Both Metric and English Engineering Units are used in the United States. English Engineering Units have a peculiarity as is demonstrated below.

Ten Pounds of Potatoes The English Engineering Unit System states that force is a dimension. The magnitude of the unit force is defined to equal that force required to support a unit mass (one pound mass) at sea level. Thus to carry (in uniform motion) ten pounds mass of potatoes a person must exert a vertical force of ten pounds force. If we apply Newton's fact for uniform motion (0 = ΣF) in the vertical direction a useful identity among units (for potatoes and everything) results.

This equation seems peculiar because there is no unknown quantity. In actuality what the equation says is profound. It says that engineers who consider the dimensions mass, length, time and force as independent and then ascribe units arbitrarily will discover the dimensions are not independent and that their units are related but with a constant. In the English engineering system (F, m, L and t defined) the relationship among units is:

In contrast, the metric system defines only mass, length and time as dimensions. Units specified are the kilogram, meter and second. Force is left to be a "derived" entity. Were metric units used in this consideration we would obtain:

With length, the metric system is superior. The English Engineering System was pragmatic in locking the unit mass to be a unit force. It sure facilitates hydraulic engineering calculations.