Days supply calculator
Days supply = total quantity ÷ amount used per day. Pick what's being dispensed and the calculator shows every step, so you can check your own work.
For “as needed” (prn), use the most doses allowed in a day.
Days supply
20 days
- 1 per dose × 3 doses a day = 3 tablets a day
- 60 tablets ÷ 3 a day = 20 days
Worked examples
60 tablets, 1 tablet three times daily
60 ÷ 3 a day = 20 days
100 mL, 5 mL twice daily
100 ÷ 10 mL a day = 10 days
#24, 1 tablet every 4 hours as needed
24 ÷ 6 (the most a day) = 4 days
#150, 1 capsule five times daily
150 ÷ 5 a day = 30 days
#60, 5 tablets once daily
60 ÷ 5 a day = 12 days
10 mL eye drops, 1 drop in both eyes twice daily
200 drops ÷ 4 a day = 50 days
Inhaler with 120 puffs, 2 puffs twice daily
120 ÷ 4 a day = 30 days
10 mL vial of U-100 insulin, 40 units daily
1,000 units ÷ 40 = 25 days
Easy places to slip
- For as-needed (prn) prescriptions, use the most doses the patient could take in a day.
- Many pharmacy systems only work out days supply up to 4 times a day. For 5 or more, pick “Five times daily”, “Six times daily” or “Other” here. For tapers, use the taper calculator.
- Both eyes (OU) or both ears (AU) doubles the drops used each day.
- U-100 insulin has 100 units in every mL, so a 10 mL vial holds 1,000 units.
- Convert teaspoons to mL before dividing: 1 teaspoon is 5 mL.
A study aid: always follow your pharmacy's policy and the pharmacist for the days supply you bill.
Practice the math until it's automatic
Gutta's Order Entry lessons teach days supply, quantity, dosing and IV rates with worked examples, then quiz you on them.
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