Drug Calculations
Unit Conversions
Units
NB. prior to performing calculations, always convert all values the same units
Gram equivalents
3
1g = 1ml = 1cm
Drug Dilutions and Concentrations

Ratios = unit of solute to units of solvent or grams of solvent in mls of solution (because 1g = 1ml)
e.g. 1:10,000 adrenaline = 1 unit adrenaline to every 10,000 units saline or 1g adrenaline in every 10,000ml saline

% solutions = grams of solvent per 100ml solute (because 1ml = 1g)
e.g. 0.01% adrenaline = 0.01g adrenaline per 100ml saline (or 1g adrenaline per 10,000ml)

Moles = weight (g) / molecular mass (u)
e.g. Magnesium’s atomic number (and therefore molecular mass) is 24 so 1 mole of magnesium is 24g
Molarity (M) = number of moles of a chemical in 1L of solution
i.e. 1 molar = molecular mass (u) per litre
e.g. 1 molar solution of magnesium consists of 24g magnesium dissolved in 1L water

Weight-Dosage Calculations
Infusion Rate Calculations
Infusion time/rate
Infusion set drop rate
© 2013 Dr Christopher Mansbridge at www.OSCEstop.com, a source of free OSCE exam notes for medical students’ finals OSCE revision
Number of Tablets/Liquids Needed
Calculate first
What you need to give (tablets/ml) = dose you want / what you’ve got (dose per tab or dose per ml)
© 2013 Dr Christopher Mansbridge at www.OSCEstop.com, a source of free OSCE exam notes for medical students’ finals OSCE revision