The formula
Desired (D) is the dose ordered. Have (H) is the dose on hand, on the label. Quantity (Q) is the form the dose on hand comes in: 1 tablet, 1 capsule, or a volume such as 5 mL. The amount to give is D ÷ H × Q.
D and H must be in the same unit before you divide. If the order is in g and the label in mg, convert first.
Example: tablets
Order: metoprolol tartrate 50 mg PO. Available: 25 mg tablets.
- D = 50 mg, H = 25 mg, Q = 1 tablet
- 50 ÷ 25 × 1 = 2 tablets
Example: oral liquid
Order: acetaminophen 240 mg PO. Available: 160 mg per 5 mL.
- D = 240 mg, H = 160 mg, Q = 5 mL
- 240 ÷ 160 × 5 = 7.5 mL
Example: injection under 1 mL
Order: hydromorphone 0.5 mg IV push. Available: 2 mg/mL.
- D = 0.5 mg, H = 2 mg, Q = 1 mL
- 0.5 ÷ 2 × 1 = 0.25 mL (1 mL syringe)
Example: with a conversion
Order: metformin 1 g PO. Available: 500 mg tablets.
- Convert: 1 g = 1,000 mg
- 1,000 ÷ 500 × 1 = 2 tablets
When to use another method
Desired over have is quick for a single-step dose. For IV rates, weight-based drips and problems with several conversions, dimensional analysis keeps all the steps in one line and makes errors easier to spot.
Frequently asked questions
What does D/H × Q stand for?
Desired dose divided by the dose you have, times the quantity the dose on hand comes in.
Can you use D/H × Q for IV drips?
Only for the first step. A drip rate also needs the time and the drop factor, so use the gtt/min formula or dimensional analysis.