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Medication Units & units/mL Calculations
Calculate units, units/hr and units/kg/hr using the exact current units/mL concentration while keeping product-specific units, device instructions and rounding separate.
MedMaths Learn pathway
Specialised Dosing Calculations
Tackle calculation systems that use units or body-surface-area dosing rather than the standard mg/kg and mg/mL patterns.
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Medication Units & units/mL Calculations
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BSA Dosing (mg/m²)
Keep BSA in m² separate from an order in mg/m², then calculate the total medication amount and apply supplied protocol rules separately.
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Medication Units & units/mL CalculationsSome medicines are measured in units of biological activity rather than mass. Units do not convert to milligrams, and units of one medicine are not comparable with units of another — but the volume calculation follows the same pattern as any concentration problem.
Some medicines are measured in units of biological activity rather than mass. Units do not convert to milligrams, and units of one medicine are not comparable with units of another — but the volume calculation follows the same pattern as any concentration problem.
Formula
volume = units prescribed ÷ concentration in units/mL
Worked example
A prescription supplies 450 units. The available product is labelled 100 units/mL.
- 1450 ÷ 100 = 4.5
- 2Sense check: 450 units is four and a half times 100 units.
4.5 mL
Common mistake
Trying to convert units into milligrams. There is no general conversion factor, and units are specific to the product.
Treat units as their own measurement family and never convert them to mass.
BSA Dosing (mg/m²)A dose written per square metre is scaled by body surface area instead of weight. Structurally it is the same two-stage calculation as mg/kg dosing: multiply the supplied rate by the supplied BSA to get a dose, then convert that dose into a volume.
A dose written per square metre is scaled by body surface area instead of weight. Structurally it is the same two-stage calculation as mg/kg dosing: multiply the supplied rate by the supplied BSA to get a dose, then convert that dose into a volume.
Formula
dose = dose per m² × BSA in m²
Worked example
A supplied dose of 60 mg/m² for a patient whose BSA is supplied as 1.7 m², from a preparation of concentration 25 mg/mL.
- 1Dose: 60 × 1.7 = 102 mg
- 2Volume: 102 ÷ 25 = 4.08
102 mg, given as 4.08 mL
Common mistake
Using body weight in place of BSA. They are different quantities in different units, and the prescription states which one applies.
The BSA figure is supplied by the question or calculated separately — the dosing step simply multiplies by it.