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One inch or several inches is a unit of length that primarily appears in English-speaking countries like the United States, Great Britain, or Canada. Through international standardization, one inch is equivalent to exactly 25.4 millimeters. The official standard symbol for inch are the letters in, though in daily routine these two letters are often shortened to a double quote like ".
Used by the United States and other major countries, inches are also commonly referenced in countries across Europe, although it's not their official unit of length. Prominent examples include screen sizes, wrench sizes, and electronic components. Also in many other parts of industry, manufacturing, and engineering, this unit is used constantly.
The inch has been standardized since 1930 as exactly 25.4 mm, and this definition has been universally adopted in all applications requiring precision.
Both units of length are part of an international system of units and therefore standardized. This is why we can say that the relation between inch to millimeter is always the same. In the year 1930, one inch was fixed to exactly 25.4 millimeters.
To convert from inches to millimeters, you multiply the value by 25.4, and the result is the same length in millimeters. To convert from millimeters to inches, you divide the value in millimeters by 25.4. The result is the same length in inches.
Example: 5 inches = 127.0 mm, and conversely, 127 mm = exactly 5 inches. For fractional inches like 3/8", multiply 0.375 × 25.4 = 9.525 mm.
Yes! The modern inch is absolutely standardized. In 1930, the British Standards Institution adopted an inch of exactly 25.4 mm. The American Standards Association followed suit in 1933. By 1935, this definition was adopted by 16 countries, and it became the worldwide standard.
However, historically there were variations:
Today, if someone refers to an "inch" in any technical or commercial context, they mean exactly 25.4 mm. This standardization is crucial for global manufacturing and trade.
Millimeter is a unit of length that is normed in the international system of units (SI). It is one-thousandth of a meter (1 mm = 0.001 m). The SI system has established itself as the leading system worldwide, and its use is required by law in most countries.
The meter was redefined in 1983 as the distance light travels in a vacuum in 1/299,792,458 seconds. Therefore, one millimeter is the distance light travels in 3.33564095198152 × 10⁻¹⁴ seconds.
Millimeters are the standard unit for precision measurements in engineering, manufacturing, and science worldwide.
In the Anglo-American system, there are numerous units of length beside the inch. One inch is the same as:
Compared to the international system of units, one inch correlates to:
The inch remains prevalent in:
Even in metric countries, many products use inch specifications for compatibility with global standards.
The term "inch" comes from the Latin uncia, meaning "one-twelfth" (of a foot). The Latin uncia was adopted into Old English as ynce, which evolved into "inch." The vowel changed from "u" to "i" and the consonant from "c" to "ch" over centuries.
Interestingly, uncia also gave us the word "ounce," originally meaning one-twelfth of a Roman pound (libra). Both measurements share the concept of being one-twelfth of a larger unit.
The first recorded definition of an inch was in 1324 by King Edward II of England, who defined it as "three grains of barley, dry and round, placed end to end lengthwise." This was later standardized to 25.4 mm.
The survey inch is exactly 25.400051 mm, part of the US survey foot system. It coexists with the standard inch (25.4 mm) and is used only for land surveying to maintain consistency with historic property records.
The square inch is a unit of area (in²). One square inch = 645.16 square millimeters. It's commonly used for measuring small areas like cross-sections, display densities (pixels per inch), and material strength.
The cubic inch is a unit of volume (in³). One cubic inch = 16,387.064 cubic millimeters or approximately 16.387 ml. It's used for engine displacement (e.g., 350 cubic inch V8 engine) and small container volumes.
Inches are ubiquitous in many industries:
Even metric countries often see inch specifications on imported goods and international standard components.
The first reference to an inch dates to 1120 AD in Great Britain, where it was used to measure injuries. The legal definition evolved over centuries:
This standardization enabled the modern global manufacturing ecosystem we rely on today.
"I'll not budge an inch, boy..."
"Cold! If the thermometer had been an inch longer we'd have frozen to death."
"Inch by inch, life is a cinch. Yard by yard, life is hard."
"Give somebody an inch, and he will take a mile."
"Now we're doing the grinding, sometimes frustrating work of delivering change - inch by inch, day by day."