Metal measuring tapes are the most commonly used tools for measuring distances. They are inexpensive, easy to use, and are available in English or metric units. Most tapes used in construction are graduated in units of 1/16 inch or millimeters. Accuracy depends on the quality of manufacturing, how they are maintained, and correctness of use.
The National Institute of Standards and Technology publishes tolerances for metal tapes in its Handbook 44, Section 5.52.
| Maintenance and Acceptance Tolerances, in Excess and in Deficiency, for Metal Tapes | |
| Nominal interval from zero, ft | Tolerance, in |
| 6 or less | 1/32 |
| 7 to 30, inclusive | 1/16 |
| 31 to 55, inclusive | 1/8 |
| 56 to 80, inclusive | 3/16 |
| 81 to 100, inclusive | 1/4 |
From NIST Handbook 44, Section 5.52, p. 5 - 12.
http://ts.nist.gov/WeightsAndMeasures/h44 - 07.cfm.
The NIST Handbook 44, Section 10.3 gives the following rules for the reading of indications on graduated scales if it is desired to read or record values only to the nearest graduation. If the indicator is between two graduations, but is closer to one graduation than it is to the other, 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." In most cases readings can be no more accurate than the smallest graduation.
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