of course a liter isn't 1 cubic meter. it's 1000cc, while a cubic meter is 1,000,000cc. that's just math.

There's only one definition of million, because "mil" means thousandth.

Also, the metric system generally uses Joules rather than calories. A Joule is the energy it takes to exert a force of 1 Newton over the distance of 1 meter. An example would be lifting about 102 grams up 1 meter.

These non-metric units that science uses all the time are conveniences based on constants we can measure or estimate. An AU is the distance from the Earth to the Sun. A sol is the time it takes for Mars to rotate once about its axis. There's nothing wrong with making up constants that make your numbers easier to explain.

While our calendar isn't a base-10 system, the second is defined by the metric system and that's the definition everyone uses.

While I'm American, and I'm used to feet, inches, pounds, miles, gallons, and cups, it is easy to understand the convenience of the Metric system. How many inches are in a mile? Well, it's 5280 feet, and there are 12 inches in a foot, so that means there's 63,360 inches in a mile. For the record, it's easier to figure how many centimeters are in a mile.

A mile is officially defined as 1609.344 meters, so all you have to do is multiply by 100. There are 160,934.4 centimeters in a mile. There are 100,000 centimeters in a kilometer.

Pounds, by the way, are a unit of force, not mass. Or rather, they used to be. So now they're a unit of mass, but you can still refer to pounds of force. So a pound of force is the force of gravity on a one pound mass. To quote Wikipedia,

Note, however, unlike the other systems the force unit is not equal to the mass unit multiplied by the acceleration unit[11]—the use of Newton's second law, F = m ⋅ a, requires another factor, gc, usually taken to be 32.174049 (lb⋅ft)/(lbf⋅s2).

source

Yikes. The metric system is certainly better.