1. Drug Calculations

2. Aims of this session review the importance of drug calculations
review formulas used for drug calculations
work through an examples booklet

3. Nurses need an understanding of basic calculations:
Fractions
Decimals
Percentages
SI Units; (e.g. millilitre-ml; milligram-mg)
Conversion of metric units
Dilution of solutions

4. Fractions Fractions are made up of numerators and denominators
Numerator is the top number
Denominator is the bottom number
An improper fraction is one in which the numerator is larger than the denominator e.g. 5 3
A proper fraction has a numerator smaller than the denominator e.g. 1 4

5. Improper fractions can be converted to mixed numbers, e.g.
5 = 1 2 or 32 = 4 4

6. Mixed numbers are partly whole numbers, partly fractions e.g. 15 or 41

7. Mixed numbers can be converted to improper fractions e.g.
15 = 13 or 41 = 9

8. Decimals
Generally used to mean a number that includes a decimal point.
e.g has 2 decimal places
0.625 has 3 decimal places
62.5 has 1 decimal place
To the right of the decimal point are tenths, hundredths, thousandths etc

9. Percentages This is the number of parts per hundred parts.
e.g. 9% = 9 parts per 100 parts
3.5% = 3.5 parts per 100 parts

10. Units of Weight 1 milligram (mg) = 1000 micrograms (mcg)
1000 milligrams (mg) = 1 gram (g)
1000 gram (g) = 1 Kilogram (Kg)

11. Milligrams are sometimes expressed in terms of grams:
500mg = 0.5g
250mg = 0.25g
10mg = 0.01g
1mg = g

12. Examples of conversions from milligrams to micrograms:
0.1mg = 100 micrograms
0.2mg = 200 micrograms
0.25mg = 250 micrograms
0.4mg = 400 micrograms
0.6mg = 600 micrograms

13. GRAMS TO KILOGRAMS KILOGRAMS TO GRAMS Divide by 1000
Move decimal point 3 places to the LEFT
E.G g = 4 kg
KILOGRAMS TO GRAMS
Multiply by 1000
Move decimal point 3 places to the RIGHT
E.G. 4kg = 4000 g

14. Percentage Calculations
Measurement of concentration commonly used is percentage.
Example – a 1% volution is:
1g in 100ml = 1% w/v (solid in liquid)
1g in 100g = 1% w/w (solid in solid e.g.
ointment)
1ml in 100ml = 1% v/v (liquid in liquid)

15. 1% solution = 1g in 100ml
1000mg in 10ml
10mg in 1ml
50mg in 5ml

16. Drug Dosages - Injections
Formula
Strength Required X Volume of stock solution = Volume
Stock Strength Required

17. Strength Required X Volume of stock solution = Volume
Example
Cortisone 40mg is required ampoule contains 50mg/2ml. What Volume is required?
40mg x 2ml x 2 = 8 or 1.6ml
50mg
Formula
Strength Required X Volume of stock solution = Volume
Stock Strength Required

18. Workbook

19. Drug Dosages – Tablets/Mixtures
Formula
Strength Required X Volume of stock solution = Amount
Stock Strength Required

20. Strength Required X Volume of stock solution = Amount
Example
30mg tablets Phenobarbitone.
How many are required for a dose of 45mg?
45mg x 1 = 1.5 tablets
30mg
Formula
Strength Required X Volume of stock solution = Amount
Stock Strength Required

21. Workbook

22. Drug Dosages – Intravenous Infusion Rates
Formula
Volume (ml) x Drops = Rates (drops/minute)
Time (hours)

23. Example
800ml fluid to be given over 5 hours. Giving set delivers 15 drops/ml. What rate is required?
800 X x = 40 drops/minute
5 hrs
Formula
Volume (ml) x Drops = Rates (drops/minute)
Time (hours)

24. Workbook

25. Any Questions