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Medication Calculation

In paediatric practice medication dosages are calculated based on the child’s weight making the dosages much more precise. It is important that the child’s most recent weight in kilograms is entered into the health information system so that drug dosage calculations are accurate.

Use the Hospital for Sick Children Electronic Formulary, which is now part of Lexicomp, to check if the prescribed dose is within a therapeutic range. Another source of information is the electronic CPS (eCPS) found on the hospital internal webpage.

Use the online IV Medication Administration Guidelines also found in Lexicomp on hospital internal webpage to check reconstitution and stability guidelines, administration guidelines, implications and side effects.

Dosage Calculation

Dosages may be calculated based on:

  • Milligrams/kg (e.g. Furosemide)
  • Micrograms/kg (e.g. Fentanyl)
  • Units/kg (e.g. Heparin)
  • Body surface area (BSA) mg/m2

Medication Calculation (milligrams/kg)

A 20kg patient has been ordered Ampicillin 500mg IV q6h.

(a) Validating ordered dose to make sure it is within the therapeutic range:

The recommended dose of Ampicillin is 100-200mg/kg/day  divided q6h, and the nurse must ensure that the dose prescribed is within therapeutic range by calculating the minimum and maximum dosages.

Minimum Dose

100mg x 20kg = 2000mg/kg/day divided q6h

500mg per dose

Maximum Dose

200mg x 20kg = 4000mg/kg/day  divided q6h

1000mg per dose

The child would receive 20kg x 100 mg/kg/day = 2000 mg/day  divided into four doses (24 hours divided by 6 hours) or 500 mg/dose. Therefore, the prescribed dose is within the therapeutic range.

(b) Correct medication reconstitution:

Prescribed dose/ mg available) x mL diluted in = Volume of medication to be administered

Calculation Example 1

Example of Medication Label:


For IM use: Add 1.8mL Sterile Water for Injection to yield approximately 2mL (250mg/mL).  For IV use: Further dilute to 10mL (50mg/mL). Shake Well. Use within 1 hour of reconstitution.

Physician Order: Give 500 mg of Ampicillin IV q6h

The instructions on a 500mg vial of Ampicillin indicate to reconstitute with 1.8ml of sterile water to provide 250mg of Ampicillin per 1 ml.

To administer the desired dose of 500 mg will require to draw 2 ml of already reconstituted Ampicillin (500mg / 250mg) x 1ml = 2ml 


Calculation Example 2

Example of Medication Label:

Ibuprofen 100mg/5mL suspension for oral  use:


 This child now has a temperature of 39.0C, and the physician has ordered Ibuprofen as an antipyretic. Physician Order: Give 200mg (10mg/kg) of Ibuprofen PO Q6h prn for fever

The instructions on the bottle of Ibuprofen indicate that there is 100 mg of Ibuprofen in 5 ml of liquid, or as you can calculate 20mg per 1 ml.

To administer 200 mg, you will need to draw 10mL of Ibuprofen (200 mg x 1ml) / 20 mg = 10 ml

At this time please complete the medication quiz.  The link to the medication quiz is located in the box to the right side of your screen. Please review the College of Nurses of Ontario practice standard entitled Medication. Click here to view this document.


References: College of Nurses of Ontario (2005). Practice Standards: Medication and SickKids Policies and Procedures: Medication Administration