HESI A2 Math Formulas & Conversions Cheat Sheet

The HESI A2 Math section gives you 50 questions in 50 minutes. You'll need to work fast and accurately. Here's every formula and conversion you need to memorize.

Section Overview

The HESI A2 Math section tests basic arithmetic, fractions, decimals, ratios, proportions, percentages, and measurement conversions. There is no advanced algebra or calculus. However, the tight time limit (1 minute per question) means you must have these formulas memorized — not calculated from scratch.

🔢 Fractions & Decimals

📐 Ratios & Proportions

This is the exact format you'll see on nursing dosage questions. Practice until it's automatic. These math skills also come in handy when studying for the HESI A2 Biology section, where you may need to calculate concentrations and ratios.

💯 Percentages

⚖️ Metric Conversions (MUST MEMORIZE)

Metric conversions make up a huge portion of the HESI math section. Commit this table to memory:

Pro tip: To convert within the metric system, remember King Henry Died By Drinking Chocolate Milk — Kilo, Hecto, Deka, Base, Deci, Centi, Milli. Moving right means multiply by 10; moving left means divide by 10.

Download the printable one-page cheat sheet with all these conversions →

🔤 Order of Operations (PEMDAS)

🏥 Dosage Calculation Formula

This is the most nursing-relevant formula on the exam. You will absolutely see at least 2–3 questions using it. Understanding dosage math is also essential for the HESI A2 Anatomy & Physiology section, where medication administration concepts overlap:

Example: The doctor orders 500 mg. The available tablets are 250 mg each.
500 / 250 × 1 tablet = 2 tablets

5 Time-Saving Tips for Test Day

1. Estimate first — before calculating, estimate the answer to eliminate obviously wrong choices.
2. Convert fractions to decimals when comparing or adding — it's usually faster.
3. Cross-multiply immediately whenever you see a proportion problem.
4. Memorize the conversions table above — at least 5–8 questions will test these directly.
5. Practice with a timer — take our free HESI A2 practice quiz to build speed under pressure. To see how to fit this practice into your schedule, review our HESI A2 study timeline.

🏛️ Roman Numeral Conversions (The 7 Symbols & Subtraction Trap)

Many students are surprised to discover that Roman numerals are actively tested on the HESI A2 Math section. While the ATI TEAS recently phased them out, Elsevier consistently includes 2 to 4 Roman numeral conversion questions on every HESI exam because medical prescriptions and pharmaceutical dosages still occasionally use them.

These are essentially guaranteed points if you memorize the 7 fundamental symbols and understand the subtraction rule.

Step 1: Memorize the 7 Core Symbols

Roman numerals do not use place value (units, tens, hundreds). Instead, values are combined through addition and subtraction of 7 standard symbols:

Step 2: Master the Two Golden Rules

Rule 1 (Addition): If symbols are identical or strictly decrease in value from left to right, simply sum their values:

• III = 1 + 1 + 1 = 3
• XVI = 10 + 5 + 1 = 16
• CLX = 100 + 50 + 10 = 160
• MDCC = 1000 + 500 + 100 + 100 = 1,700

Rule 2 (The Subtraction Trap): You are never permitted to repeat a symbol more than three times consecutively (e.g., 4 is never "IIII", and 40 is never "XXXX"). Instead, place a smaller symbol immediately before a larger symbol to subtract it:

• IV: 5 − 1 = 4
• IX: 10 − 1 = 9
• XL: 50 − 10 = 40
• XC: 100 − 10 = 90
• CD: 500 − 100 = 400
• CM: 1,000 − 100 = 900

Rounding Rules

You'll encounter rounding in multiple contexts — dosage calculations, statistical questions, and unit conversions. Remember these rules:

• 5 or greater — round up. Example: 3.456 rounded to one decimal = 3.5
• 4 or less — round down. Example: 3.432 rounded to one decimal = 3.4
• Whole number rounding: 1,847 rounded to the nearest hundred = 1,800
• In dosage context: Always follow the rounding instruction in the question. If it says "round to the nearest tenth," do exactly that — no more, no less.

Word Problems: The HESI Format

Nearly half of HESI A2 Math questions are word problems. They test whether you can extract the math from a real-world scenario. Here's a reliable 4-step approach:

1. Read the entire problem first. Don't start calculating until you understand what's being asked.
2. Identify what's given and what's wanted. Write down the numbers and their units.
3. Set up the equation. If it's a proportion, cross-multiply. If it's a percentage, use the percentage formula.
4. Solve and check units. If the question asks for milligrams, your answer must be in milligrams — not grams.

Example word problem: "A patient needs to take 750 mL of fluid over 6 hours. How many mL per hour should the patient drink?"

Least Common Multiple (LCM) and Greatest Common Factor (GCF)

These appear frequently when adding fractions or simplifying answers:

Common Mistakes to Avoid

These are the errors our HESI tutors see students make most frequently during practice sessions. Many of these same conversion pitfalls apply in the HESI A2 Chemistry section as well:

• Forgetting to simplify fractions. If your answer is 6/8, reduce it to 3/4. The HESI answer choices will always use simplified fractions.
• Mixing up metric direction. Kilo to milli is multiply by 1,000,000 — not divide. Use the King Henry mnemonic every time.
• Misreading the question. "How many MORE tablets" is different from "How many tablets TOTAL." Read carefully.
• Not converting units before calculating. If the question gives you kilograms but asks for grams, convert first, then calculate.
• Rounding too early. Keep full decimal precision throughout your calculation and only round at the final step.

Frequently Asked Questions