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📏 Math · Measurement and Data · Lesson 7 of 7 · 6 min

Converting units: US standard and metric

💊 Weights in pounds, doses in milliliters, heights in inches: nurses convert units all day, and so does the TEAS.

💡 Key idea

Multiply by a conversion fraction that cancels the old unit; smaller units mean bigger numbers.

🧠 The idea

Memorize the key CONVERSION FACTORS: 1 in = 2.54 cm, 1 kg = 2.2 lb, 1 lb = 16 oz, 1 ft = 12 in, 1 yd = 3 ft, 1 mile = 5,280 ft, 1 cup = 8 fl oz, 1 fl oz ≈ 30 mL, 1 tsp = 5 mL, 1 tbsp = 15 mL, 1 L = 1,000 mL. Within the METRIC system, move by powers of 10: kilo = 1,000, centi = 1/100, milli = 1/1,000, so 1 kg = 1,000 g and 1 m = 100 cm. Use DIMENSIONAL ANALYSIS: multiply by a fraction equal to 1 with the unit you want on top and the unit you are canceling on the bottom. Sanity check: converting to a smaller unit gives a bigger number. Temperature uses a formula, not a factor: °F = (9/5 × °C) + 32 and °C = (°F − 32) × 5/9.

✏️ Worked example

A patient weighs 165 lb. What is the weight in kilograms?
Step 1: Use 1 kg = 2.2 lb, so the conversion fraction is (1 kg / 2.2 lb).
Step 2: 165 lb × (1 kg / 2.2 lb) = 165 ÷ 2.2 kg.
Step 3: 165 ÷ 2.2 = 75.
Check: kilograms are bigger units, so the number should be smaller. 75 < 165 ✓
Answer: 75 kg.

📌 Takeaways

  • 1 kg = 2.2 lb, 1 in = 2.54 cm, 1 tsp = 5 mL, 1 tbsp = 15 mL, 1 fl oz ≈ 30 mL.
  • Put the unit you want on top of the conversion fraction.
  • °F = 9/5 °C + 32.

📖 Terms in this lesson

Conversion factor: A ratio equal to 1 that changes units without changing the amount; for example, 1 kg / 2.2 lb converts 44 lb to 20 kg.

Metric prefixes: Prefixes that scale metric units by powers of 10: kilo- = 1,000, centi- = 1/100, milli- = 1/1,000; for example, 1 L = 1,000 mL.

✅ Check yourself

Question 1 of 3

A patient weighs 154 lb. What is the weight in kilograms? (1 kg = 2.2 lb)

  1. 70 kg
  2. 338.8 kg
  3. 151.8 kg
  4. 77 kg
Show the answer and rationale

Answer: A

Divide by 2.2: 154 ÷ 2.2 = 70 kg. Multiplying by 2.2 gives 338.8, which goes the wrong way; 77 halves the weight; 151.8 subtracts 2.2 instead of dividing.

Question 2 of 3

A bottle holds 2.5 L of saline. How many milliliters is that?

  1. 2,500 mL
  2. 25,000 mL
  3. 25 mL
  4. 250 mL
Show the answer and rationale

Answer: A

1 L = 1,000 mL, so 2.5 × 1,000 = 2,500 mL. The values 25 and 250 move the decimal too few places, and 25,000 moves it one place too many.

Question 3 of 3

A child's temperature is 40 °C. What is it in °F? (°F = 9/5 °C + 32)

  1. 104 °F
  2. 4.4 °F
  3. 54.2 °F
  4. 72 °F
Show the answer and rationale

Answer: A

9/5 × 40 = 72, then 72 + 32 = 104 °F. Seventy-two forgets to add 32, 54.2 uses 5/9 in place of 9/5, and 4.4 applies the °F-to-°C formula in the wrong direction.

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