How We Calculate

Every RefillDay result can be checked by hand. This page lists the formulas, rounding rules and assumptions the calculators use, so you can see exactly where a number came from.

Days supply

Days Supply = Quantity ÷ Amount Used per Day, rounded down

We round down because a partial day doesn't cover all of that day's doses. Whatever is left after the last full day is shown as "left over." This matches how US pharmacies record days supply for tablets and capsules.

Dates

• The fill or start date is day 1 of the supply.
• Supply end date = fill date + days supply − 1: the last day covered.
• Estimated refill date = fill date + days supply: the first day without medication, which pharmacies often treat as the due date.
• Planning date = estimated refill date − your reminder buffer.

Dates are calculated as whole calendar days, never as clock times, so time zones and daylight-saving changes can't shift a date. Month lengths and leap years come from the calendar itself: February has 29 days in 2028 but 28 in 2027 and 2100. "Today" is your device's local date.

Missed doses, extra doses and pill counts

• Missed doses are added back to the quantity (the pills are still in the bottle). Extra doses are subtracted. Both are counted in pills.
• A pill count replaces the estimate entirely. We assume you counted before taking that day's doses, and the run-out date is the count date plus the whole days the count covers.
• When a saved medication has a count from an earlier day, we subtract your daily amount for each day since, so the list stays current.

Status labels

Each label has its own wording and icon, so the meaning never depends on color alone.

Other calculators

• Changing doses: we count day by day from the start date, subtracting that weekday's dose until there isn't enough for the next one. Tapers add each step's daily amount × days.
• Quantity needed: we count whole doses in the period (once weekly for 30 days is 5 doses) and round up to whole tablets.
• Tablets by strength: dose ÷ strength. We flag results that aren't whole tablets, or halves for tablets, because those doses can't be made accurately.
• Liquids: usable volume = bottle × (1 − loss allowance). Days supply = usable volume ÷ mL per day, rounded down. 1 tsp = 5 mL, 1 tbsp = 15 mL.
• Refill too soon: day = days supply × threshold, rounded up; date = fill date + that day. The thresholds are examples for comparison, not any plan's actual rule.
• Refills remaining: covered days = (refills + 1) × days supply. If you enter the date written, refills dated after the expiration (written date + validity in months) are excluded.
• Prior authorization timeline: decision deadline = time the plan received the request + the federal maximum for your coverage, request type and urgency (hours are counted as elapsed time; days as calendar days). Sources: 42 CFR 423 subpart M (Medicare Part D), Social Security Act §1927(d)(5) and 42 CFR 438 (Medicaid), 29 CFR 2560.503-1 (employer plans), 45 CFR 147.136 and 156.122 (Marketplace plans). We show the federal maximum; states and plans can be faster.
• Copay savings: days in the period = months × 365 ÷ 12; fills needed = days ÷ days per fill, rounded up; cost = fills × cost per fill; cost per day = cost per fill ÷ days per fill.
• Dosage (mg/kg): kg = lb × 0.45359237; per-dose rate: mg per dose = kg × mg/kg; per-day rate: mg per day = kg × mg/kg, divided by doses per day. Any maximum you enter replaces a higher result. mL = mg ÷ (mg ÷ mL of the liquid strength), rounded to 0.1 mL with the exact value shown.

What we don't calculate

• Doses. RefillDay never suggests how much medication to take. The mg/kg calculator only does the arithmetic for a dose rate your prescriber or the product label gave you, and it contains no drug information.
• Insurance approval. We can't see your plan's rules or claims history.
• Inhalers, insulin, eye drops, creams, patches and other products measured in puffs, units, drops or grams.

Spotted something that looks wrong? Recheck your entries first; most surprises come from a fill date or a per-day amount. Your pharmacist can always confirm the days supply on record.