Shop Note

Measure Line Diameter in Inches or Millimeters

Mike Graff

Make and use a tool that will yield measurements directly in either metric (millimeters) or imperial (inches) units. Skip the metric-imperial and/or imperial-metric conversion process entirely.

The cross-sectional shape or dimension of the tool does not matter. It is the number of turns of line around the tool needed to fill a defined space on the tool that does matter. Your tool can be made of dowel, square stock or even a pencil as long you're working to fill a measured space on the tool. A piece of material about 6 inches/15 cm in length works well. The term 'dowel' is used in the following discussion.

Carefully measure and mark two segments on the dowel - 1 inch at one end, and two centimeters (which is made up of 20 millimeters) at the other end. Do NOT use the ends of the dowel as the ends of the measured segments - instead, mark both ends of each segment on the dowel, starting about a finger width in from end of the dowel as illustrated below.

     -------------------------------------------------------------
   End    |xxxxxxxxxxxx|                    |xxxxxxxxxxxxxxx|    End
   End    |xxxxxxxxxxxx|                    |xxxxxxxxxxxxxxx|    End
     -------------------------------------------------------------
           2 cm                                1 inch
         (metric)                            (imperial)

Decide whether you want to know line diameter in inches or millimeters. Wrap line around the appropriate end of the dowel until the measured segment of the dowel is filled, being careful to keep the line snugged on the dowel and against itself as you wind it onto the dowel. ROLL line off its spool and onto the dowel in order to avoid changing the 'natural' twist of the line - especially with small diameter line.

Count the number of passes of line between the marks defining the segment.

Grab your calculator.

For diameter in INCHES, divide one (1) by the number of turns counted on the dowel. For example, if you count 35 turns of line in the one inch segment marked on the dowel, divide 1 by 35 to get .02857 - the diameter of the line is .02857 inches.

For diameter in MILLIMETERS, divide 20 (the number of millimeters in two centimeters) by the number of turns counted on the dowel. For example, if you count 28 turns of line in the two centimeter segment marked on the dowel, divide 20 by 28 to get .7143 - the diameter of the line is .7143 millimeters.

NOTE 1: This is a manual diameter measuring technique that is subject to variation from several sources - amount of (and variation in) tension used when rolling line onto the dowel, change in line twist induced by the operator while rolling line onto the dowel, humidity and moisture content of the line, and so forth. What to do? Repeat the measuring process two or three times and average the results.
NOTE 2: If you treat your line with beeswax, diluted white glue, shoe polish, stain, or other substances, then measure the line AFTER it has been treated.
{Mike Graff}

NOTE 1:	This is a manual diameter measuring technique that is subject to variation from several sources - amount of (and variation in) tension used when rolling line onto the dowel, change in line twist induced by the operator while rolling line onto the dowel, humidity and moisture content of the line, and so forth. What to do? Repeat the measuring process two or three times and average the results.
NOTE 2:	If you treat your line with beeswax, diluted white glue, shoe polish, stain, or other substances, then measure the line AFTER it has been treated.

Back?