While the metric system is easy to learn and work with in comparison to the US customary system (also referred to as the 'inch-pound' system, or more ambiguously as the 'English' system), we still use both, and since we use both, it follows that we must sometimes compare measures between the two.

This and the following sections concentrate on giving you the ability to convert between inch-pound and metric measures, when the comparison is likely to come up in daily life. You're not likely to find grains to grams, but miles to kilometers, kilograms to pounds, and milliliters to ounces, along with other similar operations, are all covered.

Because the US and metric systems are based on different things, it isn't typical to get a clean conversion from one to the other. A 'precise' conversion can often involve 8+ decimal digits; for instance, the best number given for converting pounds into kilograms is 0.45359237. This is inexpedient for human calculation, and the resulting products are as cumbersome to display as they were to calculate. For daily use, we need a means of coming up with smaller numbers.

I consulted the National Institute of Standards and Technology (NIST) for their guidance in this area. They mention two procedures, one for 'technical documents or specifications', and another methodology recommended for commercial purposes. Upon investigation, I decided to aim for the latter, primarily because it seems to actually produce more precise results, and secondly because it is in line with the numbers you will see in daily life.

It is important to clarify that the system I use is not codified. It works in a way like the NIST document describes, and produces numbers that look like the ones companies use, but I have no way of knowing for sure that this is 'the' system. However, it works for my program, and if I can explain it to a computer, a human should be able to work it out more readily, and so this might be useful for your life.

Begin by taking the number you want to convert, and multiplying it by the conversion factor. You will need to choose an appropriate number of significant digits (a "significant digit" not including 0s at the beginning or end of the number) to multiply. For many (but not all!) numbers you're likely to work with, three should suffice (e.g., 48 * 4.92). With a few small numbers you might be able to get by with less (4 * 4.9); others, including but not limited to large numbers, require more digits (767 * 4.928).

After you've multiplied, if there are any decimals, begin the rounding process. All the whole digits should be preserved. If the number has fewer than three total digits, include the number of decimal digits required to take it up to three. Keep the decimal point in the appropriate place, follow the number with a space and the appropriate symbol, and you're done.

There are numerous weight measures in the inch-pound system and the metric system, but for the purpose of this section, we will focus on the far-most-common ones, pounds and kilograms.

• lb -> kg: • by 0.45359237
• kg -> lb: • by 2.204622622

For small numbers, multiplying by the first two significant digits is good enough, but you'll eventually have to start using three. This makes these more difficult than the hardest multiplication problems you have faced in these exercises up to this point.

The first kg > lb number to require three digits is 44. Every fifth number after this (49, 54, etc.) requires three, up until 88, which also requires three, forming a new pattern. This appears to gradually expand until all numbers require three digits.

The first lb > kg number to require three digits is 15. While there is a resulting pattern, it is not obvious and is of limited utility in solving these problems.

Due to this, you will likely find converting to pounds to be the easier exercise.