Dosage Calculator: Desired Over Have × Vehicle
Enter the ordered dose and the strength on hand. The calculator converts units when they differ, then shows Desired ÷ Have × Vehicle one step at a time.
Shows the work. Answer, formula, and numbered steps, rounded the way nursing exams expect. For study and exam practice, not for patient care.
Formula
Amount = desired ÷ have × vehicle.
Fill in the fields and press Calculate to see the answer and every step.
For study and exam practice, not for patient care.
Rounding: Tablets round to a whole tablet, or the nearest half for a scored tablet. Volumes of 1 mL or more round to the tenth; under 1 mL, to the hundredth.
Terms: Desired over Have, Ratio and proportion, Dimensional analysis, mEq, PRN
Worked examples
Lisinopril 5 mg PO is ordered. Available: lisinopril 2.5 mg tablets.
- Desired dose
- 5 mg
- Have
- 2.5 mg
- Give as
- Tablets
- Scored tablet
- Yes
Formula Amount = desired ÷ have × vehicle.
- Tablets = desired dose ÷ tablet strength.
- 5 mg ÷ 2.5 mg = 2 tablets.
- The answer is a whole number of tablets: 2 tablets.
Answer 2 tablets
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
FAQ
What is the Desired over Have formula?
Amount to give = Desired ÷ Have × Vehicle. Desired is the ordered dose, Have is the strength on hand, and Vehicle is what that strength comes in: 1 for a tablet, or the mL on the label for a liquid (5 for 250 mg/5 mL).
What if the order and the label use different units?
Convert the order to the label's unit first (g to mg, mg to mcg), then divide. The calculator does this and shows the conversion as its own step.
Can I give part of a tablet?
Only a scored tablet can be split, and only in half. If the answer is not a whole or half tablet, check the order with the prescriber or pharmacist.