Metric Vs. SAE Sizes

Written by selma leathem
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Metric Vs. SAE Sizes
In the United States, tools come in SEA and metric sizes. (wrench image by Edsweb from

There are two standards of measurements for tools sold in the United States today: metric and SAE. Measurement standards are accepted physical quantities against which others are calibrated and compared to. Having measurement standards in place ensures widespread compatibility, better accuracy and fairness and transparency in trade.

The Metric System

The metric system is in wide use across most of the world except for the United States. The metric system has been in use in Europe for at least a century and the United Kingdom underwent metrication around 2000. The goal of the metric system was to produce units that could be duplicated by observers in different locations to improve accuracy. The meter is the unit of measure of length, and today it is defined to be the distance that light travels in a vacuum in 1/299,792,458 seconds. In the metric system, larger and smaller units of the same physical quantities are related to the fundamental units by multiples of 10 or 1/10. The millimetre is equal to 0.001 meters and the centimetre is 0.01m.

Imperial and US Units of Measure

English units were first used by the British and then a variation was adopted in the United States in a system referred to as U.S. measurements, which are still in use today. Imperial units grew out of English units after the Americans adopted the U.S. measuring system. There are many units in common between the Imperial and U.S. systems, such as length. The fundamental units of length are the mile, yard, foot and inch. There are 12 inches in a foot and one inch is equivalent to 0.039 millimetres. There are 5,280 feet in a mile. The yard was redefined in 1959 to have agreement between the Imperial and U.S. measurements. It’s value today is 0.9144 meters.

SAE Units

SAE measurement standards are those defined by the society of automotive engineers. This society defines standards for tools and parts used in both the automotive and aerospace industries. The units used for tools are select rational values of inches, where a rational number is an integer or a fraction.

Conversion Between SAE and Metric Units

It is possible to theoretically convert between SAE and metric units, but there may not necessarily be an exact equivalent tool available in the alternative units. To convert from metric to SAE, multiply the value in millimetres by 25.4, and to do the opposite conversion multiply the SAE value by 0.039. As an example, if you have a bolt that is 3/8-inch in SAE units and you have metric wrenches then you will want to convert it to millimetres: The value 3/8-inch is equal to 9.5mm, which does not precisely match the values that metric wrenches have in millimetres.

Metric or SAE Tools for Cars

A common application of tools is to repair or maintain cars. U.S. cars are assembled with nuts and bolts that use SAE measurements while imported cars use the metric system. So the tool kit that you need will depend on the make of car. It is possible to buy tool kits that contain both SAE and metric tools.

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