Days Supply Calculator
Calculate days supply the way a pharmacist does, for every dosage form: tablets, liquids dosed in mg or mL, inhalers with priming, insulin and weekly pens, eye drops and nasal sprays, creams by fingertip unit, patches and steroid tapers. Get the billing days supply, the quantity for 30 or 90 days, and a supply calendar.
Written and fact-checked by Manzoor Ahmad, Pharm-DLast reviewed October 2, 2026
Areas treated each application
Taper steps (units per day for how many days)
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Billing and dates (optional)
Choose the dosage form, enter what was dispensed and the directions, and see the days supply. Nothing is stored.
Days supply
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Supply calendar
Against plan maximum
Usable share dispensed
Quantity to dispense for a target supply
Reverse calculation, rounded up to whole packages where packages cannot be split.
What is days supply?
Days supply is the number of days a dispensed quantity of medicine will last when taken exactly as directed. The basic formula is days supply = quantity dispensed ÷ quantity used per day. Sixty tablets taken one twice a day is a 30-day supply.
Pharmacies submit days supply with every insurance claim, and it drives refill timing, plan limits and audits. The simple formula works for tablets, but insulin priming, inhaler priming sprays, eye drop counts, fingertip units for creams, patch change schedules and tapering doses all need extra steps. This days supply calculator handles each of them and shows the billing days supply, the exact value, and the quantity needed for 28, 30 or 90 days.
How to use the days supply calculator
- Pick the dosage form from the eight tabs. Each one asks only for what that form needs.
- Enter what was dispensed: tablets, mL, inhalers, pens or vials, bottles, grams, patch boxes or a taper quantity.
- Enter the directions: the amount per dose and how often. For as-needed (PRN) directions, use the maximum allowed per day, which gives the shortest correct days supply.
- Set a fill date and plan maximum to see dates, a supply calendar and a warning if the claim would exceed the plan limit.
- Use the reverse calculation to find the quantity to dispense for a 28, 30 or 90-day supply, rounded to whole packages where needed.
Once you know the days supply, the prescription refill calculator turns it into your earliest insurance refill date, a request-by reminder and a sync plan for several medicines.
Days supply formulas by dosage form
Tablets and capsules
Including half tabletsDays supply = quantity ÷ (units per dose × doses per day)
Ninety half-tablets? Enter 0.5 units per dose. Billing days supply is rounded down to whole days, because a partial day of medicine is not a full day of therapy.
Oral liquids, including mg doses
Convert mg to mL firstmL per dose = dose in mg ÷ (strength mg ÷ strength mL)
A child prescribed 250 mg of amoxicillin from a 400 mg/5 mL suspension takes 3.125 mL per dose. Twice a day from a 100 mL bottle, the volume lasts 16 days, but reconstituted amoxicillin is usually discarded after about 14 days, so the days supply is limited by that date, not the volume. Enter the discard period and the calculator applies it automatically.
Inhalers
Metered-dose inhalersDays supply = inhalers × (puffs per inhaler − priming puffs) ÷ (puffs per dose × doses per day)
Many metered-dose inhalers need several test sprays when first used. Those puffs never reach the patient, so they come off the usable count.
Insulin pens and vials
Two limits, whichever comes firstDays per device = the smaller of: units in device ÷ (daily units + priming units × injections) or the in-use limit
This is where most days supply errors happen. Pens usually need a 2-unit safety shot before every injection, and opened insulin often must be discarded after 28 days whether or not it is empty. Ignoring either limit overstates the days supply and leaves patients short.
Eye drops, ear drops and nasal sprays
Drops are estimated per mLDays supply = (bottle mL × drops per mL × bottles) ÷ (drops per site × sites × doses per day)
About 20 drops per mL is the usual working estimate, but real drop sizes vary by product and bottle. Nasal sprays use the labeled number of sprays per bottle minus priming sprays.
Creams and ointments (fingertip units)
One adult fingertip unit is about 0.5 gDays supply = grams dispensed ÷ (fingertip units × 0.5 g × applications per day)
A fingertip unit is the amount squeezed from a standard tube along the end segment of an adult index finger. The calculator adds up the areas you tick using the classic fingertip unit guide.
Transdermal patches
Count changes, not patchesDays supply = (patches ÷ patches worn at a time) × days between changes
Ten 72-hour patches cover 30 days. Ten patches changed twice a week cover 35 days.
Tapering doses
For steroid tapers and other step-down regimens, add each step. A 21-tablet methylprednisolone pack taken as 6, 5, 4, 3, 2 and 1 tablets on successive days is a 6-day supply. The calculator also flags when the quantity dispensed runs out before the taper ends.
Insulin days supply chart
Days supply at common daily doses, assuming one injection a day, 2 units of priming per pen injection and a 28-day in-use limit.
| Daily dose | One 300-unit pen | Box of 5 pens | One 1000-unit vial | Vials for 30 days |
|---|---|---|---|---|
| 10 units | 25.0 days | 125 days | 28 days | 2 |
| 15 units | 17.6 days | 88 days | 28 days | 2 |
| 20 units | 13.6 days | 68 days | 28 days | 2 |
| 30 units | 9.4 days | 46 days | 28 days | 2 |
| 40 units | 7.1 days | 35 days | 25 days | 2 |
| 50 units | 5.8 days | 28 days | 20 days | 2 |
| 60 units | 4.8 days | 24 days | 16 days | 2 |
| 80 units | 3.7 days | 18 days | 12 days | 3 |
| 100 units | 2.9 days | 14 days | 10 days | 3 |
Swipe sideways on a phone. More injections a day mean more priming waste; use the calculator for your exact regimen.
Fingertip unit guide for adults
| Area | Fingertip units | Approximate grams |
|---|---|---|
| Face and neck | 2.5 | 1.25 g |
| Front of trunk | 7 | 3.5 g |
| Back and buttocks | 7 | 3.5 g |
| One arm | 3 | 1.5 g |
| One hand, both sides | 1 | 0.5 g |
| One leg | 6 | 3 g |
| One foot | 2 | 1 g |
Based on the adult fingertip unit guide by Long and Finlay. Children need smaller amounts.
Why accurate days supply matters
Refill timingPlans use days supply to decide when the next refill is allowed. Overstate it and patients run out before insurance will pay.
Plan limitsMany plans cap claims at 30, 34 or 90 days. A days supply over the limit is rejected, even if the quantity is a single unbreakable package.
AuditsIncorrect days supply is one of the most common findings in pharmacy benefit manager audits, especially for insulin, inhalers, eye drops and creams, and can lead to claim recoupments.
Patient safetyA correct days supply helps spot overuse, such as a rescue inhaler refilled far too often, or underuse that signals missed doses.
A pharmacist’s checklist for tricky days supply
After years of checking claims and labels, these are the situations I see trip people up most often.
As-needed directions
Base the days supply on the maximum daily dose in the directions. “One to two tablets every 6 hours as needed” means up to 8 a day. If the directions have no maximum, ask the prescriber to add one.
Unbreakable packages
Inhalers, pens, tubes and eye drop bottles cannot be split. Calculate the real days supply from the whole package; do not shrink it to match a plan limit. If it exceeds the limit, the plan may allow a package-size override.
Dose changes and titrations
For medicines started low and increased, such as many GLP-1 pens and some antidepressants, calculate each phase separately using the taper tab in reverse: enter each step’s daily amount and days.
Weekly and less frequent doses
A once-weekly tablet dispensed as 4 tablets is a 28-day supply, not 4 days. Choose once a week in the frequency list, or use the fixed-dose pen option for weekly injections.
Refrigerated and reconstituted products
Always check the beyond-use or discard date. Antibiotic suspensions, some eye drops once opened, and insulin all have limits that shorten the true days supply.
Days supply calculator FAQ
Divide the quantity dispensed by the amount used per day. For example, 60 tablets taken one twice a day is 60 ÷ 2 = 30 days. Pharmacies round the result down to whole days for billing.
Add priming units to the daily dose (usually 2 units per pen injection), divide the units in each pen or vial by that total, cap each device at its in-use limit (commonly 28 days), then multiply by the number of devices. For example, a 300-unit pen at 20 units a day plus 2 units of priming lasts about 13.6 days.
It depends on the dose. With one injection a day and 2 units of priming, five 300-unit pens last about 125 days at 10 units a day, 68 days at 20 units, 46 days at 30 units and 35 days at 40 units.
Subtract any priming puffs from the labeled number of puffs, multiply by the number of inhalers, then divide by puffs per day. A 200-puff inhaler with 4 priming puffs used 2 puffs twice a day lasts 49 days.
Using the common estimate of 20 drops per mL, a 5 mL bottle holds about 100 drops. Actual counts vary by product and technique, so treat the result as an estimate.
Estimate grams per application with fingertip units (about 0.5 g each for adults), multiply by applications per day, then divide the grams dispensed by that daily amount. Treating both hands twice a day uses about 2 g a day, so a 30 g tube lasts 15 days.
Use the maximum daily dose allowed by the directions. For one to two tablets every 6 hours as needed, the maximum is 8 a day, so 40 tablets is a 5-day supply.
A standard 21-tablet methylprednisolone dose pack taken as 6, 5, 4, 3, 2 and 1 tablets on six consecutive days is a 6-day supply.
Divide the number of patches by the number worn at once, then multiply by the days between changes. Ten patches changed every 72 hours is a 30-day supply.
Days supply is normally rounded down to whole days, because a partial day of medicine does not provide a full day of treatment. Package-based products such as inhalers and pens are calculated from the whole package.
Multiply the daily amount by 90. For one tablet twice a day that is 180 tablets. For packaged products, divide by the usable amount per package and round up to whole packages.
An overstated days supply can block refills and leave patients without medicine, while an understated one allows refills too early. Both are common audit findings and can lead insurers to recover payment from the pharmacy.
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About the author
Manzoor Ahmad, Pharm-D
Pharmacist, health content writer and founder of DexoCalc
Manzoor is a Doctor of Pharmacy with more than nine years of experience writing about supplements, weight management and medication safety. He runs a community pharmacy where days supply for insulin, inhalers, eye drops and creams is checked on claims every working day. He built this calculator from the original published equations and checks every formula against worked examples before release.
Sources and references
- National Council for Prescription Drug Programs (NCPDP). Telecommunication Standard: Days Supply field definition and billing guidance.
- Long CC, Finlay AY. The finger-tip unit: a new practical measure. Clinical and Experimental Dermatology. 1991;16(6):444-447.
- U.S. Food and Drug Administration. Prescribing information for insulin pens and vials: priming and in-use storage periods.
- U.S. Food and Drug Administration. Prescribing information for metered-dose inhalers: priming instructions and labeled actuations.
- Code of Federal Regulations, Title 21, Part 1306. Prescriptions for controlled substances.
This days supply calculator is an educational and cross-checking tool. Plan rules and product labeling vary, and the pharmacist’s professional judgment and the product’s official labeling take precedence. It is not a diagnosis and not a substitute for advice from your doctor, pharmacist or dietitian. Never change how you take a medicine to make a supply last longer without speaking to your prescriber or pharmacist.