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newSchoolWorks.com › VoTechWorks › Health & Personal Services › Pharmacy Technician › Pharmacy Calculations and Order Entry › Lesson 2
HLT-PHAR41-03 · Lesson 2 · 15 clock hours · theory

Measurement systems and conversions

Objective. Convert accurately within metric units and between household and metric measures, including weight in pounds and kilograms.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by a licensed pharmacist or by a certification or approval body. It is a study draft and may contain errors. Do not use it to make decisions about any patient or medication. Always follow your instructor, your supervising professional, and current official references and regulations.

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Why Measurement Systems Matter in Pharmacy

Every prescription and medication order includes a quantity—a dose, a volume, a weight. As a pharmacy technician, you must convert between measurement systems quickly and accurately. A calculation error of even a small amount can result in a patient receiving too little medication to be effective or too much, causing serious harm. This lesson covers the two systems you will use most: the metric system and household measures, plus the critical conversion between pounds and kilograms.

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The Metric System

The metric system is the standard for pharmacy practice in the United States. It is a decimal-based system, meaning every unit relates to the next by a factor of 10. Three metric base units are essential in pharmacy:

Common Metric Prefixes

| Prefix | Abbreviation | Meaning | Example |

|--------|-------------|---------|--------|

| kilo- | k | 1,000 × base | 1 kg = 1,000 g |

| (base) | g, L | 1 | 1 g, 1 L |

| milli- | m | 1/1,000 of base | 1 mg = 0.001 g |

| micro- | mc or μ | 1/1,000,000 of base | 1 mcg = 0.001 mg |

Converting Within the Metric System

To move between metric units, multiply or divide by powers of 10. A simple way to remember the direction:

Key equivalents to memorize:

Example logic: To convert 2.5 g to milligrams, you are going from a larger unit to a smaller unit, so multiply by 1,000: 2.5 × 1,000 = 2,500 mg.

To convert 750 mL to liters, you are going from a smaller unit to a larger unit, so divide by 1,000: 750 ÷ 1,000 = 0.75 L.

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Household Measures

Patients at home often use household utensils rather than metric equipment. Pharmacy technicians must know how household volumes relate to metric volumes so that labels and patient instructions are accurate.

Common Household-to-Metric Volume Equivalents

| Household Measure | Abbreviation | Metric Equivalent |

|-------------------|-------------|-------------------|

| 1 teaspoon | tsp | 5 mL |

| 1 tablespoon | tbsp | 15 mL |

| 1 fluid ounce | fl oz | 30 mL |

| 1 cup | c | 240 mL |

| 1 pint | pt | 473 mL |

| 1 quart | qt | 946 mL |

| 1 gallon | gal | 3,785 mL |

> Clinical note: Because patients vary in the actual size of their kitchen spoons, oral syringes or calibrated cups should always be recommended for liquid medications.

Converting Household to Metric

Use a proportion or dimensional analysis. For example, if a prescription reads "give 2 teaspoons," the metric dose is:

2 tsp × 5 mL/tsp = 10 mL

If a child's antibiotic bottle holds 150 mL and the dose is 1 tablespoon twice daily, the supply will last:

1 tbsp = 15 mL → dose = 15 mL; total daily = 30 mL → 150 mL ÷ 30 mL/day = 5 days

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Weight: Pounds and Kilograms

Many drug doses—especially for pediatric patients, chemotherapy, and weight-based medications—are calculated in milligrams per kilogram (mg/kg). Patient weights recorded in the United States are frequently reported in pounds (lb), so you must convert to kilograms (kg) before calculating a dose.

The Essential Conversion Factor

1 kg = 2.2 lb

Example: A patient weighs 154 lb.

154 lb ÷ 2.2 = 70 kg

Example: A premature infant weighs 1.8 kg.

1.8 kg × 2.2 = 3.96 lb

Why This Matters

If a dose is ordered as 10 mg/kg and the patient weighs 44 lb:

1. Convert: 44 lb ÷ 2.2 = 20 kg

2. Calculate dose: 20 kg × 10 mg/kg = 200 mg

Using the pound weight without converting would result in a dose of 440 mg—more than twice the correct amount.

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Strategies for Accuracy

1. Always write units at every step. Canceling units is the clearest way to check your work.

2. Use dimensional analysis (factor-label method): Set up fractions so unwanted units cancel.

3. Estimate first: Ask yourself whether your answer is reasonable before accepting it.

4. Double-check direction: Are you converting to a bigger or smaller unit? Will the number go up or down?

5. Memorize the core equivalents listed in this lesson; they appear on the PTCE and in daily practice.

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Summary of Key Conversions

| Conversion | Relationship |

|---|---|

| Grams ↔ milligrams | × or ÷ 1,000 |

| Milligrams ↔ micrograms | × or ÷ 1,000 |

| Liters ↔ milliliters | × or ÷ 1,000 |

| Teaspoon ↔ mL | 1 tsp = 5 mL |

| Tablespoon ↔ mL | 1 tbsp = 15 mL |

| Fluid ounce ↔ mL | 1 fl oz = 30 mL |

| Pounds ↔ kilograms | ÷ or × 2.2 |

Mastering these relationships is foundational. Every future topic in pharmacy calculations—dosing, IV flow rates, compounding—builds on accurate unit conversion.

Key terms10

Metric systemA decimal-based measurement system using grams, liters, and meters as base units; the standard system in US pharmacy practice.
Gram (g)The base metric unit of weight or mass used in pharmacy.
Liter (L)The base metric unit of volume used in pharmacy.
Milligram (mg)One one-thousandth of a gram (0.001 g); one of the most common units for expressing drug doses.
Microgram (mcg)One one-thousandth of a milligram (0.001 mg); used for potent drugs where very small amounts are required.
Milliliter (mL)One one-thousandth of a liter (0.001 L); the standard metric unit for liquid medication volumes.
Kilogram (kg)One thousand grams; the unit used to express patient body weight for weight-based dosing calculations.
Household measuresCommon volume units used in homes (teaspoon, tablespoon, fluid ounce, cup) that must be converted to metric units for accurate pharmacy practice.
Dimensional analysisA problem-solving method in which units are treated as factors that can be multiplied and canceled, ensuring the answer has the correct unit.
Conversion factorA ratio that expresses the equivalence between two units (e.g., 1 kg = 2.2 lb) and is used to convert from one unit to another.

Worked examples4

1. Convert 3.5 grams to milligrams.
  1. Identify the relationship: 1 g = 1,000 mg.
  2. You are converting from a larger unit (grams) to a smaller unit (milligrams), so multiply by 1,000.
  3. 3.5 g × 1,000 mg/g = 3,500 mg.
Answer: 3,500 mg
2. A prescription calls for 0.25 L of a solution. How many milliliters is that?
  1. Identify the relationship: 1 L = 1,000 mL.
  2. You are converting from a larger unit (liters) to a smaller unit (milliliters), so multiply by 1,000.
  3. 0.25 L × 1,000 mL/L = 250 mL.
Answer: 250 mL
3. A child's prescription reads 'give 2 teaspoons.' How many milliliters should the parent measure?
  1. Identify the relationship: 1 tsp = 5 mL.
  2. Set up dimensional analysis: 2 tsp × (5 mL / 1 tsp).
  3. The tsp units cancel: 2 × 5 mL = 10 mL.
Answer: 10 mL
4. A patient weighs 176 pounds. What is the patient's weight in kilograms? (Use 1 kg = 2.2 lb.)
  1. Identify the relationship: 1 kg = 2.2 lb, so to convert pounds to kilograms, divide by 2.2.
  2. Set up the calculation: 176 lb ÷ 2.2 lb/kg.
  3. 176 ÷ 2.2 = 80 kg.
Answer: 80 kg

The math in this lesson1

Learners convert within metric units (3.5 g = 3,500 mg) and between household, metric, and pounds-kilograms.
N-Q.1
Brush up in the math halls at newSchoolWorks.com:
In class: Conversion ladders · Pound/kilogram practice · Liquid dose conversions
Materials: Calculators, conversion charts
Check: Quiz ≥ 90%

Practice check28

Answer each question, then check it. Nothing is recorded; this is your own practice.

Order Entry and Processing
1. Convert 125 mg to micrograms.
Order Entry and Processing
2. A patient weighs 44 lb. What is the weight in kilograms? (1 kg = 2.2 lb)
Order Entry and Processing
3. How many milliliters are in 1 teaspoon(s)? (1 tsp = 5 mL)
Order Entry and Processing
4. Convert 250 mg to micrograms.
Order Entry and Processing
5. A patient weighs 198 lb. What is the weight in kilograms? (1 kg = 2.2 lb)
Order Entry and Processing
6. How many milliliters are in 1.5 teaspoon(s)? (1 tsp = 5 mL)
Order Entry and Processing
7. Convert 500 mg to micrograms.
Order Entry and Processing
8. A patient weighs 110 lb. What is the weight in kilograms? (1 kg = 2.2 lb)
Order Entry and Processing
9. Convert 750 mg to micrograms.
Order Entry and Processing
10. A patient weighs 66 lb. What is the weight in kilograms? (1 kg = 2.2 lb)
Order Entry and Processing
11. Convert 12.5 mg to micrograms.
Order Entry and Processing
12. A patient weighs 22 lb. What is the weight in kilograms? (1 kg = 2.2 lb)
Order Entry and Processing
13. How many milliliters are in 2 teaspoon(s)? (1 tsp = 5 mL)
Order Entry and Processing
14. Convert 2.5 mg to micrograms.
Order Entry and Processing
15. Convert 0.5 mg to micrograms.
Order Entry and Processing
16. A patient weighs 55 lb. What is the weight in kilograms? (1 kg = 2.2 lb)
Order Entry and Processing
17. A patient weighs 88 lb. What is the weight in kilograms? (1 kg = 2.2 lb)
Order Entry and Processing
18. How many milliliters are in 3 teaspoon(s)? (1 tsp = 5 mL)
Order Entry and Processing
19. A patient weighs 132 lb. What is the weight in kilograms? (1 kg = 2.2 lb)
Order Entry and Processing
20. A patient weighs 33 lb. What is the weight in kilograms? (1 kg = 2.2 lb)
Order Entry and Processing
21. How many milligrams are in 0.5 grams?
Order Entry and Processing
22. A liquid medication label states the bottle contains 0.473 L. How many milliliters does the bottle contain?
Order Entry and Processing
23. A patient is instructed to take 1 tablespoon of cough syrup every 6 hours. How many milliliters is each dose?
Patient Safety and Quality Assurance
24. A child weighs 66 pounds. What is the child's weight in kilograms? (Use 1 kg = 2.2 lb.)
Patient Safety and Quality Assurance
25. A physician orders amoxicillin 25 mg/kg/day for a child who weighs 44 lb. What is the correct total daily dose? (Use 1 kg = 2.2 lb.)
Order Entry and Processing
26. Which of the following correctly converts 2,500 mcg to milligrams?
Order Entry and Processing
27. A prescription requires 6 fluid ounces of a topical solution. How many milliliters must the technician measure?
Patient Safety and Quality Assurance
28. A pharmacy technician is entering an order and notices the prescriber wrote the patient's weight as 90 kg but the patient's chart lists the weight as 90 lb. If a weight-based dose is calculated using 90 kg instead of the correct kilogram value for 90 lb, what type of error is most likely to occur?
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