Carl’s “MM to Inches” for Woodturners

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.

MMApproximate SizeExact SizeFractionNotes
1mm1/25 Inch0.03937 Inches0.0400 = 1/25 
2mm1/16 Inch0.07874 Inches0.0625 = 1/16 
3mmJust short of 1/8 inch0.11811 Inches0.1250 = 1/8 
4mmLittle over 1/8 inch0.15748 Inches0.1250 = 1/8 
5mm3/16 Inch0.19685 Inches0.1875 = 3/16 
6mmJust short of 1/4 Inch0.23622 Inches0.2500 = 1/4 
7mmLittle over 1/4 Inch0.27559 Inches0.2500 = 1/4 
8mm5/16 Inch0.31496 Inches0.3125 = 5/16Magic Size
9mmJust short of 3/8 Inch0.35433 Inches0.3750 = 3/8 
10mmLittle over 3/8 Inch0.39370 Inches0.3750 = 3/8 
11mm7/16 Inch0.43307 Inches0.4375 = 7/16 
12mmJust short of 1/2 Inch0.47244 Inches0.5000 = 1/2 
13mmLittle over 1/2 Inch0.51181 Inches0.5000 = 1/2 
14mm9/16 Inch0.55118 Inches0.5625 = 9/16 
15mmJust short of 5/8 Inch0.59055 Inches0.6250 = 5/8 
16mm5/8 Inch0.62992 Inches0.6250 = 5/8Magic Size
17mmJust short of 11/16 Inch0.66929 Inches0.6875 = 11/16 
18mmJust short of 3/4 Inch0.70866 Inches0.7500 = 3/4 
19mmLittle under 3/4 Inch0.74803 Inches0.7500 = 3/4 
20mmJust short of 13/16 Inch0.78740 Inches0.8125 = 13/16 
21mmLittle over 13/16 Inch0.82677 Inches0.8125 = 13/16 
22mmJust short of 7/8 Inch0.86614 Inches0.8750 = 7/8 
23mmLittle over 7/8 Inch0.90551 Inches0.8750 = 7/8 
24mm15/16 Inch0.94488 Inches0.9375 = 15/16 
25mm1 Inch0.98425 Inches1.0000 = 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

InchesCMMM
1/4″0.646.4
1/2″1.2712.7
3/4″1.9119
1/8″0.323.2
3/8″0.959.5
5/8″1.5915.9
7/8″2.2222.2
InchesCMMM
1/16″0.161.6
3/16″0.484.8
5/16″0.797.9
7/16″1.1111.1
9/16″1.4314.3
11/16″1.7517.5
13/16″2.0620.6
15/16″2.3823.8
InchesCMMM
1/32″0.080.8
3/32″0.242.4
5/32″0.44
7/32″0.565.6
9/32″0.717.1
11/32″0.878.7
13/32″1.0310.3
15/32″1.1911.9
InchesCMMM
17/32″1.3513.5
19/32″1.5115.1
21/32″1.6716.7
23/32″1.8318.3
25/32″1.9819.8
27/32″2.1421.4
29/32″2.323
31/32″2.4624.6

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