Units of measurement: weight

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:

Units of measurement: volume

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:

Converting from larger to smaller units

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.

• For example, to convert 2 kg to g:

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

• To convert 3.4 L into mL.

 3.4 L x 1000= 3400 mL

Or move the decimal point 3 places to the right: 3.4 L  ► 3400 mL

Converting from smaller to larger units

To convert from smaller to larger units, divide by 1000 or move the decimal point three places to the left.

• To convert 300 mg into g:

300mg/1000= 0.3 g

OR move decimal point three places to the left 300 mg ►0.3 g

• To convert 1700 mL into L:

1700 mL/1000 = 1.7 L

OR move the decimal point three places to the left

1700 mL ►1.7 L

Time calculations

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:

• 60 seconds (sec) = 1 minute (min)
• 60 minutes = 1 hours (hr)
• 24 hours = 1 day
• 7 days = 1 week
• 52 weeks = 1 year

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.

Further resources

There are lots of online and printed resources to help you develop your skills to calculate medication dosages:

Formula to calculate dosages

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:

• The dose and form in which the drug is available. For example, the drug could be presented in mg/tablet or capsule, or mg diluted in solution (mg/ml). The volume of the drug can be in tablets/capsules or vials (mL).
• Ensure that all dosages are expressed in the same units of measurement (i.e. mgc, mg, g). In some cases, you will need to convert them (see section on units of measurement for further information)

Dose = Stock Required    x    Volume 

              Strength                       1

In the next section, this formula will be applied to calculate oral (tablets) and volume doses.

Calculating oral 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

Calculating volume doses

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.

Calculating dosages by patient weight

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.

Fluid calculation in drop rate

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