Calculation
Chapter 48 Dosage Calculation
– Basic medication dose conversion and calculation skills are essential for
providing safe nursing care.
– Nurses are responsible for administering the correct amount of medication
by calculating the precise amount of medication to give. Nurses can use
three different methods for dosage calculation: ratio and proportion,
formula (desired over have), and dimensional analysis.
TYPES OF CALCULATIONS
– Solid oral medication
– Liquid oral medication
– Injectable medication
– Correct doses by weight
– IV infusion rates
STANDARD CONVERSION FACTORS
– 1 mg = 1,000 mcg
– 1 g = 1,000 mg
– 1 kg = 1,000 g
– 1 oz = 30 mL
– 1 L = 1,000 mL
– 1 tsp = 5 mL
– 1 tbsp = 15 mL
– 1 tbsp = 3 tsp
– 1 kg = 2.2 lb
– 1 gr = 60 mg
GENERAL ROUNDING GUIDELINES
– ROUNDING UP: If the number to the right is equal to or greater than 5,
round up by adding 1 to the number on the left.
– ROUNDING DOWN: If the number to the right is less than 5, round down
by dropping the number, leaving the number to the left as is.
– For dosages less than 1.0: Round to the nearest hundredth.
o For example (rounding up): 0.746 mL = 0.75 mL.
• The calculated dose is 0.746 mL. Look at the number in the
thousandths place (6). Six is greater than 5. To round to
hundredths, add 1 to the 4 in the hundredths place and drop
, the 6. The rounded dose is 0.75 mL.
o Or (rounding down): 0.743 mL = 0.74 mL.
, • The calculated dose is 0.743 mL. Look at the number in the
thousandths place (3). Three is less than 5. To round to the
hundredth, drop the 3 and leave the 4 as is. The rounded
dose is 0.74 mL.
– For dosages greater than 1.0: Round to the nearest tenth.
o For example (rounding up): 1.38 = 1.4.
• The calculated dose is 1.38 mg. Look at the number in the
hundredths place (8). Eight is greater than 5. To round to
the tenth, add 1 to the 3 in the tenth place and drop the 8.
The rounded dose is 1.4 mg.
o Or (rounding down): 1.34 mL = 1.3 mL.
• The calculated dose is 1.34 mL. Look at the number in the
hundredths place (4). Four is less than 5. To round to the
tenth, drop the 4 and leave the 3 as is. The rounded dose is
1.3 mL.
Solid dosage
– Example: A nurse is preparing to administer phenytoin 0.2 g PO every 8 hr.
The amount available is phenytoin 100 mg/capsule. How many capsules
should the nurse administer per dose? (Round the answer to the nearest
whole number. Use a leading zero if it applies. Do not use a trailing zero.)
USING RATIO AND PROPORTION
STEP 1: What is the unit of measurement the nurse should
calculate? Capsules
STEP 2: What is the dose the nurse should administer? Dose to administer =
Desired
0.2 g
STEP 3: What is the dose available? Dose available = Have
100 mg
STEP 4: Should the nurse convert the units of
measurement? Yes (g ≠ mg).
Set up an equation.
, 1g 0.2 g
=
1,000 mg X mg
Solve for X
X mg = 200mg
Or you can use your knowledge of equivalents.