Dosage calculations: conversions, IV flow rates & weight-based dosing

Calculation questions are gifts if you are systematic and traps if you rush, because the wrong answers are usually the results of predictable mistakes — forgetting to convert units, dividing the wrong way round, using the wrong drop factor, or losing a decimal place. The examiner is testing a reproducible METHOD, not arithmetic genius, and almost every item resolves to one of a small set of shapes: how many tablets/mL to give for an ordered dose, how fast to run an infusion (mL/hour on a pump, or drops/minute by gravity), how much powder-diluent to draw up after reconstitution, what a weight-based dose should be, or whether an ordered dose is SAFE.

A single method that works for all of them:

• •CONVERT first. Put every quantity into the same unit system before you do anything else — grams to milligrams, milligrams to micrograms, litres to millilitres, pounds to kilograms, hours to minutes. Most errors are born here.
• •IDENTIFY what is asked and its unit. Read what the answer must be (tablets? mL? mL/hour? gtt/min?) and let that steer which formula you use.
• •SET UP the formula so unwanted units cancel and the unit you want remains. Write it out — do not do it in your head.
• •CALCULATE, then ROUND appropriately for the form (whole tablets are not split unless scored; pump rates and gtt/min are usually whole numbers).
• •SANITY-CHECK. Ask 'is this plausible?' A dose needing 8 tablets, half a vial pushed as one dose, or an infusion at an impossible rate is a red flag to recheck, not to administer.

The safety overlay the exam always rewards: a calculation is never finished at the number. If the maths says give many tablets or a large volume of a high-alert drug, STOP and re-verify — recalculate, check the order, and for high-alert drugs (insulin, heparin, opioids, paediatric doses, IV infusions) have a second qualified nurse INDEPENDENTLY check the calculation before you give it.