Desired Over Have (D/H × V): The Basic Dosage Formula
Shows the work. Every example is computed with the formula and numbered steps. For study and exam practice, not for patient care.
Desired over Have: The formula method: amount to give = Desired (ordered dose) ÷ Have (dose on hand) × Vehicle (the form or volume the dose on hand comes in).
Desired over Have (D/H × V, sometimes D/H × Q for quantity) is the most common dosage formula:
- D, Desired: the dose ordered.
- H, Have: the dose on hand, from the label.
- V, Vehicle: what that dose comes in, 1 for a tablet or capsule, or the mL on the label for a liquid.
Amount to give = D ÷ H × V. D and H must be in the same unit, so convert first (g to mg, mg to mcg) when they differ.
It is one of three methods taught for the same problems; ratio and proportion and dimensional analysis reach the same answer.
Order: promethazine 30 mg IM. Available: promethazine 50 mg/mL.
- Desired dose
- 30 mg
- Have
- 50 mg
- Vehicle
- 1 mL
- Give as
- mL (liquid or injection)
Formula Amount = desired ÷ have × vehicle.
- mL = desired dose ÷ concentration.
- 30 mg ÷ 50 mg/mL = 0.6 mL.
- 0.6 mL is exact, so no rounding is needed.
Answer 0.6 mL
Sources
- Clinical Calculations: With Applications to General and Specialty Areas — Elsevier (Kee, Marshall, et al.)
- Calculate with Confidence — Elsevier (Morris)