About Units of Measure

Units of measure in specifications are of two types:

  • Measurements, which can be exact or nominal
  • Sizes, which, like shoe sizes, may sometimes have only the vaguest relationship to the actual measurements

Sizes are usually standardized in a reference document, usually the same document that standardizes other features of the relevant product.

The U.S. still uses inch-pound dimensional systems even though use of SI ("metric") has been mandated by the federal government for government projects for many years. The three documents listed are those that BSD recommends for standardization of units of measurement with international systems. These documents should be used in the order presented -- the first is the most fundamental, while the other two address the construction industry and construction documents in particular.

Unit of Measure "Switch"

SpecLink allows the user to switch between Inch-Pound (I-P) and Metric (SI) units of measure using a switch at Summary Info. To make this happen, each dimension must be built as a UOM choice or a Multiple Choice with the UOM option. The latter is preferred, as it can include a fill-in option for the user to use if none of the built-in options are relevant.  If you add dimensional requirements and wish to have those swtich too, you'll need to make the conversions yourself.

"Inch-Pound": Inch-pound is traditional U.S. measurements, like feet and inches, pounds and tons. There is no reference standard for preferred units.

"Metric": We follow IEEE/ASTM SI 10 and ASTM E 621 in both preferred units and style.

The metric options shown in SharedAttributes are the preferred units. All ASTM E 621 units are included in the spreadsheet.

Sizes: A size is not a unit of measure and may have inch-pound and metric that are not equivalent. Sizes are defined by some standard -- find the standard and understand the issues.

Gages: Include both the gage and the inch thickness for I-P -- e.g. "xx gage, yy inch (zz mm)"

Significant Digits: Don't use more significant digits than are reasonable. (See Significant Digits.)

Unitless Numbers: Some physical properties or quantities that occur in SpecLink do not have associated units because they express a ratio, a coefficient or an arbitrarily defined value. For example, u-value (and its inverse, R-value) or “coefficient of friction” do not have units associated with them. Also, properties like Shore hardness (ASTM D 2240) are unitless because they depend on a particular tool or test procedure to determine the numerical quantity of the property.

General Style and Formatting

Spell out all inch-pound units.

Do not use superscripts or subscripts; these do not translate from font to font. Use squared, cubic, etc.

Spell out "plus" and "minus" when used with temperature and tolerances.

When a compound dimension is involved, state as "2 by 4 inches" not "2 x 4 inches".

Common Errors in Source Documents

When a number in a source document doesn't include a unit of measure, always check whether it is truly a unitless number. Very often, the author simply assumes that because he has given the test method, that you can figure out what the unit of measure is supposed to be.

When doing a conversion, first be sure that the Inch-Pound unit is correct. Many authors, particularly manufacturers, are sloppy or ignorant and don't give the full or "true" unit of measure. In particular, the fact that "pounds" can refer to either "weight" (SI = mass) or force (SI = newton) confuses many Americans. There are some definitions in the spreadsheet that will help.

Another common error of other writers is failing to include all the parts of a compound unit. Pounds per square foot may be stated as simply "pounds" or "pound" as in "15-pound felt". In fact, the latter began life as felt of 15 pounds per square (100 square feet) weight but has evolved into a "size" (it's now significantly less than 15 pounds per square).

# is a symbol that is sometimes used for pounds. If that is the case, use the unit pound instead.