Square Inch (in²)
The square inch is the area of a square with sides of exactly one inch, equal to approximately 6.45 square centimeters. It is the small-area workhorse of the American and British measurement systems, appearing wherever surfaces are too small for square feet but too large for fractions of an inch. Pressure in the imperial system is measured in pounds per square inch, universally abbreviated PSI, making the square inch fundamental to tire gauges, hydraulic systems, and compressed gas cylinders. Screen area, printing resolution, material strength, and gasket dimensions all commonly use square inches in the United States. The unit is intuitive for anyone familiar with inches: if you can picture a one-inch square, you already know a square inch.
Definition
One square inch is the area of a square with sides of exactly one inch, where one inch is exactly 25.4 millimeters. This makes one square inch exactly 6.4516 square centimeters, 645.16 square millimeters, or approximately 0.00064516 square meters. There are exactly 144 square inches in one square foot and 1296 square inches in one square yard.
History
The square inch derives from the inch, which has roots stretching back to ancient Rome, where the uncia was one twelfth of a foot. The English inch was standardized over centuries, with various monarchs attempting to fix its length. The international yard and pound agreement of 1959 defined the inch as exactly 25.4 millimeters, which fixed the square inch at exactly 645.16 square millimeters or 6.4516 square centimeters. In American industry, the square inch became deeply embedded through its role in pressure measurement. Pounds per square inch became the standard pressure unit in American engineering, automotive, and aviation contexts. Print resolution in dots per inch, another critical application, derives from the inch and implicitly involves the square inch when calculating total pixel counts for printed images.
Common Uses
Pressure measurement in PSI is the most consequential application. Tire pressure, blood pressure in some older instruments, water pressure, and compressed gas ratings all use pounds per square inch. Print resolution uses dots per inch, and total print area is calculated in square inches. Pizza sizes in the United States are often compared by calculating the area in square inches from the diameter. Material specifications for steel, aluminum, and other metals often state tensile strength in pounds per square inch. Electronics use square inches for circuit board area calculations.
Did You Know? Facts About Square Inch
- A 16-inch pizza has an area of about 201 square inches, while two 12-inch pizzas have a combined area of about 226 square inches. Two smaller pizzas give you more pizza than one large one.
- Standard atmospheric pressure at sea level is about 14.7 PSI, meaning the atmosphere pushes on every square inch of your body with 14.7 pounds of force.
- The human thumbnail covers roughly one square inch, making it a convenient built-in reference for the unit.
- A 300-DPI printer places 90,000 dots in every square inch of paper. A 1200-DPI laser printer packs 1,440,000 dots into the same square inch.
- There are exactly 144 square inches in a square foot. The number 144 is 12 squared, which follows from there being 12 inches in a foot.