Drug Dosage Calculator

Drug dosage calculation determines the correct amount of medication to administer based on patient weight, prescribed dose, and drug concentration. Formula: Volume = (Desired Dose × Weight) / Concentration.

Volume (mL) = (Prescribed Dose (mg/kg) × Patient Weight (kg)) / Stock Concentration (mg/mL). For a 70 kg patient prescribed 2 mg/kg from a 10 mg/mL solution: (2 × 70) / 10 = 14 mL.

Dose Given (mg) = Volume Administered (mL) × Stock Concentration (mg/mL). If you give 14 mL of a 10 mg/mL solution: 14 × 10 = 140 mg.

Stock concentration is the amount of drug per unit volume in the available preparation. It is usually written on the vial or ampule label, e.g., 50 mg/mL, 10 mg/mL, or 1,000 U/mL.

Concentration is key. For heparin 25,000 U in 500 mL = 50 U/mL. To give 1,000 U: 1,000 / 50 = 20 mL. For insulin 100 U/mL: to give 10 U = 10 / 100 = 0.1 mL.

mg/kg dosing depends on patient weight and is used for most pediatric drugs and weight-based protocols. Fixed dosing uses a standard amount regardless of weight, common for adult medications (e.g., one tablet).

Dilution reduces the concentration of a stock solution by adding diluent (NS, D5W, or sterile water). Formula: C1V1 = C2V2, where C1 and V1 are initial concentration and volume, and C2 and V2 are final concentration and volume.

Use the dilution formula C1V1 = C2V2. For example, to make 50 mL of 2 mg/mL from 10 mg/mL stock: V1 = (2 × 50)/10 = 10 mL stock + 40 mL diluent.

IV flow rate (mL/hr) for drug infusion = (Dose (mg/kg/min) × Weight (kg) × 60) / Concentration (mg/mL). For 5 mcg/kg/min in 70 kg using 1600 mcg/mL: (5 × 70 × 60) / 1600 = 13.125 mL/hr.

Infusion Rate (mL/hr) = (Desired Dose (mcg/kg/min) × Weight (kg) × 60 min/hr) / Concentration (mcg/mL). Ensure units are consistent by converting mg to mcg (1 mg = 1000 mcg).

The rule of 6 simplifies: weight (kg) × 6 = mg of drug added to make 100 mL. Then 1 mL/hr = 1 mcg/kg/min. Example: 70 kg × 6 = 420 mg in 100 mL; at 10 mL/hr = 10 mcg/kg/min.

Medication errors cause significant patient harm. Accurate dosage calculation prevents underdosing (therapeutic failure) and overdosing (toxicity). Always double-check calculations using independent verification.

Common errors include decimal point misplacement (e.g., 0.5 mg vs 0.05 mg), unit conversion mistakes (mg vs mcg), confusing weight-based and fixed dosing, and miscalculating volume from concentration.

Use the 5-step method: (1) Read the order carefully, (2) Check drug concentration on label, (3) Verify patient weight, (4) Calculate using the formula, (5) Have a second person independently verify the calculation.

Pharmacokinetics is the study of drug movement through the body: absorption (how drug enters), distribution (where it goes), metabolism (how it breaks down), and excretion (how it leaves). It guides dosing regimens.

Bioavailability is the fraction of an administered dose that reaches systemic circulation unchanged. IV drugs have 100% bioavailability. Oral drugs have lower bioavailability due to first-pass liver metabolism.

Vd is the theoretical volume needed to contain the total drug dose at the measured plasma concentration. Large Vd (> 4.5 L for a 70 kg adult) indicates extensive tissue distribution.

Half-life is the time for drug concentration to decrease by 50%. It determines dosing frequency. A drug with t½ = 4 hours reaches steady state in ~20 hours and requires q4–6h dosing.

Steady state is achieved when drug intake equals elimination, usually after 4–5 half-lives. At steady state, plasma concentrations remain consistent between doses, and therapeutic levels are maintained.

A loading dose is a higher initial dose to quickly reach therapeutic levels. Loading Dose = Vd × Desired Concentration / Bioavailability. For digoxin: loading dose 10–15 mcg/kg.

Maintenance dose replaces the amount of drug eliminated since the last dose. Maintenance = Clearance × Desired Steady-State Concentration. It is typically lower than the loading dose.

Renal impairment reduces drug excretion, requiring dose reduction or extended intervals. Use creatinine clearance (CrCl) to adjust doses of renally eliminated drugs (e.g., vancomycin, gentamicin, enoxaparin).

Hepatic impairment reduces drug metabolism, requiring dose reduction for hepatically cleared drugs (e.g., morphine, benzodiazepines, phenytoin). Monitor liver function and adjust based on clinical response.

The therapeutic window is the concentration range between the minimum effective concentration and the minimum toxic concentration. Narrow window drugs (e.g., lithium, warfarin, digoxin) require careful monitoring.

High-alert medications have a high risk of causing significant harm when used in error. Examples include insulin, heparin, opioids, chemotherapy agents, potassium concentrates, and vasopressors.

For a 15 kg child prescribed 10 mg/kg of amoxicillin from a 250 mg/5 mL suspension: Dose = 15 × 10 = 150 mg. Concentration = 250/5 = 50 mg/mL. Volume = 150/50 = 3 mL.

Use a syringe (not a graduated cylinder or teaspoon) for volumes under 10 mL. Oral syringes provide accuracy to 0.1 mL. For IV medications, use tuberculin or 1 mL syringes for doses under 1 mL.

First calculate the hourly volume: Volume/hr = (Dose/hr) / Concentration. Then convert to gtts/min: gtts/min = (Volume/hr × Drop Factor) / 60. For 23 mL/hr with 60 gtts/mL: (23 × 60)/60 = 23 gtts/min.

Unit conversions change between measurement systems: 1 g = 1000 mg, 1 mg = 1000 mcg, 1 kg = 2.2 lbs, 1 tsp = 5 mL, 1 tbsp = 15 mL. Incorrect conversion is a common cause of medication errors.

Use standardized formulas, reliable references (drug formulary, pharmacy), independent double-checks, smart pump drug libraries, and computerized physician order entry (CPOE) to minimize calculation errors.