Conversion factors make use of the relationship between two units or quantities expressed in the fractional form. The *factor-label method* , also called dimensional analysis, changes one unit to another by using conversion factors.

Conversion factors are helpful when you need to compare two measurements that are not in the same units. If given a measurement in meters and the map only reads in kilometers, you have a small problem. You could guess or use the conversion factor of 1 km/10m^{3}. Look at the conversion below.

0.392m x 1 km/10m^{3 }= 0.392 x 10^{3 }km

= 3.92 x 10^{-4}

If you have centimeters and need to know the answer in inches, then use the conversion factor 1 inch/2.54 cm. (Once again, I need to reiterate that certain scientific data just needs to be committed to memory.) This particular conversion factor has been as available to me as my name, since my sophomore year of college. Do you remember the article I wrote about how silly the measuring system is in America? I just buckled down with flashcards and memorized several conversion factors. I needed to make my life easier. Face the facts; If you plan a career in the health sciences, there is no escaping just knowing the information. Forget learning what you need to know for the test. If you know *all* of the information about your subject matter, the test becomes irrelevant. You will conquer the test!

Your flashcards should look like this.

1 inch = 2.54 cm

You will be amazed at how simply looking at this data will plant it into your memory. Remember the songs that we hate, but know all of the words because the radio station plays them so much. I have teenagers. Need I say more? There are many songs that I wish I could forget the lyrics. Repetition works. I know we are in the day of high technology. I create marvelous Power Point presentations, but still receive greater command of any information that I physically write.

914 cm x 1 inch/2.54 cm = 360 inches (since 914 has 3 significant digits)

Converting measurements can also be a two-step process.

mg à g–à kg

liters-àquarts-à gallons

miles per hour -à liters per minute

Look at the two step conversions below.

Example 1

2461 mg-à ? kg

mg à g–à kg

1mg = 10^{-3} g; 1 kg = 10^{3} g (conversion factors) You must have these to begin.

Even if a professor allows you to have this information available during a test, it would look foolish for a young intern to need to peek at a conversion factor to determine a medication dosage. You will administer many doses of medications in your career. Make it easy and re-learn this information if you are rusty. Professionals are not born, but rather created. These days we have to continue to reinvent ourselves.

2461 mg x 10^{-3} g/mg x 1 kg/10^{3} g

= 2461 x 10^{-6 }kg = 2.461 x 10^{-3} kg

Example 2

8.47 liters à ? gallons

liters-àquarts-à gallons

1.06 qt/liters; 1 gal.4 qt (conversion factors)

(I still chuckle about running away from home in the third grade because I thought feet, cups, drops, and pinches were ridiculous! Learn to enjoy the more organized way to measure, by units of ten.)

8.47 liters x 1.06 qt/liter x 1 gal/4 qt = 2.24 gallons

Example 3

70 miles per hour àmeters/minute

Miles/hour à km/hour; à m/hrà m/min

1.61 km/mi; 10^{3} m/km x 1 hr/60 min (conversion factors)

70 mi/hr x 1.61 km/mi x 10^{3} m.km x 1 hr/60 min

=1878.33 m/min = 1.9 x 10^{3} m/min

SI derived units are obtained by combining SI base units.

Habits can be difficult to break. If you travel internationally you will see just how rusty you can be when every distance is measure in meters and kilometers instead of miles. We will review temperatures during the next lecture, quickly complete our survey of bones, and spend an immense amount of time on blood. You will do well with this highly technical information after this hefty review. I hope have not been too bored! Have a great day.