1/64 of an inch fraction display more useful than decimal fraction

Jul 20, 2010 at 6:01 AM
Edited Jul 20, 2010 at 6:03 AM

U.S. Customary rulers and tape measures are marked in 1/64" (one-sixty-fouth of an inch) units and its multiples (1/32", 1/16", 1/8", 1/4" and 1/2").  Displaying the closest 1/64" multiple as a fraction would be very useful for real world measurements.  For example, 500mm is currently displayed as 1'7.685".  U.S. Customary rulers, however, are not marked to find 0.685"; instead, they are marked to help find 11/16" (aka 44/64" simplified).  The fractional display is also the U.S. Customary way to show dimensions on construction drawings.  If Unit Converter added this feature, it would really stand out from the rest of the crowd.  Thanks!

Jul 23, 2010 at 5:19 AM

Thanks. I have not known of such convention, although I have been staying in the U.S. for more than three years :)


To implement properly, could you please give me some reference? I have a few questions about the conversion, but it would be better for me to have some knowledge before I ask questions.

Jul 28, 2010 at 9:10 AM
Edited Jul 28, 2010 at 10:36 AM

Probably the most official reference is the U.S. National Institute of Standards (NIST) Handbook 44. Section 5.52 Linear Measures (http://ts.nist.gov/WeightsAndMeasures/Publications/upload/Linear.doc ) says in part:

S.1. Units. ‑ A linear measure may be in total length, and the total length may be subdivided in any or all of the following:

(a) inches and binary submultiples of the inch;
(b) feet;
(c) yards and multiples of yards.

A 1‑yd measure may be graduated, in addition, to show 1/3‑yd and 2/3‑yd subdivisions.  A flexible tape may be graduated in tenths or hundredths of a foot, or both tenths and hundredths of a foot.  (Any other subdivisions are allowable only on measures of special purposes and when required for such purposes.)"
The NIST Handbook 44, Appendix A, Section 10 "Rounding Off Numerical Values" will probably be useful too. It gives the following guidance:
Obviously, if the indicator is between two graduations but is closer to one graduation than it is to the other adjacent graduation, the value of the closer graduation is the one to be read or recorded.
In the case where, as nearly as can be determined, the indicator is midway between two graduations, the odd-and-even rule is invoked, and the value to be read or recorded is that of the graduation whose value is even.

Section T.2 of the same document specifies the tolerances that various length measuring devices need to maintain.

Beyond the official statements though, practical usage might be more informative. Tape measures and rulers for construction and daily use are marked using inches and binary submultiples of the inch down to 1/32" with each foot marked as well. AFAIK, tape measures marked in tenths and hundredths of a foot are rare and used only by surveyors and pencil & paper drafting for scale conversion.

This blog article describes various tape measure markings (the "talking numbers" section was new to me):

The follow-on article explains more tape measure markings:

Here are some additional references with pictures showing typical U.S. Customary tape measures. Note that they are only sub-divided down to 1/32" and are representative of typical tools. Finer gradation, 1/64" rulers are found rarely and are used in more precise work such as fine cabinet making.

Shows a combo SI + US Customary tape measure:

Another combo SI + US Customary tape measure:

Searching for "tape measure" at Amazon.com shows more examples.

On the drawings side, there are lots of references to ASME Y14.5 as THE standard, however, copies cost $169. Here are some example drawings that illustrate common practice:

Understanding Construction Drawings (Google Books)

Plumbing drawing