Things You Didn't Know About the Inch
From Roman twelfths and barleycorns to screen diagonals and shoe sizes
The inch is everywhere — screen sizes, rainfall measurements, lumber dimensions, shoe sizes. But this tiny unit carries a surprisingly rich history that stretches from ancient Rome to the pixels on the device you're reading this on.
A Twelfth of a Foot
The word "inch" comes from the Latin "uncia", meaning "one-twelfth." The same root gives us the word "ounce" (1/12 of a Roman pound). The Roman foot (pes) was divided into 12 unciae, making each uncia roughly what we now call an inch — about 24.7 mm, slightly shorter than the modern inch of 25.4 mm.
The choice of 12 divisions was no accident. It was a deliberate mathematical decision that would echo through millennia.
The Thumb Connection
Across European languages, the word for "inch" is literally the word for "thumb":
| Language | Word for Inch | Meaning |
|---|---|---|
| Dutch | duim | thumb |
| French | pouce | thumb |
| Swedish | tum | thumb |
| Italian | pollice | thumb |
| Spanish | pulgada | from pulgar (thumb) |
| Czech | palec | thumb |
This cross-linguistic pattern strongly supports the theory that the inch was originally the width of an adult man's thumb — a measurement everyone carried with them. The average adult male thumb breadth is, in fact, remarkably close to one inch.
King David I of Scotland (reigned 1124–1153) formalized this: he defined the inch as the breadth of a man's thumb at the base of the nail. Some accounts describe an even more scientific approach — averaging the thumbs of a small, medium, and large man, an early attempt at a reproducible standard.
Three Barleycorns Make an Inch
Medieval England had another creative answer to standardization: barleycorns.
A statute from the reign of King Edward II (early 14th century) defined the inch as the length of three barleycorns laid end to end. This wasn't just an abstract decree — it had lasting consequences that survive to this day.
The barleycorn (roughly 8.47 mm or 1/3 of an inch) is still the basis of British and American shoe sizes. Each shoe size differs from the next by exactly one barleycorn. That's why shoe sizes go up in increments of 1/3 inch — a direct link to a 700-year-old English statute.
Not All Inches Were Equal
Before the 1959 international agreement, the "inch" varied significantly by country:
| Country | Name | Length (mm) |
|---|---|---|
| England | inch | 25.4 (modern) |
| France | pouce du Roi | ~27.07 |
| Prussia | Zoll | ~26.15 |
| Vienna | Wiener Zoll | ~26.34 |
| Sweden | tum | ~24.74 |
| Spain | pulgada | ~23.22 |
| Portugal | polegada | ~27.5 |
| Amsterdam | duim | ~25.68 |
The French pouce du Roi (the King's thumb) at 27.07 mm was notably larger than the English inch — derived from the pied du Roi (King's foot) of about 324.84 mm. It was used until the metric system replaced it after the Revolution.
The 1959 Agreement: Pinning Down the Inch
On July 1, 1959, six nations — the United States, United Kingdom, Canada, Australia, New Zealand, and South Africa — jointly agreed:
1 inch = 25.4 millimeters exactly
This is not an approximation. It is an exact value by international agreement. Before 1959, the U.S. and British inches differed by about 2 parts per million — trivial for daily use, but enough to matter in precision surveying.
The U.S. retained the older definition for surveying (the "U.S. survey inch" of ~25.40005 mm) until it was officially retired on January 1, 2023.
The Inch in Your Pocket
The inch quietly dominates the technology in your life:
Screen Sizes
Television, monitor, laptop, and phone screens are measured as the diagonal of the display. This convention dates to the era of cathode-ray tubes (CRTs), when the measurement was the outer diameter of the round tube — not the visible screen area. The convention stuck even as screens became rectangular and flat.
A "65-inch TV" and a "6.7-inch phone" use the same measurement logic, in inches, everywhere in the world — even in fully metric countries.
Pixel Density
Pixels per inch (PPI) describes screen resolution. Apple's Retina Display, introduced in 2010 with the iPhone 4, was marketed at 326 PPI. The related term DPI (dots per inch) is technically for printers — standard print resolution is 300 DPI for high quality.
The legacy number 72 PPI traces back to the original Macintosh in 1984, chosen because it mapped to 72 typographic points per inch — itself a centuries-old printing standard.
Hard Drive Form Factors
Hard drives are named by their platter width in inches: 3.5-inch (desktop), 2.5-inch (laptop), 1.8-inch (iPods). These sizes are nominal — a "3.5-inch" drive is actually 4 inches wide; the 3.5 refers to the approximate platter diameter.
The Inch in Construction: Nothing Is What It Seems
The construction industry uses inches everywhere, but the numbers are often misleading:
- A "2×4" board does not measure 2 inches by 4 inches. After drying and planing, it's actually 1.5 × 3.5 inches. The nominal dimensions refer to the rough-cut size before processing.
- A "1-inch pipe" (NPS 1) has an outside diameter of 1.315 inches. The nominal size originally approximated the inside diameter, but as wall thicknesses varied, the numbers drifted from any physical measurement.
- Drywall comes in thicknesses of 1/4, 3/8, 1/2, and 5/8 inch.
- Bolt sizes below 1/4 inch use a numbered gauge (#0 through #12) rather than fractional inches. Thread pitch is expressed as threads per inch (TPI) — a uniquely imperial concept.
Three Ways to Subdivide an Inch
Three competing paradigms coexist in inch-using countries, sometimes on the same job site:
Fractional inches — the traditional system: halves, quarters, eighths, sixteenths, thirty-seconds, sixty-fourths. A standard tape measure marks to 1/16 inch (~1.6 mm). This is the language of woodworking and construction.
Decimal inches — used in machining. A machinist's micrometer reads in thousandths of an inch, called a "thou" or "mil." Tolerances are often in ten-thousandths (0.0001 in), confusingly called a "tenth" in shop jargon — a "tenth" is actually one ten-thousandth.
Metric — the rest of the world. The tension between these systems famously contributed to the loss of NASA's Mars Climate Orbiter in 1999, a $125 million spacecraft destroyed because one team used pound-force-seconds while another expected newton-seconds.
The Inch in Weather
Rainfall in the United States is measured in inches of depth. One inch of rain falling on one acre produces approximately 27,154 gallons (~102,790 liters) of water, weighing roughly 113 tons.
Snowfall is measured in inches of accumulated depth. The commonly cited rule is 10 inches of snow ≈ 1 inch of rain, but this varies enormously: light powder can be 50:1, while heavy wet snow can be 3:1.
Barometric pressure is reported in inches of mercury (inHg) in the U.S. Standard atmospheric pressure is 29.92 inHg. This convention traces to Evangelista Torricelli's mercury barometer, invented in 1643.
Why Twelve? The Duodecimal Advantage
The Roman choice to divide the foot into 12 parts — giving us the 12-inch foot — was mathematically deliberate:
| Base | Divisors | Count |
|---|---|---|
| 12 | 1, 2, 3, 4, 6, 12 | 6 divisors |
| 10 | 1, 2, 5, 10 | 4 divisors |
Twelve can be evenly divided into halves (6), thirds (4), quarters (3), and sixths (2) — all whole numbers. Ten cannot divide evenly by 3, 4, or 6.
For tradespeople, this was an enormous practical benefit. Need to divide a foot-long board into three equal pieces? Each piece is exactly 4 inches — clean and simple. In a decimal foot of 10 units, one-third is 3.333... — impractical for a craftsperson working by hand.
This is why base-12 appears repeatedly throughout history:
- 12 inches in a foot
- 12 hours on a clock face
- 12 months in a year
- 12 pence in a pre-decimal British shilling
- 12 items in a dozen
- 144 items (12 × 12) in a gross
The Dozenal Society of America (founded 1944) has long argued that base-12 is superior to base-10 for general arithmetic — and the humble inch is their best exhibit.
Inches by the Numbers
- There are exactly 63,360 inches in a statute mile
- A U.S. quarter coin is 0.955 inches in diameter — very close to one inch, making it a handy rough reference
- In typesetting, 1 pica = 1/6 inch and 1 point = 1/72 inch
- One inch of water column (inH₂O) is a unit of pressure (~249 pascals), used in HVAC systems
- The symbol for inches is the double prime (″), though a straight double quote (") is commonly used as a substitute
- 5′ 11″ means 5 feet, 11 inches — single prime for feet, double prime for inches
The inch may be small, but its influence is anything but. From the width of a Roman thumb to the diagonal of your smartphone screen, this ancient twelfth-of-a-foot has proven remarkably resistant to replacement — perhaps because in a world of abstract standards, there's something satisfying about a unit you can still measure with your thumb.