Square Mil

What Is Square Mil?

A square mil is a unit used to measure very small areas. It is mostly used in engineering and electronics to describe the size of tiny parts like wire cross sections.

Definition

A square mil is the area of a square where each side is 1 mil long.

• 1 mil is 0.001 inch or one thousandth of an inch.
• So 1 square mil is 0.001 inch by 0.001 inch.

In math form.

• 1 square mil = (0.001 inch) × (0.001 inch) = 0.000001 square inch.
• 1 square mil = 1 × 10⁻⁶ square inch.

In metric units this is a very tiny area.

• 1 square mil ≈ 6.4516 × 10⁻¹⁰ square meter.
• 1 square mil ≈ 0.00064516 square millimeter.

History / Origin

The word mil comes from the Latin word mille which means thousand. A mil is one thousandth of an inch.

Engineers in countries using inches needed a simple way to talk about very small sizes. Instead of writing 0.001 inch each time, they used the word mil. From this idea they created square mil for small areas.

Square mils became common in older electrical and mechanical drawings. Today newer units are often used, but square mils still appear in some industries and older documents.

Symbol & Abbreviation

Common ways to write square mil include.

• sq mil
• mil²
• Sometimes just the words square mil in full

There is no official SI symbol because it is not a metric or SI unit. When writing technical work it is helpful to write square mil at least once in full so readers know what unit you mean.

Current Use Around the World

Square mil is not a common everyday unit. It is mostly used in special technical areas.

• Electrical and electronic engineering. To describe very small cross sectional areas of conductors and small parts on circuit boards.
• Mechanical and manufacturing drawings. In older blueprints and design documents where inch based units are used.
• United States and some other inch using countries. It appears more often where the inch system is standard.

However, another related unit called the circular mil is more common for wire sizes. In many modern designs square millimeters and square inches are now preferred since they match better with global standards.

Example Conversions

Here are some simple conversion facts.

• 1 square mil = 0.000001 square inch.
• 1 square inch = 1,000,000 square mils.
• 1 square mil ≈ 0.00064516 square millimeter.
• 1 square millimeter ≈ 1550 square mils.

Worked examples.

1. 
   

   Convert 500 square mils to square inches.

   
   

   Use 1 square mil = 0.000001 square inch.

   
   

   500 square mils = 500 × 0.000001 = 0.0005 square inch.

   
   
2. 
   

   Convert 2000 square mils to square millimeters.

   
   

   Use 1 square mil ≈ 0.00064516 square millimeter.

   
   

   2000 square mils ≈ 2000 × 0.00064516 ≈ 1.29032 square millimeters.

   
   
3. 
   

   Convert 1 square millimeter to square mils.

   
   

   Use 1 square millimeter ≈ 1550 square mils.

   
   

   So 1 mm² ≈ 1550 square mils.

   
   

Related Units

Square mil is related to many other area units.

• Square inch (in²). Main inch based area unit. 1 in² = 1,000,000 square mils.
• Square millimeter (mm²). Small metric area unit. Often used instead of square mil in modern designs.
• Square centimeter (cm²). Larger metric unit than square millimeter. 1 cm² = 100 mm².
• Square foot (ft²). Bigger inch based unit used for rooms and floors.
• Circular mil. Area of a circle with diameter 1 mil. Common in wire sizing. Different from square mil, but used in similar fields.

FAQs

Is square mil a metric unit

No. Square mil is based on the inch, not on the meter. It is part of the inch pound system, not the metric system.

Where would I see square mil used today

You may see square mils in older engineering books, old circuit board drawings, and some electrical or mechanical standards written in the United States.

What is the difference between square mil and circular mil

A square mil is the area of a square with sides of 1 mil. A circular mil is the area of a circle with a diameter of 1 mil. They are different sizes and are used in different formulas.

Why not always use square millimeters instead

Many modern designs do use square millimeters. But some industries still follow older inch based rules and standards, so they keep using units like square mil and circular mil for consistency.

How small is a square mil in real life

It is extremely small. Several thousand square mils together are still only about the same area as a single square millimeter, which itself is about the size of the tip of a sharp pencil.

Can I use square mil in school math problems

You can, but most school math problems use square centimeters, square meters, or square inches. Square mil is more for special technical work than for everyday classes.