Omni Calculator logo

Prescription Refill Calculator

Created by Łucja Zaborowska, MD, PhD candidate
Reviewed by Anna Szczepanek, PhD and Adena Benn
Last updated: Jan 18, 2024

The prescription refill calculator relieves you from the mundane task of computing days supply and the amount of medicine needed for a given therapy. It will also relieve you from the next refill date calculations. 💊

Dig into our prescription refill calendar and make your life a little bit easier!

Ps. If you wish to perform more complicated dosage calculations, visit our dosage calculator or the pediatric dose calculator.

We try our best to make our Omni Calculators as precise and reliable as possible. However, this tool can never replace professional medical advice, diagnosis, or treatment. All information on this website is for informational purposes only and is not intended to and should not serve as a substitute for medical consultation. Always seek the advice of a health provider.

How to calculate day supply for liquids?

Day supply for liquids calculations? Let's break it down.

  1. Find out the total volume of your drug.
  2. Look at your prescription.
    • Find out how often you take the drug.
    • Learn what's the amount you need to take for a single dose.

If your single dose amount is in volume, proceed to step 4.

  1. If your amount is given in mg.

    • Get to know your drug's concentration and calculate your dose volume:

    Dose volume = Dose(mg) / Concentration(mg/mL)

  2. Finally, use the equation:

    Days supply = Total volume / (Dose volume × Frequency)

Rx refill calculator vs OTC refill calculator

Let's explain the main difference between Rx drugs and OTC drugs.

  • OTC stands for the over-the-counter - it means that the drug in question is easily accessible, there are no restrictions, and the pharmacy will sell it to everyone. However, the OTC label doesn't mean that the drug is entirely harmless, and you can take it without any precautions!

  • Rx means prescription drug that a doctor must prescribe for a specific patient and a selected disease. These drugs are usually stronger than OTC medicines and may pose a greater risk to a patient if misused.

Both types of drugs can be taken regularly and may require a refill - the Rx drugs are more difficult to obtain because of a required consultation with a doctor.

How to calculate day supply for tablets?

The day supply calculations for tablets are relatively easy to compute. Let's get down to it:

  1. Find out the total number of tablets you have.
    Add the number of tablets from all the packages you got.

  2. Check your prescription/instructions.

    • Find out how often you need to take your medicine, e.g., once a day.
    • Discover how many tablets you need to take for a single dose.
  3. Use the following equation:

    Days supply = Total no. of tablets /(Tablets per dose × Frequency)

💡 Looking for tools dedicated to a specific medication? Start with levothyroxine dosage calculator or the adderall dosage calculator!

How to use the medication refill calculator?

The medicine refill calculator allows you to perform three different kinds of pill refill calculations:

  • Next refill date;
  • Expected days supply of tablets; and
  • Expected days supply of liquid drugs.
  1. Choose the section of your interest - Refill date or Days supply.
    • If you chose the Days supply, choose whether your drug's liquid or in the form of tablets/capsules.
  2. Each of the variants requires specific input:
    • Refill date: fill date and days supply;
    • Days supply: the total amount of the drug, required dosage, and frequency.
  3. Once you have added these values, our calculator will tell you the length of time your medication will last.
  4. Our tool works both ways. It could calculate the length of time your medication will last or, given a specific time(expected supply), dosage, and frequency, it would return the amount of medication you need. Do not hesitate to play with it a little!


How to perform 30 day prescription refill calculations?

It just couldn't be easier!

  1. Write down the date of your first dose.
  2. Add 29 days to the date you've just written.
  3. Obtained date is the last dose of your medication.
  4. The next refill period should begin the day after this date (+30 days).

Always remember that the number of days in each month may vary!

How to perform 90 day prescription refill calculations?

Let's make it nice and simple:

  1. Remember the date of your first dose.

  2. Add 89 days to the date you've just written.

    Last dose date = First dose date + 89 days

  3. Calculated date marks the last dose of your medication.

  4. The next refill period should begin the day after this date (+90 days).

Always remember that the number of days differs from month to month.

How to perform 28 day prescription refill calculations?

28 days equal 4 weeks - this calculation is probably the easiest to perform with the use of the calendar!

  1. Find out the date of your first dose.

  2. Use the equation below:

    Last dose date = First dose date + 27 days

  3. You've just calculated your last dose date. Your next refill period should start a day after that (+28 days).

*Always remember that the number of days varies from month to month.

Łucja Zaborowska, MD, PhD candidate
Refill date
Fill date
Days supply
Next refill
Days supply
Drug type
Tablets / capsules
Total number of tablets
once per day
Expected supply
Always double-check the calculations, the dosing information and contraindications on the leaflet. Consult your health provider before administering any medication. This tool cannot replace real medical consultation.
Check out 26 similar dosage calculators 💊
Adderall dosageAleve dosageBenadryl… 23 more
People also viewed…

Black Friday

How to get best deals on Black Friday? The struggle is real, let us help you with this Black Friday calculator!

Fetal weight percentile

The fetal weight percentile calculator allows you to estimate your child's growth rate in comparison to the general population.

Pizza size

Make the best pizza choice with our Pizza Size Calculator – compare sizes and prices for the perfect order!

Vaccine queue Russia

Калькулятор Очереди на вакцинацию показывает когда Вы, вероятнее всего, получите в России вакцину от COVID-19.