Using Formulae to Work Out Drug Dosages - dummies

By Claire Boyd

Formulae can make your life much easier when working out doses of tablets and pills, IV drip rates and more, as long as you know how they work. Here are six common formulae you can use:

  • Working out the amount of tablets or capsules to dispense according to the prescription: Probably the most common formula:

    What you want / What you’ve got

  • Working out the amount of millilitres to draw up when administering liquid medications: Used for injectables, including bolus IV injections, and for oral elixirs, syrups and emulsions. The answer is always in millilitres:

    (What you want / What you’ve got) x Volume

  • Working out the infusion pump rates: Used in syringe drivers to set the rate of the pump for, say, insulin sliding scales. You also input these figures (volume divided by time the fluids need to go through) when administering IV bags of fluids through a volumetric pump, and the pump works out the rate for you:

    Amount of fluid (ml) / Infusion time (hours)

  • Working out IV drip rates (gravity method): Helps you calculate the rate in whole drops per minute when administering IV bags of fluid. The prescription states only the type and volume of fluid; you have to work out in which administration set to administer the fluid. Clear fluids such as sodium chloride 0.9% use an administration set delivering 20 drops per millilitre, while thickened fluids such as blood use an administration set delivering 15 drops per millilitre:

    Volume (ml) / Time in hours x Drops per ml/Minutes per hour

  • Working out how to titrate drugs according to body weight: Used in acute care, paediatrics or care of the elderly when the prescription relies on adjusting the amount according to the individual’s body weight (in kilograms for this formula):

    Weight (kg) x dose

  • Working out over how many minutes to administer a drug: Used when administering medications, such as IV bolus doses, to prevent speed shock:

    Dose prescribed / Rate