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Medical Calculations

Unlock the secrets of accurate medical calculations and discover how mastering this essential skill can save lives and enhance your medical career.

USMLE Guide: Medical Calculations


As a medical professional, it is vital to have a strong foundation in medical calculations. These calculations help in determining accurate dosages, interpreting laboratory results, and making critical clinical decisions. In this USMLE guide, we will explore the key concepts and formulas related to medical calculations that you may encounter on the United States Medical Licensing Examination (USMLE).

1. Unit Conversions

Unit conversions play a significant role in medical calculations. Here are some common conversions you should be familiar with:

  • Metric System: The metric system is widely used in healthcare. Understanding the following conversions is crucial:

    • 1 kilogram (kg) = 1000 grams (g)
    • 1 gram (g) = 1000 milligrams (mg)
    • 1 milligram (mg) = 1000 micrograms (mcg)
  • Volume Conversions:

    • 1 liter (L) = 1000 milliliters (mL)
    • 1 milliliter (mL) = 1 cubic centimeter (cc)
  • Time Conversions:

    • 1 hour = 60 minutes (min)
    • 1 minute (min) = 60 seconds (sec)

Make sure to practice these conversions regularly to become comfortable with them.

2. Dosage Calculations

Accurate dosage calculations are crucial to ensure patient safety. Here are some important concepts related to dosage calculations:

  • Dosage Calculation Formula: The basic formula used for dosage calculations is as follows:

    • Desired Dose = Ordered Dose / Available Dose
  • Example: If a medication is available in 500 mg tablets and the ordered dose is 1000 mg, we can calculate the number of tablets needed using the formula:

    • Desired Dose = 1000 mg / 500 mg = 2 tablets
  • Body Surface Area (BSA): BSA is used to calculate medication dosages based on a patient's surface area. The most commonly used formula for BSA calculation is the Dubois & Dubois formula:

    • BSA (m²) = (Height (cm) x Weight (kg)) / 3600

Understanding the concept of dosage calculations and practicing various scenarios will help you excel in this area.

3. IV Flow Rate Calculations

Calculating the intravenous (IV) flow rate is essential to administer medications accurately. Here are important points related to IV flow rate calculations:

  • IV Flow Rate Formula:

    • Flow Rate (mL/hour) = (Volume to be infused (mL) / Time (hours))
  • Example: If 1000 mL of fluid needs to be infused over 8 hours, we can calculate the flow rate as follows:

    • Flow Rate = 1000 mL / 8 hours = 125 mL/hour
  • Drip Factor: The drip factor is specific to the IV tubing used and determines the number of drops per milliliter. Common drip factors include 10, 15, and 20 drops per mL.

Understanding IV flow rate calculations and practicing with different scenarios will enhance your proficiency in this area.

4. Laboratory Result Interpretation

Interpreting laboratory results is a critical skill for medical professionals. The USMLE may include questions related to laboratory values and their significance. Here are some important laboratory values and their normal ranges:

  • Hemoglobin (Hb):

    • Normal Range: 12-16 g/dL (grams per deciliter)
  • White Blood Cell (WBC) Count:

    • Normal Range: 4,500-11,000 cells/mm³ (cubic millimeter)
  • Platelet Count:

    • Normal Range: 150,000-450,000 cells/mm³ (cubic millimeter)
  • Glucose:

    • Fasting Normal Range: 70-99 mg/dL (milligrams per deciliter)

Familiarize yourself with the normal ranges of various laboratory values to be able to interpret results accurately.


Medical calculations are an integral part of clinical practice, and a solid understanding of these concepts is crucial for success on the USMLE. By mastering unit conversions, dosage calculations, IV flow rate calculations, and laboratory result interpretation, you will be well-equipped to tackle the medical calculations questions on the exam. Practice regularly and seek additional resources to strengthen your skills in this area. Best of luck!

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