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Systems of Measurement and Conversion Factors

Unit 6 Lesson 1

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Explain the different systems of measurement

Household system
Metric system

Calculate the conversions between systems of measurement

Student Learning Outcomes

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Systems of Measurement

Two main systems used in healthcare

Household system

Customary

Metric system

Current standard

Barbour-Taylor et al., 2024

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Household System

Also called "customary system" in daily use
Units

Tablespoons
Teaspoons
Pounds
Ounces

No longer recommended due to dosing errors
ISMP advises avoiding household measurements completely
Use only when absolutely necessary with clients
Always verify with metric measurements when possible

Barbour-Taylor et al., 2024

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Household System: Common Conversions

Household Unit	Metric Equivalent
1 teaspoon	5 mL
1 tablespoon	15 mL
1 cup	240 mL
1 ounce	30 mL
1 pound	0.45 kg

Barbour-Taylor et al., 2024

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Household System: Conversion Example

Convert 4 tablespoons to mL

1 tablespoon = 15 mL

4 tablespoons × 15 mL = 60 mL

Therefore, 4 tablespoons = 60 mL

Used primarily for client education
Not for professional medication administration
Increases risk of medication errors

Barbour-Taylor et al., 2024

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Critical Thinking Question

A home health nurse is teaching a client how to measure liquid medication using a household measuring spoon. The prescription calls for 15 mL of medication. How should the nurse instruct the client to measure this dose using household measurements?

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Metric System Overview

Current standard in healthcare industry
Base units

Gram (g)
Liter (L)

Based on multiples of 10 for easy conversion
Simple conversions → Move the decimal point
Most medications prescribed using metric measurements
International standard for medication administration

Barbour-Taylor et al., 2024

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Metric System: Prefixes

Prefix	Meaning	Example
Kilo (k)	1000 × larger	1 kg = 1000 g
Base unit	Standard	gram (g), liter (L)
Milli (m)	1/1000	1000 mg = 1 g
Micro (mc)	1/1,000,000	1000 mcg = 1 mg

Barbour-Taylor et al., 2024

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Metric System: Volume Measurements

Basic unit

Liter (L) for liquids

Common units in nursing practice
Liter (L)

IV fluids, large volumes

Milliliter (mL)

Medication administration, small volumes

1 L = 1000 mL (move decimal 3 places)
1 mL = 0.001 L (move decimal 3 places)

Barbour-Taylor et al., 2024

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Metric System: Weight Measurements

Basic unit

Gram (g) for mass

Common units in medication dosing
Kilogram (kg)

Client weight

Gram (g)

Some medications

Milligram (mg)

Most medications

Microgram (mcg)

Potent medications

Barbour-Taylor et al., 2024

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Metric Conversion: Basic Rules

Moving larger to smaller

Multiply by conversion factor

Moving smaller to larger

Divide by conversion factor

Remember powers of 10 (10, 100, 1000)
For metric

Move decimal point right or left

Right = smaller units (g → mg)
Left = larger units (mg → g)

Barbour-Taylor et al., 2024

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Metric Conversion: Weight Example 1

Convert 2 kg to g

1 kg = 1000 g

2 kg × 1000 = 2000 g

Move decimal point 3 places right: 2.0 → 2000

Therefore, 2 kg = 2000 g

Barbour-Taylor et al., 2024

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Metric Conversion: Weight Example 2

Convert 1500 mg to g

1000 mg = 1 g

1500 mg ÷ 1000 = 1.5 g

Move decimal point 3 places left: 1500 → 1.5

Therefore, 1500 mg = 1.5 g

Barbour-Taylor et al., 2024

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Metric Conversion: Volume Example

Convert 0.5 L to mL

1 L = 1000 mL

0.5 L × 1000 = 500 mL

Move decimal point 3 places right: 0.5 → 500

Therefore, 0.5 L = 500 mL

Barbour-Taylor et al., 2024

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Medication Calculation Example 1

Order: 0.25 g of ampicillin

Available: 250 mg tablets

How many tablets?

Convert 0.25 g to mg: 0.25 g × 1000 = 250 mg

Compare with available strength: 250 mg = 250 mg

250 mg ÷ 250 mg = 1 tablet

Therefore, give 1 tablet

Barbour-Taylor et al., 2024

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Critical Thinking Question

A nurse is preparing a medication that is available as 250 mg in 5 mL. The order is for 0.4 g. How many mL should the nurse administer?

2 mL
8 mL
10 mL
50 mL

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Medication Calculation Example 2

Order: 1000 mcg of medication

Available: 1 mg/mL solution

How many mL?

Convert 1000 mcg to mg: 1000 mcg ÷ 1000 = 1 mg

Compare with available strength: 1 mg = 1 mg/mL

1 mg ÷ 1 mg/mL = 1 mL

Therefore, give 1 mL

Barbour-Taylor et al., 2024

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Critical Thinking Question

A nurse is preparing to administer a medication with a dosage of 2000 mcg. The medication label indicates a concentration of 1 mg/mL. How many mL should the nurse administer?

0.5 mL
2 mL
20 mL
200 mL

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Converting Between Systems

Metric to Household

1 kg = 2.2 lb

Multiply kg by 2.2

1 lb = 0.45 kg

Divide lb by 2.2

1 tsp = 5 mL
1 Tbsp = 15 mL
1 oz = 30 mL

Barbour-Taylor et al., 2024

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System Conversion Example 1

Convert 143 lb to kg

1 kg = 2.2 lb

143 lb ÷ 2.2 = 65 kg

Therefore, 143 lb = 65 kg

Important for weight-based medication dosing

Barbour-Taylor et al., 2024

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System Conversion Example 2

Convert 3 tsp to mL

1 tsp = 5 mL

3 tsp × 5 mL = 15 mL

Therefore, 3 tsp = 15 mL

Used for client education on liquid medications

Barbour-Taylor et al., 2024

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Critical Thinking Question

A nurse is teaching a client how to take liquid medication at home. The prescription is for 10 mL of medication three times daily. Which household measurement should the nurse instruct the client to use?

2 teaspoons
2 tablespoons
⅔ tablespoon
1 cup

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Metric and US Traditional Measurement Units

Measurement	Metric Unit	Metric Equivalent	US Traditional
Weight	1 kilogram (kg)	1000 grams (g)	2.2 pounds (lb)
	1 gram (g)	1000 milligrams (mg)	0.035 ounces (oz)
	1 milligram (mg)	1000 micrograms (mcg)	-
Volume	1 liter (L)	1000 milliliters (mL)	4.23 cups
Length	1 meter (m)	100 centimeters (cm)	3.28 feet (ft)
	1 centimeter (cm)	10 millimeters (mm)	0.39 inches (in)

Barbour-Taylor et al., 2024

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Household Equivalents

Volume

Household Unit	Household Equivalent	Metric Equivalent
1 cup	8 fluid ounces (fl oz)	240 milliliters (mL)
1 fluid ounce (fl oz)	-	30 milliliters (mL)
1 tablespoon (Tbsp)	3 teaspoons (tsp)	15 milliliters (mL)
1 teaspoon (tsp)	-	5 milliliters (mL)

Barbour-Taylor et al., 2024

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References

Barbour-Taylor, T., Mueller, L., Paris, D. & Weaver, D. (2024). Pharmacology for Nurses. OpenStax.org. https://openstax.org/details/books/pharmacology

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