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.
To calculate a drug dosage, use Desired ÷ Have × Quantity (D/H × Q): divide the ordered dose by the strength on hand, then multiply by the volume it comes in. For example, 250 mg ordered with 125 mg/5 mL gives 250 ÷ 125 × 5 = 10 mL. Always convert both doses to the same unit before you divide.
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.
New to this? The three dosage calculation methods — desired over have, ratio and proportion, and dimensional analysis — are compared side by side with worked examples.
Work through these common nursing dosage problems, then check the step-by-step solution. Each uses the Desired ÷ Have × Quantity (D/H × Q) method, or a weight-based dose. Great for NCLEX and med-math practice.
1. Liquid medication. An order reads 500 mg. The vial is labelled 250 mg/5 mL. How many mL do you give?
2. Tablets. An order reads 750 mg. Tablets on hand are 250 mg each. How many tablets?
3. Weight-based dose. An order reads 5 mg/kg for a patient weighing 70 kg. What is the total dose?
4. Small liquid dose. An order reads 0.1 mg. The ampoule is 0.2 mg/mL. How many mL?
5. Units. An order reads 4,000 units of heparin. The vial is 5,000 units/mL. How many mL?
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.
Confirm the ordered dose and the strength on hand are in the same unit (convert first if not), divide the desired dose by the dose you have, then multiply by the quantity it comes in — the D/H × Q method. Example: order 500 mg, have 250 mg/5 mL → 500 ÷ 250 × 5 = 10 mL.
The core nursing dosage formula is Desired ÷ Have × Quantity (D/H × Q). For weight-based orders, first multiply the per-kilogram dose by the patient's weight to get the total dose, then apply D/H × Q to find the volume or number of tablets to give.