Two everyday dose calculations with the working shown: desired over have (how much of the stocked medication to give) and weight-based dosing (mg/kg). Always verify the result independently before administering anything.
Formulas: amount to give = (dose ordered ÷ dose in stock) × quantity; total dose = weight (kg) × dose per kg. Units must match within each calculation. These tools check arithmetic only — they are not a prescribing aid, and every dose must be verified against the prescription, the patient's allergies and local policy, ideally with an independent second check.
What is the desired-over-have formula?
Amount to give = (dose desired ÷ dose you have) × quantity the stocked dose comes in. If 250 mg is ordered, the stock is 500 mg tablets, and the quantity is 1 tablet: 250 ÷ 500 × 1 = 0.5 tablet. The same formula works for liquids using millilitres.
How are weight-based (mg/kg) doses calculated?
Multiply the patient's weight in kilograms by the prescribed dose per kilogram. If the weight was measured in pounds, convert first: kilograms = pounds ÷ 2.2. The result is a total dose, which must then be checked against the drug's recommended range and maximum before it is prepared.
Why do dosage calculations need an independent check?
A misplaced decimal or mismatched unit changes a dose tenfold. Independent double-checking — a second clinician repeating the calculation from the prescription — is standard safety practice for medication administration, and an online calculator is never a substitute for it.