Metric Units & Converting Between Them

There are many metric-unit prefixes, but the usual ones required in school are these: kilo-, hecto-, deka-, deci-, centi-, and milli-. To convert between the various prefixes, and thus the variously-sized units, you just move up and down this list of prefixes, moving the decimal point as you go.

To remember the prefixes in order, you can use the following sentence:

King Henry Doesn't [Usually] Drink Chocolate Milk

The first letters of the words stand for the prefixes, with "Usually" in the middle standing for the "unit", being meters, grams, or liters. Many memory phrases omit the "Usually", and consequently students forget where the basic unit goes, messing up their conversions. Leave the "Usually" in there so you can keep things straight:

kilo- hecto- deka- [unit] deci- centi- milli-

Since each step is ten times or one-tenth as much as the step on either side, we have:

1 kilometer = 10 hectometers = 100 dekameters = [1000 meters] = 10 000 decimeters = 100 000 centimeters = 1 000 000 millimeters

Alternatively, we have:

1 milliliter = 0.1 centiliters = 0.01 deciliters = [0.001 liters] = 0.000 1 dekaliters = 0.000 01 hectoliters = 0.000 001 kiloliters

The point here is that you move from one prefix to the next by moving the decimal point one place, and filling in, as necessary, with zeroes. To move to a smaller unit (a unit with a prefix some number of places further to the right in the listing), you move the decimal place to the right that same number of places, and vice versa. Together with the prefix sentence ("King Henry..."), this makes conversion between the different metric sizes very simple.

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• Convert 12.54 kilometers to centimeters.

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• Convert 457 mL to hL.

That's all there is to metric conversions. As long as you keep the prefixes straight and remember where in the sequence the "units" name goes, you'll be fine. Just count the number of jumps and note the direction; then move the decimal point the same direction and the same number of places.