Performing Pharmacy Calculations — Practice Checklist

Step-by-Step Procedure (6 steps)

1. 1.

   Read the Sig code exactly as written and translate every abbreviation before calculating anything (e.g., "1 tab PO BID" = 1 tablet by mouth twice daily).

2. 2.

   Identify the total daily dose from the Sig (quantity per dose × doses per day).

3. 3.

   Calculate days supply: total quantity dispensed ÷ total daily dose (adjusting units if the drug is measured in mL, mg, or units rather than whole tablets).

4. 4.

   For liquid medications, convert between household and metric measures as needed (1 teaspoon = 5 mL; 1 tablespoon = 15 mL) and double-check the concentration (mg/mL) before converting a dose.

5. 5.

   For weight-based pediatric or specialty doses, confirm the patient's weight in kilograms and multiply by the ordered mg/kg dose, then verify the result is a reasonable, roundable volume or tablet quantity.

6. 6.

   Cross-check your calculated days supply or quantity against what the insurance claim or prescription actually authorizes before finalizing.

Quality Checkpoints (4)

Confirm each of these before considering the task complete.

• Sig abbreviations are translated correctly before any math is done — a misread abbreviation makes every downstream number wrong.

• Units are converted consistently (don't mix mg and mL, or teaspoons and mL, without converting).

• The final days-supply number is reasonable for the quantity and frequency prescribed — a wildly short or long days-supply is a red flag to double-check.

• Any calculation producing an unusual or very large/small dose is flagged to the pharmacist rather than assumed correct.

Avoid These Mistakes

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  Confusing "BID" (twice daily) with "q2h" (every 2 hours) or similar look-alike abbreviations under time pressure.

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  Forgetting to convert teaspoons to mL (or vice versa) before calculating a liquid days-supply.

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  Dropping or adding a decimal point in a weight-based pediatric calculation — this is one of the highest-risk calculation errors in pharmacy.

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  Calculating days supply from the bottle size instead of the prescribed quantity when they differ.