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Nursing: Medication Calculations
Common calculations required in nursing include finding volumes needed for oral or
injected doses and working out IV infusion flow and drip rates. All these calculations
require the application of various maths skills looked at in previous worksheets.
This worksheet will teach you to calculate volume required for an injected dose and oral
dose, as well as working with paediatric doses.
When medication is injected
The formula to calculate the volume required for an injection is:
The volumes are measured in millilitres (mL).
The strength required and stock strength are measured in grams (g), milligrams (mg) or micrograms (mcg).
You must make sure that the unit of measurement for the stock strength and strength required match. If they do not match, then convert them both to the smaller of the two units.
Examples:
a) Calculate the volume to be drawn up if a patient requires 1mg of bumetanide, and stock ampoule contains 2mg/4mL.
Strength required is 1mg.
The stock strength is 2mg in 4mL.
The units match so the formula becomes:
๐ฃ๐๐๐ข๐๐ =
๐ ๐ก๐๐๐ ร
2 ร
Answer: the volume to be drawn up in 2mL.
Note: After you have calculated your answer always look again at the strength required and stock strength and consider if your required volume looks reasonable.
b) An infant requires 2000 units of Calciparine. If stock ampoules contain 20000 units in 1mL, how much should you draw up?
Strength required is 2 000 units. Stock strength is 20 000 units in 1 mL So the formula becomes:
๐ฃ๐๐๐ข๐๐ = ๐๐๐๐ข๐๐๐๐๐ ๐ก๐๐๐ ร ๐ ๐ก๐๐๐ ๐ฃ๐๐๐ข๐๐ 1 = 20000 2000 ร^11 ๐๐ฟ = 20000 2000 ๐๐ฟ = 20 2 ๐๐ฟ = 10 1 ๐๐ฟ =
- 1 10 1. 0 = 0.1๐๐ฟ
Answer: the volume required is 0.1mL.
ACADEMIC SKILLS Division of Student Success
c) An adult patient was ordered 85mg of cortisone. Each stock ampoule was 5mL and contained 100mg of cortisone. What volume is required?
Strength required is 85mg.
Stock strength is 100mg in 5mL.
So the formula becomes:
๐ ๐ก๐๐๐ ร
100 ร
5 10
Answer: the volume required is 4.25mL.
d) A patient is ordered 70mg of pethidine. Find the volume required if the stock solution contains 10g of pethidine per 200mL.
Strength required is 70mg.
Stock strength is 10g in 200mL.
First change the g to mg to make the units the same:
10g x 1 000 = 10 000mg
So stock strength is 10 000mg in 200mL
๐ ๐ก๐๐๐ ร
1 =^
10000 ร
4
Answer: the volume required is 1.4mL of stock solution.
Mixtures
When administering a dose of medication in a suspension form, the formula is the same as for injections:
๐ฃ๐๐๐ข๐๐ ๐๐๐๐ข๐๐๐๐ =
๐ ๐ก๐๐๐๐๐กโ ๐๐๐๐ข๐๐๐๐ ๐ ๐ก๐๐๐ ๐ ๐ก๐๐๐๐๐กโ
ร
๐ฃ๐๐๐ข๐๐ ๐๐ ๐ ๐ก๐๐๐ 1
Examples:
a) A patient who has a sore head from an accidental fall is prescribed 1 000mg of a mild painkiller to be taken orally. The stock contains 100mg in 5mL. How much medication should be given to the patient?
Strength required is 1 000mg
Stock strength is 100mg in 5mL.
๐ฃ๐๐๐ข๐๐ =
๐ ๐ก๐๐๐ ร
100 ร
1 ๐๐ฟ =^
Answer: 50mL of painkiller should be given to the patient.
b) Bill is prescribed 800mg of erythromycin, and stock on hand contains 500mg/10mL in suspension. Find the amount of suspension required.
Strength required is 800mg.
Stock strength is 500mg in 10mL.
๐ฃ๐๐๐ข๐๐ =
๐๐๐๐ข๐๐๐๐ ร
500 ร
Answer: Bill would receive 16mL of erythromycin.
Stock strength = 125mg in 5mL.
๐ฃ๐๐๐ข๐๐ = ๐๐๐๐ข๐๐๐๐๐ ๐ก๐๐๐ ร ๐ ๐ก๐๐๐ ๐ฃ๐๐๐ข๐๐ 1 =^270125 ร^51 ๐๐ฟ =^1350125 ๐๐ฟ =^27025 ๐๐ฟ =^545 =
Answer: the child requires a dose of 10.8mL.
Exercises
1. A patient is ordered an injection of 80 mg of pethidine. Each stock ampoule contains 100 mg per 1 mL. How much will you draw up for the injection? 2. A child requires 50 milligrams of Phenobarbitone. If stock ampoules contain 200 milligrams in
2 mL, How much will you draw up?
3. What volume is required for the injection if a patient is ordered 500 mg of capreomycin sulphate, and each stock ampoules contains 300 mg/mL? 4. A patient needs 5000 mg of medication. Stock solution contains 1 g per 1 mL. What volume is required? 5. A teenager is prescribed 1000 mg of chloramphenicol. Stock on hand contains 250 mg/10 mL
in suspension. Calculate the volume required.
6. A patient is prescribed 3g of sulphadiazine, and the stock contains 600 mg/5 mL. How much sulphadiazine should be given to the patient? 7. How many 300 mg tablets should be administered for a prescribed dose of 450 mg? 8. How many 25 mg tablets are required for a prescribed dose of 0.05 g? 9. Calculate the size of a single dose if a child weighing 20 kg is ordered flucloxacillin 100 mg/kg/day, 4 doses per day. 10. A doctor orders penicillin V potassium oral suspension 56 mg/kg/day in four doses per day for a child who weighs 25 kg. The suspension available is 125 mg/5mL. What volume should be administered at each dose?
Solutions
1. 0.8 mL 2. 0.5 mL 3. 1.7 mL 4. 5 mL 5. 40 mL 6. 25 mL 7. 1.5 tabs 8. 2 tabs 9. Strength required per day = 20 kg x 100 mg each kg = 2000 mg
Strength required per dose = 2000 mg รท 4 = 500 mg
10. Strength required per day = 25 kg x 56 mg each kg = 1400 mg
Strength required each dose = 1400 mg รท 4 = 350 mg
๐ฃ๐๐๐ข๐๐ =
125 ร
For more information
Visit https://www.csu.edu.au/current-students/learning-resources/build-your-skills/academic-skills- help/numeracy for more information.
References
The following resource will provide you with further useful information on this topic:
Gatford, J. D., & Phillips, N. (2011). Nursing calculations (8th ed.). Churchill.
