🧮 NAPLEX Calculations — Formulas & Worked Examples — 2026

NAPLEX Calculations Guide 2026

Every calculation type tested on the NAPLEX — CrCl, IV flow rates, alligation, TPN, osmolarity, weight-based dosing — with step-by-step formulas, worked examples, and practice quiz.

NAPLEX exam note: You will have access to an on-screen calculator. However, knowing the formulas and setup before using the calculator saves critical time. Practice setting up calculations quickly — the setup is often harder than the math.

High-Yield NAPLEX Calculation Formulas

Calculation	Formula	Units	NAPLEX Weight
CrCl (Cockcroft-Gault)	[(140-age) x wt(kg)] / [72 x SCr] x 0.85 if female	mL/min	VERY HIGH
IV flow rate	Volume (mL) / Time (hr)	mL/hr	VERY HIGH
Weight-based dosing	mg/kg x weight (kg)	mg	HIGH
Alligation	Parts higher/lower = \|target - other conc\|	parts	MODERATE
Dilution (C1V1=C2V2)	C1 x V1 = C2 x V2	consistent	MODERATE
IBW (males)	50 + 2.3 x (inches over 60)	kg	MODERATE
IBW (females)	45.5 + 2.3 x (inches over 60)	kg	MODERATE
AdjBW	IBW + 0.4 x (ABW - IBW)	kg	STANDARD
BSA	sqrt[(ht(cm) x wt(kg)) / 3600]	m²	STANDARD

Worked Examples

Creatinine Clearance

Problem: 72-year-old male, 80 kg, SCr 1.5 mg/dL. Calculate CrCl.

CrCl = [(140 - 72) x 80] / [72 x 1.5]
= [68 x 80] / 108
= 5440 / 108 = 50.4 mL/min
(No 0.85 factor — patient is male)

Weight-Based IV Infusion

Problem: Dopamine 400 mg in 250 mL D5W. Dose: 5 mcg/kg/min. Patient: 70 kg. What is the rate in mL/hr?

Concentration = 400,000 mcg / 250 mL = 1,600 mcg/mL
Dose = 5 mcg/kg/min x 70 kg = 350 mcg/min
Rate = (350 mcg/min x 60 min/hr) / 1,600 mcg/mL = 13.1 mL/hr

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