From Barleycorns to Light: How the Inch Got More Precise Than Any King Could Imagine
Edward II's barleycorn standard was not as silly as it sounds. In a medieval economy where the most valuable commodity was wool, and where every transaction involved physical inspection of the goods, a barleycorn-based inch was reproducible anywhere barley grew — which was everywhere. A merchant in York and a merchant in Flanders could both find three barleycorns and agree, roughly, on what an inch meant. The standard was good enough for its time: the tolerance on a bolt of wool cloth was about a quarter inch per yard, and barleycorn inches could hold that.
The problems accumulated as manufacturing got more precise. By the 18th century, Britain had multiple competing inches — the London inch, the Edinburgh inch, the Irish inch — all slightly different. The Industrial Revolution punished this ambiguity. A screw cut in Birmingham had to fit a nut cut in Manchester, and if the two shops used different inches, it wouldn't. In 1824, the British Weights and Measures Act standardized the imperial inch. In 1893, the United States adopted the Mendenhall Order, which defined the inch as exactly 25.4000508 mm (1/39.37 of a meter). This was close to the British inch but not identical — the difference was about 2 parts per million, enough to cause gauge block incompatibility in precision engineering.
The 1959 agreement cleaned up the mess. Six countries agreed: 1 inch = 25.4 mm exactly. The 1893 Mendenhall inch was retired. The British imperial inch was retired. Every inch in the English-speaking world became identical. The inch became metric. And because the meter is now defined by the speed of light (since 1983), the inch is indirectly defined by the speed of light. A photon traveling in a vacuum covers 1 inch in exactly 84.725 picoseconds. That is the inch: a time interval expressed as a length. Edward II would not have understood a single word of that sentence.
The Dividing-by-12 Problem: Why Imperial Height Arithmetic Is Hard
Converting inches to feet is dividing by 12. The math is simple in principle and irritating in practice, because the remainder needs to be expressed in inches. 70 inches ÷ 12 = 5 feet with a remainder of 10 inches = 5'10". The division is exact but the notation is awkward: the prime-double-prime system (5'10") encodes two different units in a single number, and adding or subtracting these numbers requires carrying at the 12 boundary. 5'10" + 3'4" = 9'2" (70" + 40" = 110" = 9'2"). Most people cannot do this in their heads. Most people should not have to — but in the US, where height, construction, and sports all use feet and inches, they do.
The metric alternative — expressing everything in a single decimal-based unit — eliminates the carrying problem. 178 cm + 102 cm = 280 cm = 2.80 m. No division by 12. No prime-double-prime notation. The tradeoff is that metric loses the fine granularity that feet-and-inches provides: the difference between 5'9" and 5'10" is a single inch (2.54 cm), and the notation makes that clear. In metric, the difference is "175 cm" vs. "178 cm" — three centimeters, a less visually distinct step. Neither system is objectively better. Each is optimized for a different set of operations: imperial for subdivision and visual comparison, metric for addition and scaling. The inch-to-foot conversion sits at the boundary between them.
Common Inches to Feet
| Inches | Feet (exact) | Something that is this size |
|---|---|---|
| 1 in | 0.08333... ft | The width of your thumb. 1/12 of a foot. |
| 12 in | 1 ft | Standard school ruler. A foot. The base of the imperial system. |
| 18 in | 1.5 ft | Typical floor tile. 1.5 feet = 18 inches = 45.7 cm. |
| 24 in | 2 ft | Two rulers. A typical computer monitor measured diagonally. |
| 36 in | 3 ft | One yard. A baseball bat. Kitchen counter height. |
| 60 in | 5 ft | Average adult female height. 5 feet tall. |
| 72 in | 6 ft | Average adult male height. 6 feet tall. Social distancing guideline during COVID. |
Frequently Asked Questions
How do I convert inches to feet?
Divide by 12. feet = inches ÷ 12. 12 in = 1 ft. 36 in = 3 ft. 72 in = 6 ft. For a remainder in inches, take the whole number as feet and multiply the decimal remainder by 12. Example: 70 in ÷ 12 = 5.833... ft = 5 ft 10 in. A calculator simplifies this — but on a job site, most people memorize the common conversions (60 in = 5 ft, 72 in = 6 ft, 84 in = 7 ft).
Why are there 12 inches in a foot?
The ratio traces back to Roman times. The Roman foot (pes) was divided into 12 unciae — the word "inch" derives from uncia, meaning one-twelfth. The Romans inherited base-12 counting from the Babylonians and Sumerians, who counted in base-12 using the finger bones of one hand. Twelve is highly divisible — it splits evenly into halves (6"), thirds (4"), quarters (3"), and sixths (2") — making it more practical for construction and trade than base-10, which only divides evenly by 2 and 5. The 12-inch foot survived the metrication of the inch itself in 1959 because the ratio is more useful than the alternative.
Does anyone use decimal inches instead of fractions?
Yes — machinists and mechanical engineers in the US routinely use decimal inches. A dimension of 1.250" (one and a quarter inches) is standard in a machine shop. Decimal inches are easier to add, subtract, and feed into CAM software than fractional inches (1-1/4"). The decimal-inch system coexists with the fractional-inch system: architects and carpenters use fractions (1/4", 1/2", 3/4"), while machinists and engineers use decimals (0.250", 0.500", 0.750"). The inch is the same size either way. The notation tells you which profession made the drawing.
Engineering Context
In US mechanical engineering, the inch is the base unit and dimensions are given in decimal inches (1.250", not 1-1/4"). In US civil engineering, the foot is the base unit and dimensions are given in decimal feet (24.50 ft, not 24'-6"). The inch-to-foot conversion — dividing by 12 — sits at the handoff between these two domains. A steel fabricator receives a structural drawing showing a beam at station 24.50 ft. The shop drawing for the same beam is dimensioned in inches: 294". The conversion from 24.50 ft to 294 inches is multiplication by 12 (24.50 × 12 = 294.0). When the same beam appears on the architectural drawing as 24'-6", the conversion is 24 × 12 + 6 = 294". Three notations, one beam, two unit conversions. The vast majority of errors in this chain are not in the arithmetic — multiplying by 12 is easy. The errors are in misidentifying which notation is being used. A dimension of "24.5" on an unlabeled drawing is 24.50 ft to the structural engineer, 24.5 inches to the machinist, and uninterpretable to the architect (who expects dash notation). The title block must declare the convention. If it doesn't, the person reading the drawing must ask before cutting steel. For more on imperial measurement conventions in construction, see the Length Conversion Guide.
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