Household to Metric Conversions: lb to kg, tsp, tbsp and oz to mL
Nursing exams use fixed textbook factors: 1 kg = 2.2 lb, 1 tsp = 5 mL, 1 tbsp = 15 mL, 1 fl oz = 30 mL. Pounds to kilograms is the one that matters most, because weight-based doses depend on it.
The Formula
Divide pounds by 2.2 to get kilograms; multiply kilograms by 2.2 to get pounds.
How to work it out
- 1 Pounds to kilograms: divide by 2.2. A kilogram is heavier than a pound, so the number of kilograms is always smaller.
- 2 Pounds and ounces: convert the ounces to a fraction of a pound first (16 oz = 1 lb), add it on, then divide by 2.2.
- 3 Round weights in kilograms to the nearest tenth unless the question says otherwise. The rounded weight is what you carry into a dose calculation.
- 4 Teaspoons, tablespoons and ounces to milliliters: multiply by 5, 15 or 30. Going the other way, divide.
Worked Examples
An adult weighs 154 lb. What is the weight in kg?
- Pounds to kilograms, so divide by 2.2.
- 154 lb ÷ 2.2 lb/kg = 70 kg.
Answer: 70 kg
A discharge sheet says to give 2 tsp of a liquid medicine. How many mL is that?
- One teaspoon is 5 mL.
- 2 tsp × 5 mL/tsp = 10 mL.
Answer: 10 mL
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Common Mistakes
- Multiplying pounds by 2.2
That gives a number bigger than the weight in pounds, which cannot be right for kilograms. Divide.
- Treating ounces as tenths of a pound
7 lb 8 oz is not 7.8 lb. There are 16 ounces in a pound, so 8 oz is 0.5 lb and the weight is 7.5 lb.
- Mixing up the spoon sizes
A teaspoon is 5 mL and a tablespoon is 15 mL. Three teaspoons make one tablespoon.
Questions Students Ask
Why 2.2 and not 2.2046?
Every US nursing text and exam uses 2.2. Using the more precise figure can make your answer differ from the key after rounding, so stick with 2.2.
Should I round the weight before or after calculating the dose?
Round the weight to the nearest tenth of a kilogram first, then use that rounded weight. That is the convention most schools and exams follow, and it is what our lessons do.
Is a fluid ounce 30 mL or 29.57 mL?
For nursing calculations it is 30 mL. The textbook factors are deliberately rounded so they are easy to use at the bedside.
Keep practicing in lesson 2: Household and Metric
Fresh numbers every time. Ten correct answers finish the lesson. The practice round and the first five problems of lesson 1 are free.
Open lesson 2