2. Drug dosage calculation are required when the amount
of medication ordered (or desired) is different from
what is available on hand.
FORMULA –
Amount Desired x Quantity = Y
Amount on Hand
NOTE – When the medication is given in tablets the quantity= 1, Since the
amount of medication available is specified per (one ) tablet.
3. COMMON EQUIVALENTS
1 KG – 2.2 LB
1gm – 1000 mg
1L- 1000 ml
1tsp – 5ml ( tea spoon)
1tbsp – 3 tsp ( 3 x 5 = 15)( table spoon)
1 oz. – 30 ml
(ounce)
4. Example - ( Tab, Capsule)
Ques. Toprol xl, 50 mg PO, is ordered. Toprol xl is available as 100 mg per tablets.
How many tablets would the nurse administer.
Answer: Required = 50 mg Units are same
On Hand = 100 mg
Quantity = 1
50 X 1 = 0.5mg tab ( Ans.)
100
5. Example (Liquid,Suspenssion )
Ques. Dilantin- 125 mg is available as 125mg /5ml. Dilantin 125, 0.3 g PO, is ordered
.How much should the nurse administer to the patient.
Ans. Desired – 0.3g 3 x 1000 = 300 mg
( 1gm = 1000mg ) 10
On hand = 125 mg
Quantity = 5ml
300 x 5 = 12ml ( Ans.)
125
6. Drug calculation based on weight
Stage – 1
Using the formula below , calculate the total required dosage based on given the
body weight.
Weight (kg) X Dose ordered ( per kg) = Y ( Required dosage )
Stage 2 - D X Q Formula to calculate amount of medication
H
Medrol 4mg /kg is ordered for a child weighing 64.8lb . Medrol is available as
500 mg /4ml.How many ml of medication must the nurse administer?
Ans. Dose ordered ( per kg ) = 4mg
Weight = 64.8lb = ( Covert the pounds into kg ) ( 1kg = 2.2 pounds )
64.8 = 29.4kg
2.2
Stage 1 : formula put = 29.4 x4 = 117.8 mg ( Desired dose)
7. { Put the desired dose on stage 2 formula}
117.8 x 4 = 0.9 mg ( Ans)
500
8. Calculation of Intravenous Drip rate
Volume (ml ) x Drop factor (gtts /ml ) = Y ( Flow rate in gtts /min)
Time (min)
Ques; Calculate IV flow rate for 250 ml of 0,5 % dextrose to be administer over 180
min. The infusion set has drop factor of 30 gtts/ml.
Ans: Volume - 250ml 250 x 30 = 41.66 gtts/min { 42gtts/min }
Time - 180 min 180
Drop factors – 30
9. Calculation of flow rate for an infusion
pump
Formula = Volume (ml ) = Y ( Flow rate in ml/h)
Time (h)
Ques. 1200ml D5W IV is ordered to infuse in 10 hours by infusing pump.
Calculate the flow rate in milliliters per hour.
Ans. 1200 = 120 ml/ hour
10
10. Macro drip : Tubing generally 10 – 20 gtts /ml and is used to
infuse large volumes or to infuse fluid quickly.
Micro drip : Tubing delivers 60 gtts /ml and is used for small
very precise amount of fluid , as with neonates or pediatrics
patients.
11. Practice questions
1. Give 100mg of a drug three times a day the drug is available as 20 mg / 5ml . Calculate
the dose ?
2. A patient requires amisulpride 200mg .The available stock on the ward is 100 mg /ml
oral solution. What volume is required ?
3 . A patient require dihydrocodeine 30 mg. The available stock on the ward is 10 mg /5ml
.What volume is required?
4. Ordered, 1500mg of calcium carbonate for the patient .The drug is available in 250mg
per tablet. How many tablets should be given to the patient?
