Drug CalculationsDrug Calculations

Aims of this sessionAims of this session

review the importance of drug review the importance of drug calculationscalculations

review formulas used for drug review formulas used for drug calculations calculations

work through an examples bookletwork through an examples booklet

Nurses need an understanding Nurses need an understanding of basic calculations:of basic calculations:

FractionsFractions DecimalsDecimals PercentagesPercentages SI Units; (e.g. millilitre-ml; milligram-SI Units; (e.g. millilitre-ml; milligram-

mg)mg) Conversion of metric unitsConversion of metric units Dilution of solutionsDilution of solutions

FractionsFractions Fractions are made up of numerators and Fractions are made up of numerators and

denominators denominators Numerator is the top numberNumerator is the top number Denominator is the bottom numberDenominator is the bottom number An improper fraction is one in which the An improper fraction is one in which the

numerator is larger than the denominator numerator is larger than the denominator e.g. e.g. 55

33 A proper fraction has a numerator smaller A proper fraction has a numerator smaller

than the denominator e.g. than the denominator e.g. 11 4 4

Improper fractions can be converted Improper fractions can be converted to mixed numbers, e.g. to mixed numbers, e.g.

55 = = 11 22 oror 3232 = = 4 4 44

3 3 7 73 3 7 7

Mixed numbers are partly whole Mixed numbers are partly whole

numbers, partly fractions e.g. numbers, partly fractions e.g. 1155 or or

4411

8 28 2

Mixed numbers can be converted to Mixed numbers can be converted to improper fractions e.g.improper fractions e.g.

1155 = = 1313 or or 4411 = = 99

8 8 2 28 8 2 2

DecimalsDecimals

Generally used to mean a number that Generally used to mean a number that includes a decimal point.includes a decimal point.

e.g. 6.25 has 2 decimal placese.g. 6.25 has 2 decimal places 0.625 has 3 decimal places0.625 has 3 decimal places 62.5 has 1 decimal place62.5 has 1 decimal place

To the right of the decimal point are To the right of the decimal point are tenths, hundredths, thousandths etctenths, hundredths, thousandths etc

PercentagesPercentages

This is the number of parts per This is the number of parts per hundred parts.hundred parts.

e.g. 9% = 9 parts per 100 partse.g. 9% = 9 parts per 100 parts

3.5% = 3.5 parts per 100 parts 3.5% = 3.5 parts per 100 parts

Units of WeightUnits of Weight

1 milligram (mg) = 1000 1 milligram (mg) = 1000 micrograms micrograms

(mcg)(mcg)

1000 milligrams (mg) = 1 gram (g)1000 milligrams (mg) = 1 gram (g) 1000 gram (g) = 1 Kilogram 1000 gram (g) = 1 Kilogram

(Kg)(Kg)

Milligrams are sometimes Milligrams are sometimes expressed in terms of grams:expressed in terms of grams:

500mg = 0.5g500mg = 0.5g 250mg = 0.25g250mg = 0.25g 10mg = 0.01g10mg = 0.01g 1mg = 0.001g1mg = 0.001g

Examples of conversions from Examples of conversions from milligrams to micrograms:milligrams to micrograms:

0.1mg = 100 micrograms0.1mg = 100 micrograms 0.2mg = 200 micrograms0.2mg = 200 micrograms 0.25mg = 250 micrograms0.25mg = 250 micrograms 0.4mg = 400 micrograms0.4mg = 400 micrograms 0.6mg = 600 micrograms0.6mg = 600 micrograms

GRAMS TO KILOGRAMSGRAMS TO KILOGRAMSDivide by 1000Divide by 1000Move decimal point 3 places to the Move decimal point 3 places to the LEFTLEFTE.G.E.G. 4000 g = 4 kg 4000 g = 4 kg

KILOGRAMS TO GRAMSKILOGRAMS TO GRAMSMultiply by 1000Multiply by 1000Move decimal point 3 places to the Move decimal point 3 places to the RIGHTRIGHTE.G.E.G. 4kg = 4000 g 4kg = 4000 g

Percentage CalculationsPercentage Calculations Measurement of concentration Measurement of concentration

commonly used is percentage.commonly used is percentage. Example – a 1% volution is:Example – a 1% volution is: 1g in 100ml = 1% w/v (solid in liquid)1g in 100ml = 1% w/v (solid in liquid) 1g in 100g = 1% w/w (solid in solid 1g in 100g = 1% w/w (solid in solid

e.g. e.g.

ointment)ointment) 1ml in 100ml = 1% v/v (liquid in 1ml in 100ml = 1% v/v (liquid in

liquid)liquid)

1% solution = 1g in 100ml1% solution = 1g in 100ml

1000mg in 10ml1000mg in 10ml

10mg in 1ml10mg in 1ml

50mg in 5ml50mg in 5ml

Drug Dosages - InjectionsDrug Dosages - Injections

FormulaFormula

Strength RequiredStrength Required X Volume of stock solution = Volume X Volume of stock solution = Volume

Stock StrengthStock Strength Required Required

ExampleExample Cortisone 40mg is required ampoule contains Cortisone 40mg is required ampoule contains

50mg/2ml. What Volume is required?50mg/2ml. What Volume is required?

40mg40mg x x 2ml 2ml 4040 x x 22 = = 8 8 or or 1.6ml1.6ml50mg 1 50mg 1 50 1 5 50 1 5

FormulaFormulaStrength RequiredStrength Required X Volume of stock solution = Volume X Volume of stock solution = Volume

Stock StrengthStock Strength Required Required

WorkbookWorkbook

Drug Dosages – Drug Dosages – Tablets/MixturesTablets/Mixtures

FormulaFormula

Strength RequiredStrength Required X Volume of stock solution = Amount X Volume of stock solution = Amount

Stock StrengthStock Strength Required Required

ExampleExample30mg tablets Phenobarbitone.30mg tablets Phenobarbitone.

How many are required for a dose of 45mg?How many are required for a dose of 45mg?

45mg45mg x 1 = 1.5 tablets x 1 = 1.5 tablets

30mg 30mg

FormulaFormula

Strength RequiredStrength Required X Volume of stock solution = Amount X Volume of stock solution = Amount

Stock Strength RequiredStock Strength Required

WorkbookWorkbook

Drug Dosages –Drug Dosages – Intravenous Infusion Rates Intravenous Infusion Rates

Formula Formula

Volume (ml)Volume (ml) x x DropsDrops = Rates = Rates (drops/minute)(drops/minute)

Time (hours) 60Time (hours) 60

ExampleExample

800ml fluid to be given over 5 hours. Giving 800ml fluid to be given over 5 hours. Giving set delivers 15 drops/ml. What rate is set delivers 15 drops/ml. What rate is required?required?

800 800 X X 1515 800800 x x 1515 = 40 drops/minute = 40 drops/minute5 hrs 605 hrs 60 5 5 60 60

Formula Formula

Volume (ml)Volume (ml) x x DropsDrops = Rates (drops/minute) = Rates (drops/minute)Time (hours) 60Time (hours) 60

WorkbookWorkbook

Any QuestionsAny Questions