only 0.015748″?  I think not!

In the woodturning world I rarely do things to an accuracy of 1/16″ = 0.0625″.  Thus I don’t give a rat’s ass about 0.4mm.

Carl’s MM to Inches

Here is how I do the MM to inches conversion in my head.

1. Memorize the following!   Forget about the 0.4mm!

  25mm = 1″  (Close enough for Government Work)
  16mm = 5/8″  (Almost Dead On)

2. You can derived the rest.

a. If less than 5/8″ then divide by 2.

   25mm / 2 = 12mm = 1/2″   Forget about the 0.5!
   12mm / 2 = 6mm = 1/4″
   6mm / 2 = 3mm = 1/8″

b. If over 5/8″ then remember 5/8″ is 16mm and start from there.

   3/4″ is 5/8″ + 1/8″.  Thus 16mm + 3mm = 19mm = 3/4″.

c. If over 1″ and less then 2″ then start from 1″ = 25mm.

   1-1/4″ is 1″ + 1/4″.  Thus 25mm + 25mm / 4 =  25mm + 6mm = 31mm 

d. If over 2″ then the 0.4mm in 1″ equals 25.4mm starts to add up.   However, it is not really that much.  Go with the following.

      25mm = 1″
      50mm = 2″
      75mm = 3″
    100mm = 4″
    125mm = 5″
     …
     250mm = 10″  (Really 254mm = 10.0″, thus you are off by 4mm which is less than 3/16″)

     If less than 10″ then you can estimate the size as  “mm / 25 = inches” with a little error.

     Hint: Think of US money. 100mm is $1 US or 100 cents. 1″ is 1 quarter or 25 cents, 2″ is 2 quarters or 50 cents.

Handy MM to Inches Table for Real People

Remember the ones in Red in following table!  No one really agrees.  But, they are sort of the agreed on standard in the Woodturning world. (A 1/2″ gouge is 12mm.  A 1/8″ parting tool is 3mm.)  Use the ones in Green below when in doubt.

I got the above table from www.jansjewells.com. I liked the “Just Short of xx inch” column.  I added the last 2 columns.

Note: 8mm = 5/16″ is a magic size.  Because it is almost dead on!.  5/16″ set screws and 8mm set screws use the same size Allen wrench.  Thus the Blue Handle Allen wrench that comes with Oneway chucks will fit the 5/16″ set screws on your adjustable handle, etc.  Always design things to use 8mm or 5/16″ set screws rather than 1/4″ set screws.

Handy Fractional Inches to MM Table

I got the above table from https://www.ginifab.com/feeds/cm_to_inch/”.

On-line MM or CM to Fractions of an Inch

There is a nice on-line “MM or CM to Fractions of an Inch” converter at https://www.ginifab.com/feeds/cm_to_inch/”.