Desired-Over-Have Formula
Overview
Desired over have — sometimes written D/H x Q — is the single formula behind most oral and injectable dose calculations. Desired is what the prescriber ordered. Have is the strength printed on the label. Quantity is the volume or number of tablets that labelled strength comes in. Divide desired by have, multiply by the quantity, and the result is the amount to administer. Two things make it go wrong: running it before the units match, and misreading the quantity as the size of the container rather than the volume the strength is stated in.
Interpretation
Technique
Worked example: 375 mg ordered, label reads 250 mg per 5 mL
- Desired375 mg
- Have250 mg
- Quantity5 mL
- Divide then multiply375 / 250 x 5
- Answer7.5 mL
During — Monitoring
Patient Teaching
Clinical Pearl
Desired over have, times the quantity it comes in. Match the units first, or the formula will hand you a wrong answer with total confidence.