All nurses need to be competent in the calculation of medication dosages. You need to know how to calculate required dosages accurately, including doses of tablets, doses of solutions, and intravenous fluid rates and medications.
These drug calculations will require the application of some basic mathematics such as addition, subtraction, multiplication, and division. You will also need to be able to:
Before starting you may want to review your knowledge on basic mathematic calculations and fractions, percentages and decimals. There are lots of online resources to help you develop and practice your maths skills. These maths worksheets cover essential maths skills.
More resources on essential maths skills that you will need in order to effectively calculate drug dosages can be found here:
If you would like to test you rmaths skills, complete the Numeracy Success Indicator (NSI).
Need more help with Maths? Find out about the Math Skills Program
Medication dosage orders are provided and dispensed in a variety of ways. It is important that you familiarise yourself with the terminology used to measure weight and volume, and how to convert from larger to smaller units  and vice versa.
When applying formulas to calculate dosages, make sure that you are using the same units of measurement; this means that, in some cases, you will need to convert units of measurement. This section will show you how weight and volume are measured.
The gram is the basic unit of weight. The most common multiple of a gram is the kilogram, which is 1000 times greater than the gram. The most common subdivision of a gram is the milligram, which represents 1/1000 of a gram or 0.001 g. The table below shows equivalencies between units of weight:
The litre is the basic unit of volume; a subdivision of a litre is the millilitre. The table below shows the equivalence between litre and millilitre:
Because all metric units are multiples or subdivisions of the major units, you can convert units by dividing by the appropriate multiple or multiplying by the appropriate subdivision.
If you need to convert between a larger a smaller unit (for example, from kilogram to gram), you will multiply by 1000, or move the decimal point 3 places to the right.
2 kg will convert to 2000 g
2kg x 1000= 2,000g
Or move the decimal point 3 places to the right: 2kg ► 2000 g
3.4 L x 1000= 3400 mL
Or move the decimal point 3 places to the right: 3.4 L ► 3400 mL
To convert from smaller to larger units, divide by 1000 or move the decimal point three places to the left.
300mg/1000= 0.3 g
OR move decimal point three places to the left 300 mg ►0.3 g
1700 mL/1000 = 1.7 L
OR move the decimal point three places to the left
1700 mL ►1.7 L
Choose the correct option
Q1 Convert 0.69 g to mg:When working with time, you will need to be able to convert time into fractions and then decimals. This is needed in order to calculate IVT rates, for example.
First of all, make sure you are familiar with how time is measured and the equivalencies between units of time:
For instance, one hour and fifteen minutes can be expressed as a fraction:
1 ^{¼}
To convert the fraction ¼ into decimals, you will divide the top number by the bottom number in the fraction: ¼= 0.25
Thus, one hour and fifteen minutes can be expressed as 1.25 hr (1+0.25)
If you have to convert minutes into decimals, you will follow a similar procedure:
15 minutes can be expressed as a fraction of an hour: 15/60 mins
If we divide the top number by the bottom number, we obtain 0.25 hr
Remember to always include the unit of time measurement that you are using.
Convert the following into decimals:
Q1 90 minutesThere are lots of online and printed resources to help you develop your skills to calculate medication dosages:
Drugs may be administered via several routes. Drugs that are administered orally are usually in tablet, capsule or liquid form. Drugs can also be administered by injection or intravenous infusion. You will need to be able to calculate dosages for oral and liquid medications.
When using formulas, the most important step is to identify and understand what each part of the formula means. Before applying the formula to calculate medication dosages, review important concepts that will help you identify what information you need to look for, and how to use the formula.
Most drugs are available in a limited number of strengths or concentrations. To calculate medication dosages, you will use the stock required, stock strength and volume as part of the formula. It is important to understand what it is meant by strength and volume, as you will need to check this information about the drug to use it correctly in the formula.
The strength is the amount of drug in units or g, mcg, while the volume is the amount of liquid in which the drug has been diluted.
How to use proportions with liquid solutions
This video is very helpful if you need more information on how to read the stock strength and stock volume of medication (that is, the amount of drug and solution available), and how to calculate the volume of solution required.
The formula to calculate the required dose includes the stock required (prescribed by the doctor), the stock strength (amount of drug available) and volume (amount of solution available).
Before you apply the formula, take into consideration two key aspects:
Dose = Stock Required x Volume
Strength 1
In the next section, this formula will be applied to calculate oral (tablets) and volume doses.
To calculate oral doses, the formula can be simplified. The volume is not needed in this case because the drug comes in tablets.
Number of tablets = Dose ordered
Stock Strength
EXAMPLE 1: If the order is 50mg Phenergan, and the stock strength is 25mg tablets, the formula will look like this:
50 mg = 2 tablets
25 mg
EXAMPLE 2: In this example, the order is 0.125 mg Digoxin, and the stock strength is 62.5 mcg tablets.
We first have to ensure that we are using the same units of measurement for stock strength and stock required. In this case, the stock strength is presented in mcg, but the order is in mg. Therefore, you will need to convert the order dose from milligrams (mg) to micrograms (mcg) so that you have the same units:
0.125 mg X 1000 = 125 mcg
If you need to review how to convert units of measurement, see section on units of measurement above.
Once stock strength and dose ordered are expressed in the same units of measurements, the formula can be applied:
Dose = 125 mcg = 2 tablets
62.5 mcg
This activity will guide you through the process to calculate tablet doses.
Do you need to convert units of measurement?
Yes, in this example the stock available is expressed in mg, and the dose required is expressed in g.
Thus, you need to convert the order dose from g to mg. To convert from larger to smaller units:
1g X 1000= 1000 mg
The dose prescribed is now expressed in mg: 1000 mg.
Applying the formula:
Dose= Dose ordered/ Stock Strength
Dose= 1000/500= 2 tablets
Do you need to convert units of measurement?
Do you need to convert units of measurement?
No, in this example strength and dose required are expressed in mg.
Applying the formula:
Dose= Dose ordered/ Stock Strength
Dose= 25/50= 0.5 or 1/2 tablets
Do you need to convert units?
Do you need to convert units?
Yes, in this example dose required is expressed in mcg and strength is expressed in mg.
Thus, you will need to convert from smaller to larger units of measurement:
125 mcg/1000= 0.125 mg
Dose required is now expressed in mg: 0.125 mg
Applying the formula:
Dose= Dose ordered/ Stock Strength
Dose= 0.125/0.25= 0.5 or 1/2 tablets
The formula will now be applied to calculate volume doses, including the volume of stock solution. If the order is 450mg Amiodarone, and the stock strength is 150mg/3mL suspension, the formula will look like this:
Dose = Stock Required x Volume
Stock Strength 1
Dose = 450 mg x 3 mL = 9 mL
150mg 1
Remember to include the unit of measurement in the result: mL, as you are using a liquid solution of the drug.
This activity will guide you through the process to calculate volume doses.
Do you need to convert units?
Yes, in this example the dose required is expressed in mcg, and the stock strength is expressed in mg.
Thus, to convert from smaller to larger units:
120mcg/1000= 0.12 mg
Dose required is now expressed in mg: 0.12 mg.
Applying the formula:
Dose =  Stock Required

x  Volume

Stock Strength  1 
Dose =  0.12 mg

x  2 mL

0.08 mg  1 
Do you need to convert units?
Do you need to convert units?
No, in this example stock strength and stock required are expressed in mg.
Applying the formula:
Dose =  Stock Required

x  Volume

Stock Strength  1 
Dose =  100 mg

x  1 mL

= 2.5 mL 
40 mg  1 
Drug dosages are often ordered according to the patient’s weight. Therefore, you will need to take into account the weight of the patient when calculating dosages.
How to calculate a single dose based on body weight
In this case, you will need to calculate the size of a single dose based on the recommended dosage (in mg per kg per day) and the patient’s weight.
For example, a child is prescribed paracetamol at a dose of 15 mg/kg/dose, 4 doses daily. If the child’s weight is 28kg, to calculate the size of a single dose:
For each kilo of weight, the child should receive 15 mg of drug. Thus: Dose= 15mg (prescribed dose) x 28 (child’s weight in kg) = 42mg dose required
How to calculate volume doses
To calculate the drug suspension dose, you will use the formula to calculate drug dosages that uses the stock strength, stock required and volume strength.
Following the previous example: paracetamol is ordered for children at a dose of 15mg/kg/dose to a maximum of 4 doses per day. A child’s weight is 28kg, and the stock strength is 24mg/mL.
Stock required= 420 mg (this is the stock required that was calculated in the previous step using the child’s weight)
Dose = Stock Required x Volume
Stock Strength 1
Dose = 420 mg x 1 mL = 17.5 mL of suspension per dose
24 mg 1
Finally, we know that each dose is 420 mg, and the child can have 4 doses per day.
Therefore: 420 mg X 4 = 1680 mg per day. This is the maximum amount of drug that the child can receive per day.
This activity will guide you through the process to calculate doses by patient weight.
The child should receive 16 mg/kg/day. First of all, you will calculate total dosage by weight:
16 mg (recommended dosage) x 18 (child's weight in kg)= 288 mg per day
The daily dosage is 288 mg. We know that the child will receive 4 doses daily:
288 mg per day divided by 4 (number of daily doses)= 72 mg per single dose
First, we will calculate the maximum dose per day:
80 mg (maximum recommended dose per day) x 24 (child's weight in kg)= 1920 mg/day. This is the maximum safe dose that the child should receive per day.
We have calculated that the maximum dose per day is 1920 mg. To calculate the maximum safe individual dose:
1920 mg divided by 3 (number of daily doses)= 640 mg per maximum individual dose
In Intravenous Therapy, you will need to calculate the fluid rate for infusion in mL/hr. You will need to take into account the time (in hours) to set the intravenous pump. The formula to calculate the fluid rate is:
Note: The volume must be in millilitres (mL).
Example 1:
If an anaesthetist orders N/Saline 1 L to run over 8 hours, the formula will look like this:
1000 mL = 125 mL/hr. Therefore, you will set the intravenous rate at 125 mL/hr
8 hrs
Note that we converted 1 L into 1000 mL as we need to use the volume ordered in mL
Note that if the time is given in minutes, you will need to convert minutes into hours to use this information in the formula. If you need more information about converting units of time, check the Time Calculations section. Alternatively, you can use this formula:
Rate (mL/hr) = Volume (mL) x60
Time (in mins)
Example 2:
75 mL of fluid needs to be infused over 20 minutes:
Rate (mL/hr) = Volume (mL) x60
Time (in mins)
= 75 mL x 60 = 225 mL/hr
20 min
To calculate fluid drops per minute, you will use the same elements as in the previous formula, but you will have to also include the drop factor (assume a drop factor of 20 unless otherwise stated).
Note: We divide by 60 to convert the drop rate to minutes
Example:
If an anesthetist orders N/Saline 1L to run over 6 hours, the formula will look like this:
1000 mL X 20 = 55.6 dpm (that is, 56 drops each minute if you round up)
6 60
If the time is given in minutes, you can use this formula:
Example:
If a patient is to have 300 mL of dextrose 5% run through in 50 minutes:
300 mL x 20 (drop factor) = 6000 = 120 drops/min
50 minutes 50
This activity will guide you through the process to calculate fluid rates in mL/hr and drop rates
The time is given in minutes. In this example, we will convert minutes into hours:
90 minutes = 90/60 = 1.5 hrs
Using the formula:
Total volume ordered (in mL)

= fluid in mL/hr 
Time (in hours) 
120 mL/1.5 hr = 80 mL/hr
The formula to use is:
Total volume ordered (in mL)

x  drop factor

= drops per minute (dpm) 
Time (in hours)  60 
Since we have to use mL, we will convert 0.5 litre to mL:
0.5 L x 1000 = 500 mL
You need to administer 500 mL of the drug:
Total volume ordered (in mL)

x  drop factor

= drops per minute (dpm) 
Time (in hours)  60 
500 mL/12hrs x 20/60 = 41.66 X 0.33 = 13.88 = 14 drops per minute