The Core Formulas You Actually Need

Desired over have times quantity handles most oral and injectable doses: (desired dose ÷ dose on hand) × volume on hand. IV flow rate in mL/hr is total volume ÷ total hours. Drops per minute is (volume in mL × drop factor) ÷ time in minutes; the drop factor comes from the tubing (10, 15, or 20 gtt/mL for macrodrip, 60 gtt/mL for microdrip). Weight-based doses are mg/kg/day or mcg/kg/min, so convert pounds to kilograms first by dividing by 2.2. Titrated drips use (dose ordered × weight × 60) ÷ concentration in mcg/mL. Dimensional analysis works for all of them if you prefer one method: write what you want on the left, chain the conversion factors, and cancel units until only the target unit remains.

Conversions You Must Know Cold

1 kg = 2.2 lb. 1 L = 1,000 mL. 1 g = 1,000 mg. 1 mg = 1,000 mcg. 1 tsp = 5 mL. 1 tbsp = 15 mL. 1 oz = 30 mL. 1 cup = 240 mL. 1 grain = 60 mg (some references use 65; the NCLEX uses 60). Time conversions matter just as much: 1 hr = 60 min, and titration questions almost always need that factor of 60. Practice moving decimal places rather than reaching for a calculator step, because the on-screen calculator is where transcription errors happen.

Rounding and Formatting Rules That Cost Points

Read the stem for the rounding instruction and follow it exactly — 'round to the nearest whole number,' 'round to the tenth,' or 'record your answer using one decimal place.' Drops per minute and drip rates are always whole numbers because you cannot give a partial drop. Use a leading zero for values under one (0.5 mg, never .5 mg) and never use a trailing zero (5 mg, never 5.0 mg); the NCLEX tests this as a safety principle, not just style. Do not include units in the answer box unless the item asks for them, and do not round intermediate steps — carry the full value through and round only at the end.

Safety Checks the Exam Expects Before You Give the Dose

Calculation questions often hide a safety judgment. If the math produces more than two tablets, more than 2 mL for an IM injection in an average adult, or a pediatric dose above the safe range printed in the stem, the expected answer is to hold and clarify with the provider rather than administer. Always compare your calculated dose against the safe dose range when one is given — a question that supplies mg/kg/day parameters is asking you to verify safety, not just compute. Check high-alert medications twice: insulin, heparin, opioids, chemotherapy, and any pediatric weight-based drug require an independent double check.