APPENDIX F - Metric-Imperial Conversions

It should be kept in mind that the metric system is the fundamental measurement system of the United States. Since 1893, all U.S. Customary units (also often referred to as “Imperial” or “English” units—see additional info below) have been defined in terms of metric units. The easiest such definition to remember is that of the inch:

one inch = 2.54 centimeters exactly*

*Throughout this appendix, all conversion values in boldface are exact.

From the above, one can derive the equivalents of other units of length, e.g.:

Doing Conversions in the Working Constant

If both mile and kilometer distances must be marked when laying out a race course, the conversion is most easily done in the working constant:

1. Determine the working constant in counts/km or counts/mile.
2. Multiply the constant in counts/km by 1.609344 to obtain the constant in counts/mile, or:
3. Divide the constant in counts/mile by 1.609344 to obtain the constant in counts/km.

Conversion Table for Standard Distances

The following table shows how kilometer and mile distances are related but is not intended for routine measuring. It is much easier to do the conversion once in your measuring constant, as described above, than to do repeated conversions for each split. The table does show, for example, that 10 km is not exactly 6.2 miles. Courses measured to exactly 6.2 miles will not be certified as 10 kilometers!

**The marathon is defined as 42.195 km exactly.

Conversions for Steel Taping

More about U.S. Customary, “Imperial” and “English” Units

There is no really good name for the ‘system’ (such as it is) of non-metric units still used in the United States. They aren’t necessarily customary, or used in England, and differ in significant ways from the British Imperial System. In some contexts, they’re referred to simply as “inch-pound” units. The Imperial System is a reasonably well-defined set of units, established by the U.K. in 1824, sharing the same basic units of length and mass as U.S. customary units, but with significantly different sizes for some units such as the gallon, bushel, fluid ounce, and hundredweight.

Of perhaps greater interest to course measurers, two sets of sizes for the non-metric length units (inch, foot, mile, etc.) can still be found in the U.S. The definition quoted above (that 1 inch = 2.54 cm exactly) is the modern definition of the inch, as standardized in 1959 by all English-speaking countries. However, when U.S. customary units were first defined in terms of metric units in 1893, the inch was defined as the fraction 1 / 39.37 of a meter, equivalent to about 2.54000508... centimeters, or about 2 parts per million longer than the modern 2.54 cm definition. Moreover, versions of the foot and mile based on the 1893 U.S. definitions have persisted under the names “U.S. survey foot” and “U.S. survey mile.” Thus, a U.S. survey mile is about 1.609347 km, or about 3 mm longer than a standard mile of exactly 1.609344 km.

Fortunately, the differences between these values are so small (about 8 cm in a marathon) that they are insignificant compared with all the other uncertainties in measuring a race course. We recommend that all length conversions be performed using factors based on the modern definition, 1 inch = 2.54 cm exactly.