Square Inch to Square Millimeter Conversion Calculator: Free Online Tool

Convert square inches to square millimeters with our free online area converter.

From:

Square Inch

To:

Square Millimeter

How to Use the Calculator:

Enter the value you want to convert in the 'From' field (Square Inch).
The converted value in Square Millimeter will appear automatically in the 'To' field.
Use the dropdown menus to select different units within the Area category.
Click the swap button (⇌) to reverse the conversion direction.

How to Convert Square Inch to Square Millimeter

Converting Square Inch to Square Millimeter involves multiplying the value by a specific conversion factor, as shown in the formula below.

Formula:

1 Square Inch = 645.16 square millimeters

Example Calculation:

Convert 10 square inches: 10 × 645.16 = 6451.6 square millimeters

Disclaimer: For Reference Only

These conversion results are provided for informational purposes only. While we strive for accuracy, we make no guarantees regarding the precision of these results, especially for conversions involving extremely large or small numbers which may be subject to the inherent limitations of standard computer floating-point arithmetic.

Not for professional use. Results should be verified before use in any critical application. View our Terms of Service for more information.

What is a Square Inch and a Square Millimeter?

The square inch (symbol: in², sq in) is an imperial and US customary unit of area. It represents the area of a square with sides that are each one inch long.

The square millimeter (symbol: mm²) is a unit of area in the metric system. It represents the area of a square with sides that are each one millimeter long.

Note: The Square Inch is part of the imperial/US customary system, primarily used in the US, UK, and Canada for everyday measurements. The Square Millimeter belongs to the metric (SI) system.

History of the Square Inch and Square Millimeter

Derived from the inch, whose definition has varied historically but was standardized as 25.4 mm, the square inch became a common unit for small area measurements within the imperial system. It is frequently used in the US and UK for engineering and everyday measurements.

Derived from the millimeter (one-thousandth of a meter), the square millimeter arose with the metric system as a unit for measuring extremely small areas, particularly in scientific and technical fields requiring high precision.

Common Uses for square inches and square millimeters

Explore the typical applications for both Square Inch (imperial/US) and Square Millimeter (metric) to understand their common contexts.

Common Uses for square inches

Engineering & Manufacturing (US/UK): Specifying the area of small components, cross-sections, or surface areas.
Material Measurement: Measuring the area of small sheets of material (e.g., paper, fabric samples).
Pressure Measurement: Used implicitly in units like pounds per square inch (psi).
Crafts & Hobbies: Measuring fabric squares, paper sizes, or small design areas.

Common Uses for square millimeters

Engineering & Manufacturing: Specifying tolerances, cross-sectional areas of fine wires or fibers, surface areas of micro-components.
Material Science: Measuring grain sizes, or the area of microscopic features.
Electronics: Defining the area of pads or traces on printed circuit boards (PCBs).
Optics: Measuring the area of small apertures or sensor elements.

Frequently Asked Questions

Questions About Square Inch (in²)

How many square inches are in a square foot?

There are 144 square inches in one square foot (12 inches x 12 inches).

How many square centimeters are in a square inch?

One square inch is equal to exactly 6.4516 square centimeters.

Is the square inch used for large areas?

No, it's used for small areas. Square feet (ft²) or square yards (yd²) are used for room sizes, and acres (ac) or square miles (mi²) for land.

Is the square inch an SI unit?

No, it is part of the imperial and US customary systems. The corresponding metric units are square centimeter (cm²) or square millimeter (mm²).

About Square Millimeter (mm²)

How many square millimeters are in a square centimeter?

There are 100 square millimeters in one square centimeter (cm²).

Is mm² used for everyday measurements?

No, it is generally too small for everyday use. Square centimeters (cm²) or square meters (m²) are more practical for common objects.

How does mm² relate to the square meter?

One square meter contains 1,000,000 square millimeters.

Is mm² an SI unit?

It is derived from the SI base unit meter and is a standard metric unit for area.

Conversion Table: Square Inch to Square Millimeter

Square Inch (in²)	Square Millimeter (mm²)
1	645.16
5	3,225.8
10	6,451.6
25	16,129
50	32,258
100	64,516
500	322,580
1,000	645,160

All Area Conversions

Square Meter to Square Kilometer Square Meter to Square Centimeter Square Meter to Square Millimeter Square Meter to Square Inch Square Meter to Square Foot Square Meter to Square Yard Square Meter to Square Mile Square Meter to Acre Square Meter to Hectare Square Kilometer to Square Meter Square Kilometer to Square Centimeter Square Kilometer to Square Millimeter Square Kilometer to Square Inch Square Kilometer to Square Foot Square Kilometer to Square Yard Square Kilometer to Square Mile Square Kilometer to Acre Square Kilometer to Hectare Square Centimeter to Square Meter Square Centimeter to Square Kilometer Square Centimeter to Square Millimeter Square Centimeter to Square Inch Square Centimeter to Square Foot Square Centimeter to Square Yard Square Centimeter to Square Mile Square Centimeter to Acre Square Centimeter to Hectare Square Millimeter to Square Meter Square Millimeter to Square Kilometer Square Millimeter to Square Centimeter Square Millimeter to Square Inch Square Millimeter to Square Foot Square Millimeter to Square Yard Square Millimeter to Square Mile Square Millimeter to Acre Square Millimeter to Hectare Square Inch to Square Meter Square Inch to Square Kilometer Square Inch to Square Centimeter Square Inch to Square Foot Square Inch to Square Yard Square Inch to Square Mile Square Inch to Acre Square Inch to Hectare Square Foot to Square Meter Square Foot to Square Kilometer Square Foot to Square Centimeter Square Foot to Square Millimeter Square Foot to Square Inch Square Foot to Square Yard Square Foot to Square Mile Square Foot to Acre Square Foot to Hectare Square Yard to Square Meter Square Yard to Square Kilometer Square Yard to Square Centimeter Square Yard to Square Millimeter Square Yard to Square Inch Square Yard to Square Foot Square Yard to Square Mile Square Yard to Acre Square Yard to Hectare Square Mile to Square Meter Square Mile to Square Kilometer Square Mile to Square Centimeter Square Mile to Square Millimeter Square Mile to Square Inch Square Mile to Square Foot Square Mile to Square Yard Square Mile to Acre Square Mile to Hectare Acre to Square Meter Acre to Square Kilometer Acre to Square Centimeter Acre to Square Millimeter Acre to Square Inch Acre to Square Foot Acre to Square Yard Acre to Square Mile Acre to Hectare Hectare to Square Meter Hectare to Square Kilometer Hectare to Square Centimeter Hectare to Square Millimeter Hectare to Square Inch Hectare to Square Foot Hectare to Square Yard Hectare to Square Mile Hectare to Acre