# Drug Dose Calculation by Body Weight and Body Surface Area

#### Calculating dosing by weight

A physician orders azithromycin for oral suspension at a dose of 15 mg/kg/day, divided every 12 h, for a child that weighs 36 lb. calculate the volume of oral suspension, in mL, that should be administered for each dose of a 200-mg/5-mL suspension as follows:

1. Calculate the child's weight in kg:

36 lb × kg/2.2 lb = 16.4 kg

2. Multiply the weight, in kg, by the dosing rate:

16.4 kg × 15 mg/kg/day = 246 mg/day

3. Divide the total daily dose by the number of doses/day:

246 mg/2 doses = 123 mg/dose

4. Calculate the volume of each dose using ratio and proportion:

(123 mg/dose)/(200 mg/5 mL) = 3.1 mL/dose

Some calculations may also be completed using dimensional units analysis (DUA). The DUA should begin at the left end with a factor containing the numerator answer units. All units other than those in the answer should cancel. If using DUA, the preceding equation would be as follows:

#### Calculating by body surface area (human)

A physician orders rituximab at a dose of 375 mg/m2 every week for 6 weeks for a patient who is 6 ft 2 inch tall and weighs 183 lb. Calculate the volume, in mL, of 10-mg/mL rituximab injection needed to make each IV infusion dose as follows:

1. Calculate the patient’s (body surface area) BSA:

2. Multiply the BSA by the dosing rate:

2.08 m2 × 375 mg/m2 = 780 mg/dose

3. Calculate the volume of each dose using ratio and proportion:

(780 mg/dose)/(10 mg/mL) = 78 mL/dose

Available Equations: BSA = square root of [wt. (in kg) × height (in cm)/3600]; BSA = square root of [wt. (in lb) × height (in inch)/3131]

Reference: USP-NF (1160)

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