Carl’s “MM to Inches” for Woodturners

Meta Description: Carl often see Woodturners get all twisted up by 1" = 25.4mm. His advice is to forget about the 0.4mm! Carl does the MM to inches conversion in his head using this method.

Photo: Carl’s “MM to Inches” for Woodturners 1

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

Photo: Carl’s “MM to Inches” for Woodturners 2

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.

MM Approximate Size Exact Size Fraction Notes
1mm 1/25 Inch 0.03937 Inches 0.0400 = 1/25  
2mm 1/16 Inch 0.07874 Inches 0.0625 = 1/16  
3mm Just short of 1/8 inch 0.11811 Inches 0.1250 = 1/8  
4mm Little over 1/8 inch 0.15748 Inches 0.1250 = 1/8  
5mm 3/16 Inch 0.19685 Inches 0.1875 = 3/16  
6mm Just short of 1/4 Inch 0.23622 Inches 0.2500 = 1/4  
7mm Little over 1/4 Inch 0.27559 Inches 0.2500 = 1/4  
8mm 5/16 Inch 0.31496 Inches 0.3125 = 5/16 Magic Size
9mm Just short of 3/8 Inch 0.35433 Inches 0.3750 = 3/8  
10mm Little over 3/8 Inch 0.39370 Inches 0.3750 = 3/8  
11mm 7/16 Inch 0.43307 Inches 0.4375 = 7/16  
12mm Just short of 1/2 Inch 0.47244 Inches 0.5000 = 1/2  
13mm Little over 1/2 Inch 0.51181 Inches 0.5000 = 1/2  
14mm 9/16 Inch 0.55118 Inches 0.5625 = 9/16  
15mm Just short of 5/8 Inch 0.59055 Inches 0.6250 = 5/8  
16mm 5/8 Inch 0.62992 Inches 0.6250 = 5/8 Magic Size
17mm Just short of 11/16 Inch 0.66929 Inches 0.6875 = 11/16  
18mm Just short of 3/4 Inch 0.70866 Inches 0.7500 = 3/4  
19mm Little under 3/4 Inch 0.74803 Inches 0.7500 = 3/4  
20mm Just short of 13/16 Inch 0.78740 Inches 0.8125 = 13/16  
21mm Little over 13/16 Inch 0.82677 Inches 0.8125 = 13/16  
22mm Just short of 7/8 Inch 0.86614 Inches 0.8750 = 7/8  
23mm Little over 7/8 Inch 0.90551 Inches 0.8750 = 7/8  
24mm 15/16 Inch 0.94488 Inches 0.9375 = 15/16  
25mm 1 Inch 0.98425 Inches 1.0000 = 1  

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

Inches CM MM
1/4″ 0.64 6.4
1/2″ 1.27 12.7
3/4″ 1.91 19
1/8″ 0.32 3.2
3/8″ 0.95 9.5
5/8″ 1.59 15.9
7/8″ 2.22 22.2
Inches CM MM
1/16″ 0.16 1.6
3/16″ 0.48 4.8
5/16″ 0.79 7.9
7/16″ 1.11 11.1
9/16″ 1.43 14.3
11/16″ 1.75 17.5
13/16″ 2.06 20.6
15/16″ 2.38 23.8
Inches CM MM
1/32″ 0.08 0.8
3/32″ 0.24 2.4
5/32″ 0.4 4
7/32″ 0.56 5.6
9/32″ 0.71 7.1
11/32″ 0.87 8.7
13/32″ 1.03 10.3
15/32″ 1.19 11.9
Inches CM MM
17/32″ 1.35 13.5
19/32″ 1.51 15.1
21/32″ 1.67 16.7
23/32″ 1.83 18.3
25/32″ 1.98 19.8
27/32″ 2.14 21.4
29/32″ 2.3 23
31/32″ 2.46 24.6

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/”.

Photo: Carl’s “MM to Inches” for Woodturners 3