Whether you're pre-nursing, currently a nursing student, or new nurse, understanding how to calculate drug dosages is a vital concept to master.
Medication administration is a core competency for all nurses in any clinical setting. Accurately calculating dosages is a crucial skill required to safely administer medications. While there is technology to help prevent medication errors i.e. bar code scanning and smart infusion pumps, we cannot fully rely on these advancements. The simplest calculation strategy we'll go over today is the universal formula, also known as the "desired over have" method. Want the full study step-by-step dosage calculation guide? Don't forget to get yours today for 50% off!
Universal Formula: Desired Over Have Method
Desired over have = the desired amount (D) aka the dose prescribed and the amount on hand (H) or the amount you “have” available. The quantity (Q) is the form and amount in which the drug is supplied (i.e. tablet, capsule, liquid). To calculate the dose, take the desired amount and divide it by the amount on hand, then multiply it by the quantity, like this:
Example 1: Administer 250 mg of xyz med. You have xyz med in 500 mg tablets. How many tablets are you going to give?
Dose: 250 mg
Have: 500 mg
Quantity: 1 tablet
200 ÷ 500 x 1 = 0.5
Write final answer correctly: 0.5 tablets (NOT 0.50)
IV Infusion: What is the rate (mL/hr)? How many hours? How much to infuse (mL)?
What if you're asked to infuse a medication over LESS than 1 hour, such as 15 minutes, 30 minutes, or 45 minutes?
15 minutes: this is 1/4 of an hour, so 0.25 hours.
30 minutes: this is 1/2 of an hour, so 0.50 hours.
45 minutes: this is 3/4 of an hour, so 0.75 hours.
Simply divide the minutes by 60 as shown in the table below. Remember, you need to administer the milliliters per hour, so we have to have our time in hours!
Example 1: Infuse LR 500 ml over 4 hours. What is the rate?
500 ml ÷ 4 hours = 125 ml/hr
Example 2: Infuse NS 300 ml over 30 minutes. What is the rate?
300 ml ÷ 0.5 hours = 600 ml/hr
Example 3: Infuse 1 liter over 5 hours. What is the rate?
Remember we need to convert liter → ml first (add 3 zeros: 1 liter = 1,000 ml)
1,000 ml ÷ 5 hours = 200 ml/hr
Infusion Completion Times
Example 1: An IV medication was started at 0615 and will take 6 hours to infuse. What time will it be completed? Answer in military time.
1. Add 6 hours to 0615 (6:15am)
2. 6 hours from 0615 is 1215
3. Answer in military time: 1215
Example 2: You started an IV infusion of 1,000 ml NS on Monday at 1600. If the infusion rate is 100 ml/hr, what day and time will it be completed? Answer in military time.
Formula: Volume (ml) ÷ infusion rate (ml/hr) = hours to complete
1. 1,000 ml ÷ 100 ml/hr = 10 hours
2. Add 10 hours to Monday 1600 (4pm)
3. That gives us 0200 (2am) on Tuesday
4. Answer in military time: Tuesday 0200
Nursing school is hard enough, this is why I created a dosage calculation nursing study guide bundle for you! My dosage calculation study guide includes step-by-step calculations and examples from this post along with how to calculate IV drip rate (gtt/min), weight-based calculations and reconstitution medications. Get your complete 5-page dosage calculation study guide bundle today for 50% off before the sale ends!
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To help you remember anticholinergic and cholinergic drugs: Remember SEE, SPIT, SHIT (Excretion). Cholinergic drugs = " can SEE, can SPIT, and can SHIT (plus urinate)" Anticholinergic drugs = "can’t SEE, can't SPIT and can't SHIT"
Cardiac glycosides are a class of medications that are derived from plants, used to treat various cardiovascular conditions, including heart failure and certain arrhythmias. The primary mechanism of action of cardiac glycosides is to increase the force of contraction of the heart muscle.
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