HESI A2 Math Study Guide: Conversions, Ratios, and Dosage Calculations

The HESI A2 Math section intimidates many nursing school applicants, but its content is more straightforward than it appears. The exam tests practical arithmetic skills that nurses use every day: unit conversions, ratio and proportion, dosage calculations, and basic algebra. With focused preparation and the right strategies, the math section can become one of your highest-scoring areas.

HESI A2 Math Section Overview

The HESI A2 Math section contains approximately 55 questions with a time limit of 50 minutes. Questions are drawn from several categories:

• Fractions, decimals, and percentages
• Ratio and proportion
• Basic algebra
• Measurement and unit conversions
• Military time
• Roman numerals
• Dosage and calculation problems

A basic on-screen calculator is provided for most versions of the exam, but you should be comfortable solving problems by hand in case the interface is slow or unfamiliar.

Fractions, Decimals, and Percentages

These foundational skills underlie nearly every other math topic on the exam:

• Fractions: simplify to lowest terms, find common denominators for addition and subtraction, multiply across numerators and denominators, and invert and multiply to divide
• Converting between forms: divide the numerator by the denominator to convert a fraction to a decimal; move the decimal point two places to convert a decimal to a percent
• Percentage problems: remember that of means multiply and is means equals — so “what is 30% of 240?” becomes 0.30 x 240 = 72

Practice converting among all three forms quickly. Speed here saves time for harder questions later.

Ratio and Proportion

Ratio and proportion questions ask you to find a missing value when two ratios are equal. This is the backbone of medication dosage calculations:

Setup: Write the known ratio on the left and the unknown ratio on the right, then cross-multiply and solve.

Example: If 250 mg is in 5 mL, how many mL contain 375 mg?

• 250/5 = 375/x
• 250x = 5 x 375 = 1875
• x = 1875 / 250 = 7.5 mL

Always label your units and verify that the units in both ratios match. Setting up the proportion correctly is more important than arithmetic speed.

Unit Conversions You Must Memorize

The HESI A2 tests conversions within and between metric, household, and apothecary systems. Learn these cold:

Metric:

• 1 kg = 1000 g; 1 g = 1000 mg; 1 mg = 1000 mcg
• 1 L = 1000 mL

Metric to household:

• 1 kg = 2.2 lb
• 1 inch = 2.54 cm
• 1 tsp = 5 mL; 1 tbsp = 15 mL; 1 oz = 30 mL
• 1 cup = 240 mL; 1 pt = 480 mL; 1 qt = 960 mL

Temperature:

• Celsius to Fahrenheit: F = (C x 9/5) + 32
• Fahrenheit to Celsius: C = (F – 32) x 5/9

Military time conversions also appear. For times after noon, add 12 to the standard hour (e.g., 2:00 PM = 1400).

Dosage Calculation Strategies

Dosage problems combine ratio-proportion with unit conversions and are the most clinically relevant math on the exam. Two reliable methods are:

Dimensional Analysis (Factor-Label Method): Set up a chain of fractions where unwanted units cancel and only the desired unit remains. This method is self-checking because misplaced units signal a setup error before you calculate.

Desired over Have (D/H x V):

• D = desired dose (what is ordered)
• H = dose on hand (what you have)
• V = vehicle (the quantity containing H)
• Answer = (D / H) x V

Example: Order is for 500 mg of amoxicillin; you have 250 mg per 5 mL suspension. (500/250) x 5 = 2 x 5 = 10 mL.

Practice both methods until one feels automatic. On the exam, use the method that minimizes setup errors for you personally.

Study Plan and Test-Day Tips

Approach HESI A2 Math preparation systematically:

1. Memorize conversion tables first — write them from memory daily for one week until they are automatic
2. Practice dosage calculations without a calculator to build mental math fluency, then verify with a calculator
3. Take timed practice tests targeting one question per minute to build exam pace
4. Review every error: math errors almost always have a specific cause — misread problem, wrong formula, unit mismatch — identifying the cause prevents repetition
5. On test day: write your conversion table on scratch paper before starting so you do not have to recall it under pressure; show all work even for simple problems to catch arithmetic slips

The HESI A2 Math section rewards preparation far more than raw talent. Students who drill conversions and dosage setups consistently outperform those who rely on intuition alone.