Dosage Calculation For Nurses


Dosage calculation is an important aspect of medical care for health personnel, especially nurses whose job is to administer medications to hospital patients after the doctor prescribes the right treatment and medication for that particular ailment. While a dosage is the size or frequency of a dose of a medicine or drug, pathogen, or nutrient delivered as a unit, its calculation is the act of knowing the right amount of that dosage to apply to that patient’s medication over a specific period of time.

The importance of calculating dose and dosage cannot be overemphasized even as medical lame men may differ in perspective by thinking that all size fits all will be acceptable in medicine, which it doesn’t. When it comes to dosage calculations, it is important to note that there are basic dynamics that come into play, which every nurse needs to know and put into consideration before determining the right dose and dosage to administer on a patient. Failure to follow the right dosage calculation guidelines will ultimately lead to other health crises in the patient.

Nurses and the Problem of Dosage Calculation

Poor mathematical skills are a major problem confronting the nursing industry regarding nurses calculating and administering drugs with the right dose and dosage to patients. In fact, a United Kingdom Central Council for Nursing, Midwifery and Health Visiting (UKCC) council meeting in Belfast in June 2000 expressed concern at the lack of basic mathematic ability among nurses (Coombes, 2000; Duffin, 2000). Medical experts also publicly opined that dosage calculations are still a key source of drug error. Experts believe that the risk of error was unacceptably high, specifically in pediatric nursing, where calculating dosages according to body weight increases calculation complexity.

Knowing how to solve dosage and calculation problems to determine the exact amount of medication must be nurses’ and nursing students’ top priority to help stop and prevent drug error crises in the medical industry and save lives. There are different dosage uses such as prescriptions, drips (iv), flow rate set up, and syringe draws up of medications. The importance of dosage and calculation to the medical world should be reoccurring theme and caption to nurses and nursing students as its proper use to determine the amount of medication needed to administer medications is a key part of the medical process treatment of ailing patients that saves lives. Mastering the mathematical skills for proper dosage calculation is a key ability that nursing students must learn to pass nursing licensing examination (NCLEX-RN) to enable them to graduate as nurses.

Guidelines to Dosage Calculation 

One of the key guilds that nurses must consider before calculating the precise dosage of a patient medication is their weight. Dosage is often administered in milligrams and milliliters, depending on the type of medication to be administered. For instance, a certain amount of medication milligrams are given for a certain number of kilograms that match the patient’s body weight. Nurses should calculate the patient’s needed dosage by multiplying the patient’s weight by the main dosage. This means, If the patient’s weight 40 kg and the general dosage is 35 mg/kg, the nurse administers the patient 750 mg (40kg x 35 mg/kg).

Drugs in children are mostly dosed to match their body weight (mg/kg) or body surface area (BSA) (mg/m2). Nurses must ensure proper care when converting body weight from pounds to kilograms (1 kg= 2.2 lb) before calculating doses based on body weight. In universal standards, doses are often indicated as mg/kg/day or mg/kg/dose. In cases where orders are written as “mg/kg/d,” which is confusing, the nurse should inquire for further clarification from the prescriber.

Steps to Dosage Calculation 

  1. Apply Dosage Calculation Formula

Simple Equation:

  • Weight: Check the patient weight, which must be expressed in kg or lb. 
  • Dosage: Determine the prescribed amount of drug in mg per kg of body weight. 
  • Dose: Determine the total amount of medication you will administer on the patient.
  • Frequency: Determine the total duration of medication you will administer the dosage on the patient
  1. Understand the Prescribers Order: This will provide you with the information that you need to know about the patient’s prescribed dose (250mg), how often the patient takes it (once per day), and how long they will be taking it (14 days).


250gm1day x14days1 = mg total


250gm1day x14days1 =   3500mg

Conversion Factors

  • 1 kg = 2.2 lb
  • 1 gallon = 4 quart
  • 1 tsp = 5 mL
  • 1 inch = 2.54 cm
  • 1 L = 1,000 mL
  • 1 kg = 1,000 g
  • 1 oz = 30 mL = 2 tbsp
  • 1 g = 1,000 mg
  • 1 mg = 1,000 mcg
  • 1 cm = 10 mm
  • 1 tbsp = 15 mL
  • 1 cup = 8 fl oz
  • 1 pint = 2 cups
  • 12 inches = 1 foot
  • 1 L = 1.057 qt
  • 1 lb = 16 oz
  • 1 tbsp = 3 tsp
  • 60 minute = 1 hour
  • 1 cc = 1 mL
  • 2 pints = 1 qt
  • 8 oz = 240 mL = 1 glass
  • 1 tsp = 60 gtt
  • 1 pt = 500 mL = 16 oz
  • 1 oz = 30 mL
  • 4 oz = 120 mL (Casey, 2018).

Technique for Dosage Calculation

  1. Desired Over Have or Formula Method
  2. Dimensional Analysis Method
  3. Ratio and Proportion Method

Dosage Forms

The dosage form is depending on the type of administration, and it comes in several different forms. One of the most often use form is the oral administration type. However, it is not used in all medical situations. This is why other forms such as: subcutaneous, intramuscular, intraosseous, intravenous, etc., are readily available and called parenteral means. 

Also read Nursing Report Sheet – A Complete Guide

Dosage Calculation For Nurses

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