How big is Inch Ruler? | Scale of the Universe

An inch ruler is a straight measuring stick marked in inches. The one in the explorer is 12 inches long, which is exactly 1 foot, or 30.48 centimeters. The size shown is its length, rounded to 30 centimeters.

Size

An inch is defined as exactly 2.54 centimeters, and 12 inches make 1 foot. A 12-inch ruler is therefore exactly 30.48 centimeters long. No measurement is involved: the length follows from the definition of the inch.

In the United States, the National Institute of Standards and Technology (NIST) publishes these values in its Handbook 44. The handbook also holds the rules for measures used in trade, which the National Council on Weights and Measures recommends that states adopt. Under those rules, a ruler may be marked in inches and in fractions made by halving, such as halves, quarters, eighths and sixteenths of an inch. A metric ruler may be marked in centimeters and tenths of a centimeter, which are millimeters.

Size comparisons

A 12-inch ruler is about 1.3 times the width of a basketball, which is about 24 centimeters (9.4 inches) across.
Sixteen U.S. pennies, each 19.05 millimeters (0.75 inches) across, span exactly 12 inches when laid edge to edge in a row.
A person 1.7 meters (5 feet 7 inches) tall is about as tall as five and a half rulers stacked end to end.
About 29,000 rulers stacked end to end would reach the top of Mount Everest, which is 8,849 meters (29,032 feet) high.

How the inch was fixed

The United States has used an inch of exactly 2.54 centimeters since 1959. That year, a notice in the Federal Register defined the foot as exactly 0.3048 meters, which is 12 inches of 2.54 centimeters each. The foot used before, exactly 1,200/3,937 meters, was renamed the U.S. survey foot and kept only for surveying land.

The two feet differ by 2 parts per million. On a 12-inch ruler, that is about 0.6 micrometers, or millionths of a meter. That is less than a hundredth of the width of a human hair.

Over a mile, the difference grows to about 0.01 feet. Keeping two feet led to mistakes, so NIST and the National Oceanic and Atmospheric Administration retired the survey foot at the end of 2022.

How rulers are checked

Handbook 44 sets tolerances, the largest error allowed, for rulers and other measures used in trade. A rigid measure must be straight. Its marks must be at right angles to its edge and no wider than 0.75 millimeters (0.03 inches).

At the 1-foot mark, a measure already in use may be too long or too short by no more than 1/32 inch, about 0.8 millimeters. A new measure being put into use for the first time must be within half that, 1/64 inch.