Dosage calculation formula sheet

Doses

Tablets or mL = desired ÷ have × quantity. Convert the order to the label's unit first. Tablets round to the nearest half, and only scored tablets are split. Liquids round to the tenth of a mL, or the hundredth under 1 mL.

Reconstituted powders: mL = dose ÷ final concentration. Use the concentration after mixing, never the diluent volume.

IV rates

Pump rate: mL/hr = volume in mL ÷ time in hours. Minutes divided by 60 give hours. Round to a whole number unless the pump or question allows tenths.

Gravity drip: gtt/min = volume in mL × drop factor ÷ time in minutes. Macrodrip is 10, 15 or 20 gtt/mL; microdrip is 60 gtt/mL, where gtt/min equals mL/hr. Round to a whole drop.

Infusion time: hours = volume ÷ mL/hr. Convert the decimal part to minutes by multiplying by 60. Volume infused = mL/hr × hours.

Weight-based, pediatric and heparin

Kilograms = pounds ÷ 2.2. Dose = kg × mg/kg. If the order is per day, divide by the doses per day: q12h is 2, q8h is 3, q6h is 4. Safe dose: multiply both ends of the range by the weight and compare like with like.

Heparin: concentration = units ÷ mL in the bag. Units/hr = units/kg/hr × kg. mL/hr = units/hr ÷ units/mL. Bolus mL = units ÷ vial units/mL.

Critical care drips in mcg/kg/min: mL/hr = mcg/kg/min × kg × 60 ÷ concentration in mcg/mL.

Rules that apply to every problem

Round once, at the end, to the rule the question states. Write a leading zero (0.5 mL) and no trailing zero (5 mL). Write units beside every number. Check the answer's size before you commit to it: more than 4 tablets or 5 mL for an injection usually means a slip.

Before the exam

Know the conversions without looking: 1 kg = 2.2 lb, 1 g = 1,000 mg, 1 mg = 1,000 mcg, 1 tsp = 5 mL, 1 tbsp = 15 mL, 1 oz = 30 mL, 1 hr = 60 min. Know the rounding rules your program uses. Practise every problem type at least twenty times in the format the exam uses.

How to use this sheet

Print it and keep it beside you for the first week of practice. Each time you reach for it, note which formula you needed. In the second week, cover the formulas and try each problem type from memory, checking only when you are stuck. By the third week the sheet should be in a drawer. The exam will not let you bring it, and the formulas are short enough that daily drilling makes them automatic.

Critical care drips, step by step

Vasoactive drips are ordered in mcg/kg/min or mcg/min and programmed in mL/hr. Find the concentration in mcg/mL: dopamine 400 mg in 250 mL is 400,000 mcg ÷ 250 mL = 1,600 mcg/mL. Find the dose per hour: 5 mcg/kg/min × 70 kg × 60 min = 21,000 mcg/hr. Divide: 21,000 ÷ 1,600 = 13.1 mL/hr. Round to the tenth, as most pumps for these drugs accept tenths.

Nitroglycerin is usually ordered in mcg/min without weight: 10 mcg/min × 60 = 600 mcg/hr; from 50 mg in 250 mL, which is 200 mcg/mL, that is 3 mL/hr.

Infusion time and finish time

Hours = volume ÷ mL/hr. 500 mL at 75 mL/hr is 6.67 hours. Multiply the decimal by 60 for minutes: 0.67 × 60 is about 40, so 6 hours 40 minutes. Add that to the start time on the 24-hour clock for the finish time: started at 0800, 1,000 mL at 100 mL/hr finishes at 1800.