At the counter: calculate from the quantity actually dispensed and the maximum amount the directions permit the patient to use per day. For devices and non-daily products, first identify the labeled count, concentration, container limit, wear interval, or elapsed schedule. Keep that input beside the arithmetic and follow the current payer's rounding rule.
Days supply is NCPDP field 405-D5. Some workflow and reject references shorten the field to its three-digit number, 405; both identify Days Supply, while 405-D5 is the full transaction-field identifier. The calculation depends on using the correct input for the exact product, package, and directions. Each section below pairs the formula with its required inputs, example, limitations, and source. The current label, payer sheet, and clarified prescription remain authoritative.
Days supply calculation directory
Basic, non-daily, and as-needed directions
daily dosing: quantity dispensed ÷ maximum quantity used per day
non-daily dosing: number of doses dispensed × dosing interval
- Required inputs
- Dispensed quantity, complete sig, maximum daily use when PRN, and the payer or system rounding rule.
- Caveat
- For tapers or inconsistent intervals, count elapsed calendar days. Do not average a variable schedule into a fictional daily dose.
- Example
- Quantity 42, one dose every four to six hours as needed: the published CMS method uses six doses per day, producing seven days.
NCPDP's Version D and Above guidance, v73, says a non-whole result may be rounded up or down at pharmacy or system discretion unless the payer controls the method. It also warns that rounding up can create a prospective DUR reject, while rounding down can create audit exposure or leave too little medication. Retain the unrounded result and the rule used.
CMS, Pharmacy Auditing and Dispensing Job Aid: Billing Oral and Inhaled Products, December 2015; CMS, Billing Kits, December 2015; and NCPDP v73, June 2026. The CMS job aids are older published billing methods, not current universal payer rules.
Drops
(bottle mL × payer- or label-supported drops per mL) ÷ drops used per day
- Required inputs
- Bottle volume, eyes or ears treated, drops per dose, doses per day, and the conversion the payer accepts.
- Caveat
- CMS published 20 drops per mL as a 2015 billing convention. It is not a universal physical property of every bottle or formulation.
- Example
- 2.5 mL × 20 drops/mL = 50 drops. One drop in each eye twice daily uses four drops per day: 50 ÷ 4 = 12.5 days before payer rounding.
Measured drop size, single-dose-container instructions, spillage allowances, and product-labeled volume can change the correct input. NCPDP recognizes that a payer may allow spillage but does not publish one universal allowance. Record the conversion and allowance actually used.
CMS, Pharmacy Auditing and Dispensing Job Aid: Billing Other Dosage Forms, December 2015; NCPDP v73, June 2026.
Nasal sprays and course-limited otic drops
nasal spray: labeled usable spray count ÷ maximum sprays used per day
finite otic course: use the shorter of volume-based coverage and prescribed treatment days
- Required inputs
- Exact product and presentation, labeled usable spray count or bottle volume, maximum sprays or drops per day, ears treated, treatment duration, and label-specific priming instructions.
- Caveat
- Do not subtract priming from a labeled usable count unless the label directs it. Do not extend a finite treatment course merely because liquid remains.
- Example
- A fluticasone bottle labeled for 120 metered sprays, used as two sprays in each nostril once daily, provides 30 days. The cited label accounts for initial priming before stating the 120-spray usable count.
The current ofloxacin otic label illustrates the course boundary. For otitis externa in patients 13 years and older, it directs ten drops (0.5 mL) once daily for seven days. A 5 mL bottle contains more solution than that one-ear course uses, but the labeled treatment still covers seven days rather than ten.
DailyMed, fluticasone propionate nasal spray label, current version published March 9, 2026; DailyMed, ofloxacin otic solution label, updated June 11, 2026.
Insulin pens
total labeled units dispensed ÷ (maximum prescribed units per day + documented priming units)
- Required inputs
- Exact concentration, volume and pen count; maximum prescribed dose; label-directed safety-test or priming amount; and in-use discard period.
- Caveat
- Priming is device-specific and a failed safety test may need repetition. Each pen's in-use clock starts when that pen is first used.
- Example
- Five Lantus SoloStar pens contain 1,500 labeled units. At 20 units daily plus a successful two-unit safety test before each injection: 1,500 ÷ 22 = 68.18 days before payer rounding.
The current DailyMed listing for Lantus SoloStar states 300 units per pen, directs a two-unit safety test before each injection, allows that test to be repeated if insulin does not appear, and says to discard an in-use pen after 28 days. At 22 units per day, one 300-unit pen lasts about 13.6 days, so the 28-day in-use limit does not shorten this example. A different dose, concentration, presentation, or device can produce a different result.
For sliding-scale or “as directed” insulin, obtain a usable maximum daily dose under the current payer and prescription requirements; do not invent an average.
DailyMed, Lantus SoloStar (insulin glargine) label, updated June 6, 2023; Instructions for Use revised June 2022; U.S. Department of Justice, insulin-pen settlement announcement, April 11, 2025.
Metered-dose and dry-powder inhalers
labeled metered inhalations or doses ÷ maximum inhalations or doses used per day
- Required inputs
- Exact device and package, labeled counter or blister count, complete sig or clarified maximum, and any labeled discard limit.
- Caveat
- Do not calculate from gram weight. A priming counter may start above the labeled count so the labeled usable count remains intact.
- Example
- Ventolin HFA has 60- and 200-inhalation presentations. A 200-inhalation device on a documented two inhalations twice daily uses four per day: 200 ÷ 4 = 50 days.
The Ventolin HFA label directs four priming sprays but says the counter starts at 204 or 64 and reads 200 or 60 after initial priming. That makes the labeled 200 or 60 the usable calculation input. The same product name can therefore yield a different days supply when the package count differs.
No universal “rescue inhaler equals 30 days” rule follows from the label. If PRN directions do not establish a maximum the payer can use, clarify them rather than inventing one.
DailyMed, Ventolin HFA (albuterol sulfate) label and Instructions for Use.
Topical creams and ointments
quantity dispensed in grams ÷ documented maximum grams used per day
- Required inputs
- Treatment area, frequency, grams per application or another documented area method, and the dispensed grams.
- Caveat
- CMS's 2015 “Rule of Hand” says one gram usually covers four handprints. It is a published estimation method, not a universal product or payer rule.
- Example
- A documented maximum of 1.5 grams per application twice daily uses 3 grams per day. A 30-gram tube therefore calculates to 10 days.
CMS instructs pharmacies to consider both frequency and treatment area and to document calculations and prescriber clarification. No single 30-day default or grams-per-body-area table applies across payers; document the estimation method used.
CMS, Pharmacy Auditing and Dispensing Job Aid: Billing Other Dosage Forms, December 2015.
Transdermal patches
(patch count ÷ patches used per application) × labeled or prescribed wear interval
- Required inputs
- Patch count, number applied at the same time, change interval, and any labeled or prescribed off interval.
- Caveat
- “Patch count × days worn” is correct only when one patch is used per application. Multiple concurrent patches consume the package faster.
- Example
- A Butrans carton contains four systems and its current label describes continuous delivery for seven days: (4 ÷ 1) × 7 = 28 days.
As a pure arithmetic check, eight patches used two at a time and changed every three days cover four application cycles, or 12 days. Whether that regimen is clinically and contractually valid must come from the prescription, exact label, package, and payer, not from this formula.
DailyMed, Butrans (buprenorphine) transdermal system label, updated May 1, 2026.
Dose packs, kits, tapers, and intermittent dosing
days supply = elapsed calendar time covered by the complete labeled or prescribed schedule
- Required inputs
- Every step of the taper or kit, start and stop days, drug-free intervals, and the package configuration actually dispensed.
- Caveat
- Tablet count divided by an average daily dose can be wrong when the dose changes or when administration is weekly.
- Example
- A taper of 4 tablets daily for 3 days, then 3 for 3 days, then 2 for 3 days, then 1 for 3 days uses 30 tablets across 12 elapsed days.
CMS's older job aids instruct pharmacies to count the elapsed time for inconsistent dosing intervals and to account for drug-free intervals and initiation-versus-maintenance schedules. Confirm the exact current product label before using a brand-specific package schedule.
CMS, Pharmacy Auditing and Dispensing Job Aid: Billing Oral and Inhaled Products, December 2015; CMS, Billing Kits, December 2015.
Keep a reproducible calculation record
For non-tablet calculations, retain the exact product and package, labeled count or concentration, complete sig or clarified maximum, conversion or device source, discard or package limit, arithmetic, unrounded result, and submitted rounding convention. Because labels and payer requirements change, record the source used instead of relying on a memorized conversion.
The submitted value helps establish the refill interval and can contribute to refill-too-soon or plan-limit edits; the current payer response controls the applicable threshold. If a payer instructs a capped submitted value for a package, retain the complete calculated interval and use it for refill timing. The Department of Justice's 2025 insulin-pen settlement describes the risk of submitting a capped value without preserving and applying the actual interval. See the claim-reject reference for the related reject workflow.
Sources11 linked sources
- NCPDP, Telecommunication Standard Version D and Above Questions, Answers and Editorial Updates, v73, June 2026
- NCPDP, Payer Sheet Template Implementation Guide for version D.0, v18, September 2024
- CMS, Pharmacy Auditing and Dispensing Job Aid: Billing Oral and Inhaled Products, December 2015
- CMS, Pharmacy Auditing and Dispensing Job Aid: Billing Other Dosage Forms, December 2015
- CMS, Pharmacy Auditing and Dispensing Job Aid: Billing Kits, December 2015
- DailyMed, Lantus SoloStar (insulin glargine) label and Instructions for Use, revised June 2022
- DailyMed, Ventolin HFA (albuterol sulfate) label and Instructions for Use
- DailyMed, fluticasone propionate nasal spray label, current version published March 9, 2026
- DailyMed, ofloxacin otic solution label, updated June 11, 2026
- DailyMed, Butrans (buprenorphine) transdermal system label, updated May 1, 2026
- U.S. Department of Justice, settlement concerning insulin-pen days-supply practices, April 11, 2025