Why Conversions Are Critical on the HESI A2
Unit conversions account for a large portion of the HESI A2 Math section — and they show up in Biology, Chemistry, and dosage-related questions too. The good news? These are some of the most predictable questions on the entire exam. If you memorize the key conversion factors and practice the process, you can pick up easy points.
This guide provides every conversion chart you'll need, organized by type, plus worked examples and practice problems to lock in your skills.
Metric System Conversions
The metric system is the foundation of medical measurements. Memorize these prefix relationships:
| Prefix | Abbreviation | Relationship to Base Unit |
|---|---|---|
| Kilo- | k | 1,000 × base unit |
| Hecto- | h | 100 × base unit |
| Deca- | da | 10 × base unit |
| Base | g, L, m | 1 (gram, liter, meter) |
| Deci- | d | 0.1 × base unit |
| Centi- | c | 0.01 × base unit |
| Milli- | m | 0.001 × base unit |
| Micro- | mcg (μ) | 0.000001 × base unit |
High-Yield Metric Conversions for the HESI A2
| Conversion | Equivalence |
|---|---|
| 1 kilogram (kg) | 1,000 grams (g) |
| 1 gram (g) | 1,000 milligrams (mg) |
| 1 milligram (mg) | 1,000 micrograms (mcg) |
| 1 liter (L) | 1,000 milliliters (mL) |
| 1 meter (m) | 100 centimeters (cm) |
| 1 meter (m) | 1,000 millimeters (mm) |
| 1 kilometer (km) | 1,000 meters (m) |
| 1 milliliter (mL) | 1 cubic centimeter (cc) |
Memory Trick — "King Henry Doesn't Usually Drink Chocolate Milk":
Kilo → Hecto → Deca → Unit → Deci → Centi → Milli
Each step to the right = multiply by 10. Each step to the left = divide by 10.
Household-to-Metric Conversions
These bridge the gap between everyday measurements and medical/metric units — extremely common on the HESI A2:
| Household Measurement | Metric Equivalent |
|---|---|
| 1 teaspoon (tsp) | 5 mL |
| 1 tablespoon (tbsp) | 15 mL |
| 1 fluid ounce (fl oz) | 30 mL |
| 1 cup | 240 mL |
| 1 pint (pt) | 480 mL (~500 mL) |
| 1 quart (qt) | 960 mL (~1,000 mL or 1 L) |
| 1 gallon (gal) | 3,785 mL (~3,800 mL) |
| 1 ounce (oz) | 28.35 grams (g) |
| 1 pound (lb) | 0.454 kilograms (kg) or 454 g |
| 1 kilogram (kg) | 2.2 pounds (lb) |
| 1 inch (in) | 2.54 centimeters (cm) |
Worked Example
Question: A patient weighs 176 pounds. What is their weight in kilograms?
Solution:
176 lb × (1 kg ÷ 2.2 lb) = 176 ÷ 2.2 = 80 kg
Temperature Conversions
You must know both formulas and be able to convert in either direction:
| Conversion | Formula |
|---|---|
| Fahrenheit → Celsius | °C = (°F − 32) × 5/9 |
| Celsius → Fahrenheit | °F = (°C × 9/5) + 32 |
Key Temperature Reference Points
| Description | °F | °C |
|---|---|---|
| Water freezes | 32°F | 0°C |
| Normal body temperature | 98.6°F | 37°C |
| Water boils | 212°F | 100°C |
| Fever threshold (adult) | 100.4°F | 38°C |
Question: Convert 101.3°F to Celsius.
Solution:
°C = (101.3 − 32) × 5/9 = 69.3 × 5/9 = 346.5 ÷ 9 = 38.5°C
Military (24-Hour) Time
Healthcare uses 24-hour time to prevent medication errors. This appears frequently on the HESI A2:
| Standard Time | Military Time | Standard Time | Military Time |
|---|---|---|---|
| 12:00 AM (midnight) | 0000 | 12:00 PM (noon) | 1200 |
| 1:00 AM | 0100 | 1:00 PM | 1300 |
| 2:00 AM | 0200 | 2:00 PM | 1400 |
| 3:00 AM | 0300 | 3:00 PM | 1500 |
| 4:00 AM | 0400 | 4:00 PM | 1600 |
| 5:00 AM | 0500 | 5:00 PM | 1700 |
| 6:00 AM | 0600 | 6:00 PM | 1800 |
| 7:00 AM | 0700 | 7:00 PM | 1900 |
| 8:00 AM | 0800 | 8:00 PM | 2000 |
| 9:00 AM | 0900 | 9:00 PM | 2100 |
| 10:00 AM | 1000 | 10:00 PM | 2200 |
| 11:00 AM | 1100 | 11:00 PM | 2300 |
Quick rule: For PM times, add 12 to the hour (3:00 PM = 15:00 = 1500). For military times after 1200, subtract 12 to get standard time (1900 = 7:00 PM).
Roman Numerals
Roman numerals still appear in medical prescriptions and dosage orders. The HESI A2 may test basic numeral recognition:
| Roman Numeral | Value | Roman Numeral | Value |
|---|---|---|---|
| I | 1 | X | 10 |
| II | 2 | XX | 20 |
| III | 3 | L | 50 |
| IV | 4 | C | 100 |
| V | 5 | D | 500 |
| VII | 7 | M | 1,000 |
| IX | 9 | ss (special) | ½ (half) |
Rule: A smaller numeral before a larger one means subtraction (IV = 4, IX = 9). A smaller numeral after a larger one means addition (VI = 6, XI = 11).
Dosage Conversion Essentials
While detailed dosage calculations are covered in nursing programs, the HESI A2 tests the foundational conversions that underpin them:
| Conversion | Equivalence |
|---|---|
| 1 grain (gr) | 60 milligrams (mg) [or 64.8 mg exact] |
| 1 dram | 4 mL (approximately) |
| 1 drop (gtt) | Varies by dropper (standard ~0.05 mL) |
| 1 mL | ~15–20 drops (gtt) |
Percentage, Decimal & Fraction Conversions
These appear throughout the Math section and are foundational for every other conversion:
| Fraction | Decimal | Percentage |
|---|---|---|
| 1/2 | 0.5 | 50% |
| 1/3 | 0.333 | 33.3% |
| 1/4 | 0.25 | 25% |
| 1/5 | 0.2 | 20% |
| 1/8 | 0.125 | 12.5% |
| 1/10 | 0.1 | 10% |
| 2/3 | 0.667 | 66.7% |
| 3/4 | 0.75 | 75% |
| 3/8 | 0.375 | 37.5% |
| 7/8 | 0.875 | 87.5% |
Conversion Steps
- Fraction → Decimal: Divide numerator by denominator (3/4 = 3 ÷ 4 = 0.75)
- Decimal → Percentage: Multiply by 100 (0.75 × 100 = 75%)
- Percentage → Decimal: Divide by 100 (75% ÷ 100 = 0.75)
- Decimal → Fraction: Place over power of 10 and simplify (0.75 = 75/100 = 3/4)
Ratio and Proportion Method for Conversions
This is the most reliable method for solving HESI A2 conversion problems. It works for any conversion type:
- Set up the known ratio (the conversion factor)
- Set up the unknown ratio with x
- Cross-multiply and solve for x
Question: How many milliliters are in 2.5 tablespoons?
Solution:
Known ratio: 1 tbsp = 15 mL
Set up proportion: 1 tbsp / 15 mL = 2.5 tbsp / x mL
Cross-multiply: 1 × x = 15 × 2.5
x = 37.5 mL
Question: A child weighs 44 pounds. What is their weight in kilograms?
Solution:
Known ratio: 2.2 lb = 1 kg
Set up proportion: 2.2 lb / 1 kg = 44 lb / x kg
Cross-multiply: 2.2x = 44
x = 44 ÷ 2.2 = 20 kg
Practice Problems
Test yourself with these conversion problems. Answers are provided below.
- Convert 3.5 liters to milliliters.
- A patient weighs 198 pounds. Convert to kilograms (round to one decimal).
- Convert 39.2°C to Fahrenheit.
- How many teaspoons are in 30 mL?
- Convert 2045 military time to standard time.
- A prescription reads "gr v." How many milligrams is this?
- Convert 5 feet 8 inches to centimeters.
- What is 7/8 as a percentage?
- Convert 2.5 cups to milliliters.
- A medication dose is 0.25 g. How many milligrams is this?
Answer Key
- 3,500 mL (3.5 × 1,000)
- 90.0 kg (198 ÷ 2.2)
- 102.56°F ((39.2 × 9/5) + 32)
- 6 teaspoons (30 mL ÷ 5 mL per tsp)
- 8:45 PM (20 − 12 = 8, so 8:45 PM)
- 300 mg (5 × 60 mg)
- 172.72 cm (68 inches × 2.54)
- 87.5% (7 ÷ 8 = 0.875 × 100)
- 600 mL (2.5 × 240)
- 250 mg (0.25 × 1,000)
Tips for Mastering Conversions on the HESI A2
- Make a flash card set with the conversion tables above — review them daily for two weeks before your exam.
- Always use the ratio-proportion method to double-check your work; it's fool-proof.
- Watch your decimal placement — the most common errors are moving the decimal the wrong direction in metric conversions.
- Practice under timed conditions — you should be able to complete a conversion problem in under 60 seconds.
- Use estimation to verify answers — if you're converting 150 lbs to kg and get 330, that's obviously wrong (should be about 68).
For comprehensive practice with conversion questions in a realistic exam format, our HESI A2 Prep platform includes hundreds of math questions — including conversions, dosage calculations, ratios, and proportions — with detailed step-by-step explanations for every answer.