Pharmacy Calculations
Days supply, concentrations, and dosing — the arithmetic that must be right every time.
By the end of this lesson you can
- Calculate days supply for solid and liquid dosage forms
- Work with ratio strengths and percentage concentrations
- Perform weight-based dosing calculations
- Check an answer for plausibility before accepting it
Lesson Notes
Read through the key concepts before you try the challenge.
Days supply is the calculation you will do most
You are processing an insurance claim.
The days supply is wrong, so the claim rejects. You fix it and resubmit. But when days supply is wrong the other way — too long — a patient runs out early, cannot get a refill, and stops taking a medication they need.
Your task: Learn the calculations, and learn to check them before they leave your hands.
Days supply is total quantity divided by the amount taken per day. The arithmetic is simple; the errors come from misreading the sig. Two tablets twice daily is four per day, not two — and that single misreading halves or doubles the answer.
Days supply for a liquid
Amoxicillin suspension, 250 mg/5 mL. Sig: 500 mg PO TID. Dispensed: 300 mL. How many days?
- 1
Convert the dose from mg to mL.
The bottle is measured in mL and the dose is written in mg, so they must be brought to the same unit before anything else. 500 mg at 250 mg per 5 mL is 10 mL per dose.
- 2
Calculate the daily volume.
TID is three times daily, so 10 mL × 3 = 30 mL per day. This is the step where a misread frequency does its damage, so read the sig again here.
- 3
Divide the quantity dispensed by the daily volume.
300 mL ÷ 30 mL per day = 10 days.
- 4
Sanity-check against the prescription's intent.
Ten days is a normal antibiotic course, so the answer is plausible. An answer of 100 days or 1.5 days would tell you a decimal went astray — and this check catches most calculation errors in about three seconds.
Result: 10 days supply.
Match the units first, then find the daily amount, then divide — and always ask whether the answer is a plausible course of therapy.
| Written | Means | Example |
|---|---|---|
| 1:1000 | 1 gram in 1000 mL | Epinephrine 1:1000 is 1 mg/mL |
| 1% w/v | 1 gram in 100 mL | 1% lidocaine is 10 mg/mL |
| 5% w/v | 5 grams in 100 mL | 5% dextrose is 50 mg/mL |
| mg/mL | Milligrams per millilitre | Stated directly |
A prescription reads: 2 tablets PO BID. 120 tablets are dispensed. What is the days supply?
Challenge
Apply what you've learned in this lesson.
Show every step. Calculation errors hide in skipped steps.
- Calculate days supply for: 1 tab TID, 90 dispensed; 2 tabs BID, 60 dispensed; 5 mL QID, 200 mL dispensed.
- Convert to mg/mL: 0.45% saline, 2% lidocaine, 1:10,000 epinephrine.
- A child weighs 18 kg and the dose is 15 mg/kg per day divided into three doses. Calculate the amount per dose.
- For each answer above, write one sentence saying why the result is or is not plausible.
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