The drug dosage calculator helps nurses calculate the exact volume of liquid medication to administer when the ordered dose and available concentration differ. It uses the standard nursing D/H × Q formula — Desired divided by Have, multiplied by Quantity — which is the foundation of safe medication administration in clinical practice. Use this tool as a quick double-check before drawing up any liquid medication.
The drug dosage formula in practice. Each example uses Desired ÷ Have × Quantity (D/H × Q).
| Order | Available | Calculation | Give |
|---|---|---|---|
| 250 mg | 125 mg / 5 mL | 250 ÷ 125 × 5 | 10 mL |
| 500 mg | 250 mg / 5 mL | 500 ÷ 250 × 5 | 10 mL |
| 75 mg | 100 mg / 2 mL | 75 ÷ 100 × 2 | 1.5 mL |
| 1 g | 500 mg / 10 mL | 1000 ÷ 500 × 10 | 20 mL |
| 0.25 mg | 0.5 mg / 1 mL | 0.25 ÷ 0.5 × 1 | 0.5 mL |
| 40 mg | 10 mg / 1 mL | 40 ÷ 10 × 1 | 4 mL |
Before administering any medication, confirm: Right Patient · Right Drug · Right Dose · Right Route · Right Time. Always double-check high-alert medications with a second nurse.
The standard nursing drug dosage formula is D/H × Q, where D is the desired (ordered) dose, H is the dose on hand (available), and Q is the quantity (volume) available. For example, if 250mg is ordered and the available concentration is 125mg/5mL, the volume to give is (250 ÷ 125) × 5 = 10 mL.
Always double-check drug dose calculations for high-alert medications (insulin, heparin, opioids, chemotherapy), paediatric doses, and any result greater than 10 mL for a single dose. Most institutions require an independent double-check by a second nurse for these drug categories.