Amount to give = (D ÷ H) × Q
The desired-over-have rule is the workhorse of oral and injectable dosing. Divide the desired (ordered) dose by the dose you have on hand, then multiply by the quantity that strength is supplied in — 1 tablet, or the mL a liquid's strength comes in.
What each part means
| D | Desired dose — what the prescriber ordered |
|---|---|
| H | Dose on Hand — the strength available |
| Q | Quantity the strength is supplied in (1 tablet, or mL) |
Worked example
Ordered 500 mg; on hand 250 mg tablets. (500 ÷ 250) × 1 = 2 tablets.