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Dosage Calculation - Worksheet
Dosage calculation is done to calculate the required number/volume of drug(s) to administer to the
patient. The medication might be administered in two ways; tablet form and fluid form. To solve these
kinds of problems, you need good knowledge of Fractions, Long division, Decimals, and Unit
conversions.
Formula
Oral Dose:
Intravenous Dose:
Stock required is the amount of drug you need to administer to the patient. In other words, this is
what you want.
Stock strength the dosage strength available in the current stock. In other words, this is what you
have got.
** the units for SR and SS need to be same. If the units are different one unit needs to be converted
before doing any further calculations.
Example
The doctor has prescribed 50 mg of Amitriptyline to a client. There are 10 mg tablets available in
stock. How many tablets will you administer?
Answer
Stock required: 50 mg
Stock strength: 10 mg
This is an oral dose.
So, tablet dosage =
= 5 tablets
Stock Required (SR)
Stock Strength (SS)
Stock Required (SR)
Stock Strength (SS)
X Volume of stock
50 mg
10 mg
Example
A client with acute cholecystitis is to receive 90 mg Gentamycin IV. On hand are vials containing
80mg/ 2ml.How many mls should be given?
Answer
Stock required: 90 mg
Stock strength: 80 mg/2ml
Volume of stock: 2 ml
So, Dosage =
= 2.25 ml
Including unit conversion
Example
A client is ordered 0.4 mg Thyroxine. Each tablet contains 200 mcg. How many tablets will you
administer?
Answer
Stock required: 0.4 mg
Stock strength: 200 mcg
Need to convert the SR amount to mcg.
So, SR= 0.4mg= 0.4 X 1000 = 400mcg
Dosage =
= 2 tablets.
Example
A client is ordered 80mcg of Fenatyl citrate intravenously. 0.2 mg in 2 mls of liquid for IV Injection is
available. How many mls will you administer?
Answer
Stock required (SR): 80 mcg
Stock strength (SS): 0.2mg/2ml
90 mg
80 mg
X 2ml
400 mg
200 mg
Example
A child weighs 12kg and is ordered 20mg/kg/dose. Stock contains 6mg/ml. Find how many mls you
would administer.
SR = 20mg/kg/dose
SS = 6mg/ml
Child’s weight= 12kg
Dose ൌ ൬
Stock required
Stock strength
ൈ volume൰ ൈ Weight
Dose ൌ
ൈ 1 ൈ 12 ൌ 40ml
So, you should administer 40 mls.
If the dose needs to be administered more than once a day, e.g. every 6 hours or 8 hours, there
would be a small added step to find out the volume to administer.
Example
A child weighs 24kg and is ordered 12mg/kg/day every 6/24, stocked as 40mg/4ml. Find the mg per
dose, the equivalent mls and the total dose over 24hrs to be given.
SR = 12mg/kg/day
SS = 40mg/4ml
Child’s weight= 24kg
Dosage frequency 6/24, i.e. every 6 hours, so, four times a day.
Total dose ൌ ൬
Stock required
Stock strength
ൈ volume൰ ൈ Weight
Total dose ൌ
ൈ 4 ൈ 24 ൌ 28.8ml
So, mls per dose =
ଶ଼ .଼
ସ
= 7.2 mls
mg per dose= ቀ
ୗ୲୭ୡ୩ ୫
ୗ୲୭ୡ୩ ୴୭୪୳୫ୣ
ൈ SR volume ቁ
ସ
ସ
ൈ 7.2ቁ ൌ 72 mg
Practice Exercises
Question 1
Fergon Elixir contains 250 mg of Ferrous Sulphate dissolved in 8 ml of solvent. How many mls of
elixir contain 100 mg of Ferrous Sulphate?
Question 2
A client is ordered 8 mg of Trifluoperazine orally. 0.04g in 6 ml of liquid forte is available. How many
millilitres will you administer?
Question 3
A client is ordered 450 mg of Carbamazepine. Available tablets are of 150mg. How many tablets will
you give?
Question 4
A doctor has ordered 62.5mcg of Benztropine Mesylate to a client but the pharmacy has dispensed
0.125 mg tablets. How many tablets will you administer to the client?
Question 5
The client has been ordered codeine -2 tablets QID (four times a day). The stock strength of Codeine
is 30mg per tablet. How many mg of codeine will this client receive per day?
Question 6
A child weighs 25kg and is ordered 12mg/kg/day to be given TDS. Stock contains 10mg/2ml. Find
how many mls are to be given in each dose.
