A square inch is a unit of area: the area of a square 1 inch on each side. An inch is exactly 2.54 centimeters, so a square inch is exactly 6.4516 square centimeters. The explorer shows 2.5 centimeters: the length of one side of the square, rounded, not its area.

Definition

A square inch is defined through the inch. In 1959, the United States fixed the foot at exactly 0.3048 meter, which makes the inch, one-twelfth of a foot, exactly 25.4 millimeters. The National Institute of Standards and Technology (NIST) lists the conversion to metric as exact: 1 square inch is 6.4516 square centimeters, or 645.16 square millimeters.

Before 1959, the US foot was defined as 1200/3937 meter, which made the inch about 25.40005 millimeters. That is about 2 parts in a million longer than today's inch. According to NIST, the change resolved a long-standing mismatch between the definitions used by different organizations in the United States and in other countries.

Size comparisons

• A U.S. penny, 0.75 inch (19 millimeters) across, is three-quarters as wide as a square inch. Laid on the square, it covers about 44 percent of its area.
• A standard glass marble, 5/8 inch (1.6 centimeters) across, is five-eighths as wide as the square.
• A square foot, 12 inches on each side, holds exactly 144 square inches. A square meter holds about 1,550.
• An American football field, 360 by 160 feet (110 by 49 meters) including the end zones, covers about 8.3 million square inches.

Pounds per square inch

The square inch is part of a common unit of pressure, the pound per square inch, written psi. Pressure measures how hard something pushes on each bit of a surface. A pressure of 1 psi means a force of 1 pound pushing on every square inch.

Air at Earth's surface presses on everything with about 14.7 pounds per square inch. The International Space Station keeps the air inside it at about that same pressure. The metric unit of pressure is the pascal, and NIST gives 1 psi as 6,894.757 pascals. The standard air pressure at sea level, 101,325 pascals, works out to 14.7 psi.

Sources

• NIST Guide to the SI, Appendix B.8: Factors for Units Listed Alphabetically. National Institute of Standards and Technology, 2025.
• U.S. Survey Foot. National Institute of Standards and Technology.
• Coin Specifications. United States Mint, 2026.
• Tournament Rules. National Marbles Tournament.
• 2026 NFL Rulebook. National Football League, 2026.
• Air Pressure. NOAA JetStream, 2026.