Prescription refill calculator

Quantity and daily dose must both be strictly positive finite numbers; zero, negative, missing, and non-numeric values fail with invalid_input and are not billed.

What days covered means and how the next refill date is computed

This calculator uses the integer identity days_covered equals the floor of quantity divided by daily_dose. The next refill date is the fill date plus days_covered using proleptic Gregorian serial day counts. Equivalently, refill_date equals fill_date plus days_covered days.

Input contract, response shape, and error behaviour

Days covered above thirty-six thousand five hundred (one hundred years) are rejected so a microscopic daily dose cannot invent a date past year 9999. Failed validation is not billed. Send a JSON body with three required fields. The field quantity (aliases qty, quantity_dispensed, qty_dispensed, dispensed, units) is the amount dispensed and must be a strictly positive finite number or numeric string, at most one million. The field daily_dose (aliases dose, dose_per_day, daily_consumption, units_per_day, per_day) is the amount taken each day in the same unit and must likewise be a strictly positive finite number at most one million. The field fill_date (aliases date, filled_on, fill, dispensed_on) is the calendar day of the fill as a real Gregorian date in YYYY-MM-DD form with years from one through nine thousand nine hundred ninety-nine; impossible days such as the thirty-first of February are rejected. Each numeric field may arrive as a JSON number or as a numeric string; empty strings, non-finite values, and missing fields fail with invalid_input. On success the body echoes rounded quantity and daily_dose, the normalized fill_date, integer days_covered, refill_date as YYYY-MM-DD, and a formula string restating the identity. Example: quantity thirty, daily_dose one, fill_date two thousand twenty-six March first yields days_covered thirty and refill_date two thousand twenty-six March thirty-first. Example: quantity ninety, daily_dose two, fill_date two thousand twenty-six December fifteenth yields days_covered forty-five and refill_date two thousand twenty-seven January twenty-ninth, crossing the year boundary without a host clock. A successful API call costs the published $0.002 base price with unit item.

When this calculator helps, and what it will not do

Use this landing when the search is for a prescription refill date or a days-supply calculator from quantity, daily dose, and fill date, not when the visitor needs a gestational calendar, a caffeine half-life, or a recipe scaler. Pharmacy-tech classrooms can freeze a thirty-tablet once-daily fill and vary the fill date so learners see month-end and leap-day carry without a spreadsheet date serial. The tool does not apply early-refill windows, copay logic, controlled-substance limits, or remaining-refill counts from a pharmacy claim, and it does not convert tablets to millilitres or guess a default daily dose from free-text sig. Prefer numbers rather than pre-formatted strings when you control the client; string numbers are accepted only when they match a strict finite numeric pattern. Adherence worksheets can compare an intended thirty-day cycle against a twice-daily ninety-count bottle and read days_covered and refill_date directly from the JSON. Automation tests can pin expected values for a year-boundary fill, a leap-year fill, and the required invalid_input path when quantity is zero or daily dose is not a positive number. Failed tasks that return invalid_input do not consume prepaid balance, which keeps exploratory scripts cheap when learners deliberately send a zero quantity.