Free Tool
Dosage Calculator
Solve the four pharmacy math problems that appear most on the PTCB and pharmacy technician exams: weight-based dose, IV pump rate, IV drip rate, and days supply.
mg = weight (kg) × mg/kg
350 mg
For exam practice and study only. Always verify clinical doses against a current reference and your pharmacist or prescriber before dispensing.
The Four Dosage Calculations Every Pharmacy Tech Must Know
Pharmacy math is one of the highest-weighted domains on the PTCB and state pharmacy technician exams, and four calculation types account for most of the questions. Weight-based dosing scales a medication to the patient: dose in milligrams equals the patient's weight in kilograms times the ordered mg/kg. Because orders often list weight in pounds, converting to kilograms (divide by 2.2) is a common first step that trips up test takers.
Intravenous calculations come in two flavors. When an electronic pump delivers the medication, you only need the rate in mL/hr, found by dividing the total volume by the infusion time in hours. When the infusion runs by gravity through drip tubing, you instead calculate drops per minute using the tubing's drop factor: gtt/min = (volume × drop factor) ÷ time in minutes. Choosing the right formula depends entirely on whether a pump or a gravity set is in use.
Days supply ties the clinical order to the billing side of the pharmacy. It is the total quantity dispensed divided by the daily usage — dose per administration times administrations per day. An accurate days supply keeps insurance claims clean and prevents early-refill rejections, which is why the exam tests it alongside the clinical math. This calculator keeps each of the four modes separate so you can practice recognizing which formula a question is really asking for.
Frequently Asked Questions
How do you calculate a weight-based dose?
Multiply the patient's weight in kilograms by the prescribed dose per kilogram: dose (mg) = weight (kg) × mg/kg. For example, a 70 kg patient at 5 mg/kg needs 70 × 5 = 350 mg. If the weight is given in pounds, divide by 2.2 to convert to kilograms first.
What is the formula for an IV pump rate?
IV infusion pump rate in mL/hr equals the total volume to infuse divided by the time in hours: mL/hr = volume (mL) ÷ time (hr). Infusing 1,000 mL over 8 hours is 1,000 ÷ 8 = 125 mL/hr. Pumps are programmed in mL/hr, so no drop factor is needed.
How do you calculate IV drip rate in gtt/min?
For a manual (gravity) IV set, drip rate in drops per minute equals volume times the tubing's drop factor divided by the time in minutes: gtt/min = (volume mL × drop factor gtt/mL) ÷ time (min). With 1,000 mL, a 15 gtt/mL set, over 480 minutes: (1,000 × 15) ÷ 480 = 31.25, rounded to 31 gtt/min.
What is a drop factor?
The drop factor is how many drops equal one milliliter for a given IV tubing set, printed on the package as gtt/mL. Macrodrip sets are commonly 10, 15, or 20 gtt/mL; microdrip sets are 60 gtt/mL. You must use the set's actual drop factor in the drip-rate formula.
How is days supply calculated for an insurance claim?
Days supply equals the total quantity dispensed divided by the amount used per day: days = total quantity ÷ (dose per administration × administrations per day). Ninety tablets taken as one tablet three times daily is 90 ÷ (1 × 3) = 30 days. Getting this right is essential for accurate pharmacy billing.
Ready for the full Pharmacy Tech path?
Practice dosage math on real exam questions
Drill weight-based dosing, IV rates, and days-supply problems with instant explanations, then run full PTCB-style simulations in your browser.