Ratio and Proportion Method for Dosage Calculation
Shows the work. Every example is computed with the formula and numbered steps. For study and exam practice, not for patient care.
Ratio and proportion: A dosage method that sets the known ratio (dose on hand to its volume) equal to the ordered dose over the unknown amount, then cross-multiplies.
Ratio and proportion compares two equal ratios. The label gives the first (the dose on hand and the volume it is in); the order gives half of the second, and the unknown amount, x, completes it:
have ÷ vehicle = desired ÷ x, or written with colons, have : vehicle :: desired : x
Cross-multiply and solve for x. As with every method, convert first so both doses are in the same unit, and keep the units in the same positions on both sides.
The answer is the same as Desired over Have, which is the proportion already solved for x.
Ondansetron 16 mg PO is ordered. Available: ondansetron oral solution 4 mg/5 mL.
- Desired dose
- 16 mg
- Have
- 4 mg
- Vehicle
- 5 mL
- Give as
- mL (liquid or injection)
Formula Amount = desired ÷ have × vehicle.
- mL = desired ÷ have × vehicle.
- 16 mg ÷ 4 mg × 5 mL = 20 mL.
- 20 mL is exact, so no rounding is needed.
Answer 20 mL
Sources
- Clinical Calculations: With Applications to General and Specialty Areas — Elsevier (Kee, Marshall, et al.)
- Calculate with Confidence — Elsevier (Morris)