Hi Tom. I noticed that you have expressed difficulties in converting back and forth to metric units. I find it pretty simple - there are 25.4mm per inch, so half an inch is 12.7mm. 14mm and 9/16 are so close that they can be used interchangeably. If you want to know what 10mm is in thousandths of an inch, for example, divide 10 by 25.4, giving .393. Printing off a decimal equivalent chart, that is nearly 13/32. (13 divided by 32 = .406). 30.5 cm per foot.
A meter is a metric system yard, effectively one pace. 1000 meters / paces per kilometer, so that is a very human and understandable measurement. For convenience sake, you just have to remember 1.609 km per mile. 2.2 pounds per kilogram, a liter is roughly a quart, 3.73 liters per US gallon, I believe. A deciliter is a tenth of a liter. That measurement is not really used commonly, but does appear in the Imbert tables.
The volume and weight systems are linked together very nicely, based on the properties of water. One cubic centimeter of water weighs one gram, one thousand cubic centimeters of water weighs one kilogram, and has a volume of exactly one liter. All that makes for easy calculation of water tank weights, etc.
As others have said, 1HP = 750 watts (though watts aren’t metric, they are a unit of international measure). Hopefully this helps to show how useful metric units can be to calculate directly.