1. Conversions: Metric and Household Systems
Chapter 4
Conversions: Metric and Household Systems

2. Conversions between Metric and Household Systems
Review base units of the metric system

3. Pre-scribed Medications:
Ordered in a unit of weight measurement
usually gram or milligram
RN responsibility to interpret order
RN responsibility to use conversions if necessary
Administer correct amount of medication prescribed
Give example on board: ORDER: Acetaminophen 650 mg to be given. ON HAND is 325 mg tablets. HOW many??
If sam e amount in liquid: 650 mg ON HAND: 325 mg per 5 mL HOW much?

4. Identifying Need for Unit Conversion
A drug order written in a unit of measurement different from what is supplied
Usually written in one metric size (mg) but is supplied in another metric unit (g)
Encounter apothecary or household measure
Converting a unit of measure does not change amount of ordered dosage of medication! It provides an alternate expression of dosage.
Example to be done on board next slide

5. Identifying Need for Unit Conversion
Medication order versus supply on hand
Medication order: triazolam 250 mcg orally at bedtime
Supply on hand: Halcion 0.25 mg tablets
Medication order: Antacid Plus ½ fl oz after meals
Supply on hand: Antacid Plus 1 fl oz (30 mL)
First example: prescribed quantity must be converted into the unit of measure supplied ANSWER= 0.25 mg tablets; give 1 page 98-99
Second example: nurse selects correct unit of measure from 2 options provided on label before calculating ordered dose

6. Identifying Need for Unit Conversion
First example: prescribed quantity must be converted into the unit of measure supplied
Second example: nurse selects correct unit of measure from 2 options provided on label before calculating ordered dose
Therefore, calculating unit conversions are the first steps in dosage calculations

7. RULE FOR PROPORTION/RATIO
Recall Equivalents
Set up proportion for 2 equivalent ratios
Cross multiply to solve for an unknown quantity, X
Another method to perform conversions to change from one unit to another within the same measurement system is:
How do we do this?
We us ratio/proportion to convert from one unit to another, you need to follow these 3 steps

8. Conversions
Proportions and ratios are set up for solving in the same format. THIS IS ESSENTIAL. The like units must be across from each other. The units in the numerators match, and the units in the denominators match.
In a proportion, the ratio for a known equivalent equals the ratio for an unknown equivalent
Remember: a proportion compares like things to like things

9. Converting Using Ratio-Proportion
Example: How many cups are equivalent to 3 quarts?
It is known that 1 qt equals 4 cups
Set up proportion by expressing ratios as fractions
What we know: the first ratio of the proportion contains the known equivalent; 1:4
the second ratio contains the desired unit of measure and the Unknown Equivalent expressed as X 3:X
Express the ratios as fractions…
Note: X, in the denominator, is the unknown.

10. Converting Using Ratio-Proportion
Cross-multiply
Simplify: Divide both sides by number before X
Therefore, 3 qt = 12 cups
Do example 2 on the board from page 100: example 2: ask class what is the known equivalent AND then, the desired unit of measurement and the unknown equivalent

11. In review: Using ratio-proportion method: Recall known equivalent
Set up proportion
Ratio for known equivalent equals ratio for unknown equivalent
Cross-multiply to solve for unknown quantity (X)

12. Converting within Metric System
Most conversions are derived by multiplying or dividing by 1,000
Multiplying by 1,000
Move decimal three places to right
Used when converting from larger to smaller unit
Note: Sometimes you have to add zeros to hold the
places equal to the number of zeros in the equivalent.
Example: 2 grams per tablet means the same as
2,000 milligrams per tablet.
1g = 1,000 mg, so 2 x 1,000 = = 2,000 mg
On board can show ratio proportion method

13. Converting within Metric System
Most conversions are derived by multiplying or dividing by 1,000
Dividing by 1,000
Move decimal three places to left
Used when converting from smaller to larger unit
Example: Convert 20 mg to g
1g = 1000 mg, so we divide moving decimal 3 places to left; x=0.02g
Can show this in ratio proportion method too on board.

14. Converting within Metric System
Example: Convert 0.3 g to equivalent number of mg
Recall known equivalent
Equivalent is: 1 g = 1,000 mg
Set up equivalent ratios
Ask class to do this themselves before answering

15. Converting within Metric System
Cross-multiply
Answer:
Ask class to convert 50 mL to L
What is first rule….second rule…third rule…

16. Converting within Metric System
Example: Convert 50 mL to L
Recall known equivalent
Equivalent is: 1 L = 1,000 mL
Set up equivalent ratios

17. Converting within Metric System
Cross-multiply
Simplify

18. Remember:
When converting from a larger to a smaller unit of measure, the number of smaller units should be greater than original number of larger units Conversely, When converting from a smaller to a larger unit of measure, the number of larger units should be less than the original number of small units

19. Conversion Slide
Use this diagram when converting dosages within metric system
See which way the arrow points then:
1 mcg = mg (moved decimal point to the left 3 places)
2 g = 2,000 mg (moved decimal point to the right 3 places) kg g mg
mcg
Move decimal three places to left for each step
Move decimal three places to right for each step
On the board: page 101 example 2-6 (practice for converting from larger unit to smaller unit)
example 1-5 (practice for converting from smaller unit to larger unit)
Have class work on review set 13 pt 106

20. Approximate Equivalents
1 t = 5 mL
1 T = 3 t = 15 mL = fl oz
1 fl oz = 30 mL = 6 t = 2 T
1 L = 1 qt = 32 fl oz = 2 pt = 4 cups
1 pt = 500 mL = 16 fl oz = 2 cups
1 cup = 250 mL = 8 fl oz
1 kg = 2.2 lb
1 inches (in) = 2.5 cm need to know!
Infrequent and may soon be completely obsolete
Any conversions based in the houselhold/apothecary system is based on approx. equiv.
They are called approx. because it is impractical to use exact equivalents; ie,
A more exact equiv of 1 US fl oz is mL or approx. 30 mL
In the UK, 1 fl oz is mL

21. Converting Between Systems of Measurement
Use ratio-proportion method
Same steps as when converting within same system of measurement

22. Converting Between Systems of Measurement
Convert 2 fl oz to mL
-converting from a larger to smaller unit
Recall equivalent
Equivalent is: 1 fl oz = 30 mL
Set up equivalent ratio
Cross-multiply
Converting between larger to a smaller unit

23. Converting Between Systems of Measurement
Example: Convert 45 mL to t
Converting from smaller to larger unit
Recall equivalent
Equivalent is: 1 t = 5 mL
Set up equivalent ratio
how about

24. Converting Between Systems of Measurement
Cross-multiply
Simplify
Then , convert from a smaller to a larger unit of measure
On board, examples 2-3 page
THEN ask: what about kg and lb….what is the known equivalent? Set up problem for determining what 40 kg is in lbs

25. Converting Between Systems of Measurement
Approximate equivalent: 1 kg = 2.2 lb
(so our weight in kg is about half of our weight in pounds)

26. Converting Between Systems of Measurement
When converting from a smaller to a larger unit of measure, the resulting number of larger units will be less than the original number of smaller units.
When converting from a larger unit of measure to a smaller unit of measure, the resulting number of smaller units should be greater than the original number of larger units.
Always look at your answer and ask yourself if it makes sense.

27. Quick Review
Steps for using ratio-proportion method to convert within or between systems of measure:
Recall known equivalent
Set up proportion ratio
Label and match units in numerators and denominators
Cross-multiply to find value of x
Label units in answer to match unknown x
Have class work on review set 14, questions 1-30

28. Can go over odd problems from review set 14 questions 31, 32, 36, 37