Metric System

The metric system is widely used in all fields of science, medicine included. Like the decimal system it is based on powers of 10, but instead of the reference point being a decimal point, it is a unit of measurement. The most common units are meters, grams, and liters. There are other units in various fields of science (bars, amps, volts, hertz, etc) , but medicine primarily uses the common three.

  1. Meters are used for length

    • For patient heights, we use centimeters as this improves accuracy over feet and inches

    • Sometimes we may use kilometers for fitness goals

  2. Grams are used for mass or weight

    • It is important to know a patient's weight in kilograms (or grams for newborns) as some medications are based on this

    • Most medications are dosed in grams, milligrams, or even micrograms

  3. Liters are used to measure volume

    • Intravenous infusions are in liters or milliliters

    • Recording of patient intake and output is in milliliters

    • Many lab results will be in liters or deciliters (0.1 liter)

Conversions- used commonly in nursing

Meters

1 kilometer (km) = 1,000 meters (m)

1 meter (m) = 100 centimeters (cm)

1 centimeter (cm) = 10 millimeters (mm)

Grams

1 kilogram (kg) = 1,000 grams (g)

1 gram (g) = 1,000 milligrams (mg)

1 milligram (mg) = 1,000 micrograms (mcg)

Liters

1 liter (l) = 1,000 milliliters (ml)

We learned that we must move the decimal point over when converting from thousands to hundreds or tenths to thousandths. While we can still use a decimal point in the metric system, changing the prefix can eliminate the need for a decimal and make the value easier to read. For instance 0.125 milligram (mg) can be stated as 125 microgram (mcg). To convert from one prefix to another, you must move the decimal point over the proper number of places.

From kilo to base unit (no prefix), move the decimal to the right 3 places:

  • 1.5 kilogram (kg) = 1500 gram (g)

  • 0.75 kilogram (kg) = 750 gram (g)

  • 10 kilometer (km) = 10,000 meter (m)

From base unit (no prefix) to centi, move the decimal to the right 2 places:

  • 1.75 meter (m) = 175 centimeter (cm)

  • 0.856 meter (m) = 85.6 centimeter (cm)

From base unit (no prefix) to milli, move the decimal to the right 3 places:

  • 1.5 gram (g) = 1,500 milligram (mg)

  • 0.75 gram (g) = 750 milligram (mg)

  • 2.5 liter (l) = 2,500 millilter (ml)

  • 0.05 liter (l) = 50 milliliter (ml)

From milli to micro, move the decimal to the right 3 places:

  • 0.25 milligram (mg) = 250 microgram (mcg)

  • 1.5 milligram (mg) = 1,500 microgram (mcg)

Note: to convert the opposite direction, as in microgram to milligram or gram to kilogram, just move the decimal point to the left the same number of places.

Time for some practice, and these will all use the metric abbreviations (do not round answers)

Convert 1,500 mg to g

1.5 g Since moving from smaller unit to larger, move decimal to the left, in this case 3 places

Convert 1.5 l to ml

1,500 ml Since moving from larger unit to smaller, move decimal to right, in this case 3 places

Convert 1,500 mcg to g

0.0015 g This one is tricky. Since moving from small to large units, move decimal to the left, in this case 6 places as we are going from micrograms to the base unit of grams. As you can see, this number would easily be misread, hence the use of the metric system nomenclature of milli and micro

Convert 275 ml to l

0.275 l Move the decimal 3 places to the left

Convert 1.2 cm to mm

12 mm Move the decimal 1 place to the right as we are going from large to small in units and only one place- centimeter to millimeter

Convert 9.5 g to milligram

9,500 mg Move the decimal 3 places to the right

How many ml is 1,500 ml + 1.25 l

2,750 ml First convert the 1.25 l to 1,250 ml, then add the two like units together... 1,500 + 1,250 = 2,750

How many ml is 1.33 l + 1.25 l + 500 ml

3,080 ml First convert the l to ml... 1.33 l is 1,330 ml and 1.25 l is 1,250 ml; then add them all together with the 500 ml... 1,330 + 1,250 + 500 = 3,080

How many ml is half of 1 l

500 ml First convert 1 l to 1,000 ml, then divide by half... 1,000/2 = 500

In liters, what is 400 ml divided by 4

0.1 l First convert 400 ml to 0.4 l, then divide by 4... 0.4/4 = 0.1

Note: In medicine you will encounter the use of cc or cubic centimeter. This is the equivalent of a milliliter (ml). 125 cc = 125 ml, or a 10 cc syringe = a 10 ml syringe. Please refrain from using cc as it could be misread as two zeros

Next we will look at the USC System (what we use in everyday life and the kitchen) and how to convert to the Metric system