Are you feeling overwhelmed when it comes to mastering dosage calculations? You’re not alone! Many medical professionals have difficulty understanding the complex calculations needed for accurate dosages of medications or other solutions.
With some guidance, however, anyone can learn how to calculate doses from various drugs accurately, making them more confident in their work and safe from medication errors. So don’t give up just yet – read on and prepare to become an expert in Dosage Calculations.
| Abbreviations Commonly Used in Dosage | |
| Abbreviation
|
Term |
| po | by mouth (or orally) |
| susp | suspension |
| pm | as needed |
| tab | tablet |
| cap | capsule |
| q | every |
| bid | twice a day |
| tid | three times a day |
| qid | four times a day |
Table of Contents
Dose vs. Dosage
Dose, also known as absolute dose, is the quantity of medicine a patient can take at any ne time. It does not depend on time or body weight. In contrast, when the dose is based on the patient’s weight, it is referred to as a relative dose.
Dosage is the frequency at which this specified dose must be taken or administered.
So, dose is the amount of medicine, while dosage is how often it should be taken.
For example, a patient weighing 70 kilograms is prescribed 2 different medications, 5 mg of Drug X every 6 hours and 10 mg/kg of Drug Y, a relative dose or weight-based dose, every 12 hours.
Thus, for Drug X, the patient’s dose will be 5 mg, and the dosage will be 5 mg every 6 hours.
And for Drug Y, the patient’s relative dose will be 10 mg/kg or 700 mg (10 mg/kg * 70 kg), and the dosage will be 700 mg every 12 hours.
Medicine can come in either solid or liquid form. To calculate the dose of both forms, there is a slight variation. Per the American Medical Association, solid forms are expressed in metric mass units, such as milligrams (mg) or micrograms (mcg). Liquid forms are expressed in milliliters (mL).
Drug Dosage & IV Rates Calculations
Drug Dosage Calculations Drug dosage calculations are required when the amount of medication ordered (or desired) differs from what is available for the nurse to administer.
Formula:
Note: When medication is given in tablets, the QUANTITY = 1 since the number of drugs available is specified per (one) tablet.
Example 1: Toprol XL, 50 mg PO, is ordered. Toprol XL is available as 100 mg per tablet. How many tablets would the nurse administer?
Step 1: Determine your givens.
Amount desired (D) = 50 mg
Amount on hand (H) = 100 mg
tablets Quantity = 1
Step 2: Plug what you know into the formula and simplify.
Therefore, the nurse would administer 0.5 of a tablet.
Example 2: 1200 mg of Klor-Con is ordered. This medication is only available at 600 mg per tablet. How many tablets should the nurse give?
Step 1: Determine your givens.
Amount desired (D) = 1200 mg
Amount on hand (H) = 600 mg
tablets Quantity = 1
Step 2: Plug what you know into the formula and simplify.
Therefore, the nurse would administer two (2) tablets.
The same formula can be used for dosage calculations where the medication is available as an amount per certain volume.
In these types of calculations, the volume available on hand is QUANTITY.
Example 3: Dilantin-125 is available at 125 mg/5 mL. Dilantin-125, 0.3 g PO, is ordered. How much should the nurse administer to the patient?
Step 1: Determine your givens.
Amount desired (D) = 0.3 g
Amount on hand (H) = 125 mg
Volume available = 5 mL
Step 2: Convert 0.3 g to mg (since the ordered dose is in grams, but the drug is available in milligrams).
0.3 g x 1,000 mg/g = 300 mg
Step 3: Plug what you know into the formula and simplify.
Therefore, the nurse should administer 12 mL of Dilantin-125 to the patient.
Dosage Calculations based on Body Weight.
Dosage calculations based on body weight are required when the dosage ordered and administered depends on the patient’s weight. For example, many pediatric drugs are ordered and given per weight (usually in kg). Dosage calculations based on body weight are calculated in two main stages.
Stage 1: Using the formula below, calculate the total required dosage based on given the body weight.
Stage 2: Apply the (D/H)x Q formula to calculate the amount of medication to administer.
Example 1: Medrol 4 mg/kg is ordered for a child weighing 62.8 lb. Medrol is available at 500 mg/4mL. How many milliliters of medication must the nurse administer?
Step 1: Determine your givens.
Amount desired (D) = 4 mg/kg
Weight of patient = 62.8 lb
Amount on hand (H) = 500 mg
Volume available = 4 mL
Step 2: Convert the weight from pounds to kilograms.
62.8 lb ÷ 2.2 lb/kg = 28.5 kg
Step 3: Calculate the total required dosage (amount desired) based on the patient’s body weight.
28.5 kg x 4 mg/kg = 114 mg
Step 4: Plug what you know into the formula and simplify.
Therefore, the nurse should administer 0.9 mL of Medrol to the patient.
Example 2: A doctor orders 400 mg of Amoxil to be taken by a 30 lb toddler every 12 hours. The medication label indicates that the desired dosage range for this age group is 25-50 mg/kg per day. Is this doctor’s order within the selected range?
Step 1: Determine your givens.
Weight of child = 30 lb
Desired dosage range (low) = 25 mg/kg per day
Desired dosage range (high) = 50 mg/kg per day
Dosage ordered = 400 mg
Step 2: Convert the weight from pounds to kilograms.
30 lb ÷ 2.2 lb/kg = 13.6 kg
Step 3: Calculate the desired dosage range based on the patient’s weight.
Weight (kg) x Dosage Range (per kg) = Y (Desired dosage range)
13.6 kg x 25 mg/kg = 340 mg (low)
13.6 kg x 50 mg/kg = 680 mg (high)
Step 4: Calculate the medication the doctor has ordered for one day or 24 hours.
24 hr ÷ 12 hr = 2
The doctor has ordered the medication to be given 2 times per day. Every dose is 400 mg. 400 mg x 2 = 800 mg Therefore, the doctor has ordered 800 mg daily.
Calculation of Intravenous Drip Rates
In these types of calculations, for a given volume, period, and drop factor (gtts/mL), the required IV flow rate in drops per minute (gtts/min) is calculated.
Note: Since a fraction of a drop is impossible to give to a patient, it is usual to round the answers to the nearest whole number.
Formula: [Volume (mL)/ Time (min) ]x Drop Factor (gtts/mL) = Y (Flow Rate in gtts/min)
Example 1: Calculate the IV flow rate for 250 mL of 0.5% dextrose to be administered over 180 minutes. The infusion set has a drop factor of 30 gtts/mL.
Step 1: Determine your givens.
Volume (mL) = 250 mL
Time (min) = 180 min
Drop factor (gtts/mL) = 30 gtts/mL
Step 2: Plug in what you know into the formula and simplify.
[250 mL ÷ 180 min] x 30 gtts/mL = 41.66 gtts/min
Rounded to the nearest whole number, this IV flow rate is 42 gtts/min.
Example 2: The infusion set is adjusted for a drop factor of 15 gtts/mL. Calculate the IV flow rate if 1500 mL IV saline is ordered to be infused over 12 hours.
Step 1: Determine your givens.
Volume (mL) = 1500 mL
Time (min) = 720 min
Drop factor (gtts/mL) = 15 gtts/mL
Step 2: Convert 12 hours into minutes.
12 h x 60 min/h = 720 min
Step 3: Plug in what you know into the formula and simplify.
(1500 mL ÷ 720 min) x 15 gtts/mL = 31.25 gtts/min
Rounded to the nearest whole number, this IV flow rate is 31 gtts/min.
Calculation of Flow Rate for an Infusion Pump
Infusion pumps do not have a calibrated drop factor. The flow rate depends on the volume of fluid ordered and the time of infusion.
Formula: [Volume (mL)/ Time (h)] = Y (Flow Rate in mL/h)
Example 1: 1200 mL D5W IV is ordered to infuse in 10 hours by infusion pump. Calculate the flow rate in milliliters per hour.
Step 1: Determine your givens.
Volume (mL) = 1200 mL
Time (h) = 10 h
Step 2: Since the volume is given in mL and the time is given in hours, the flow rate can be calculated in one step using the formula.
Volume (ml) / Time (h) = (Flow Rate In mL/h)
1200 mL ÷ 10 h = 120 mL/h
Therefore, the IV flow rate is 120 mL/hr.
Example 2: 600 mL of antibiotic is to be infused over 180 minutes by an infusion pump. Calculate the flow rate (mL per hour).
Step 1: Determine your givens.
Volume (mL) = 600 mL
Time (min) = 180 min
Step 2: Convert the time from minutes to hours.
180 min ÷ 60 min/h = 3 h
Step 3: Plug what you know into the formula and simplify.
Volume (mL) ÷ Time (h) = Flow Rate in mL/hr
600 mL ÷ 3 h = 200 mL/h
Therefore, the IV flow rate is 200 mL/hr.
Mrs. Marie Brown has been a registered nurse for over 25 years. She began her nursing career at a Level I Trauma Center in downtown Chicago, Illinois. There she worked in the Emergency Department and on the Surgical Intensive Care Unit. After several years, she moved to the Midwest and continued her nursing career in a critical care setting. For the last 10 years of her nursing career, Mrs. Brown worked as a flight nurse with an air ambulance service. During this time, she cared for patients throughout the United States.