6 Days Supply Calculations

The basic calculation

Days supply is the number of days a dispensed quantity lasts when used according to the prescribed directions. Calculate it by determining the amount used each day and dividing the dispensed quantity by that daily amount:

The dispensed quantity and daily use must be expressed in the same units. First determine the daily use, then divide. For directions that allow variable or as-needed use, use the maximum permitted daily use when the calculation calls for maximum use.

Tablets and capsules

For tablets and capsules, multiply the number of units taken per dose by the number of doses taken per day. Divide the quantity dispensed by that daily total.

For example, a prescription for $1\frac{1}{2}$ tablets twice daily, with $90$ tablets dispensed, has daily use of $1\frac{1}{2} \times 2 = 3$ tablets per day. The days supply is $90 \div 3 = 30$ days.

For as-needed directions, calculate using the maximum frequency permitted. Directions for $1$ tablet every $4$ to $6$ hours as needed permit up to $6$ tablets per day. With $42$ tablets dispensed, the supply is $42 \div 6 = 7$ days.

Liquid medications

For liquids, divide the total volume dispensed by the volume used per day. If a dose is stated in milligrams while the product concentration is stated in milligrams per milliliter, first convert the dose to milliliters.

For example, a product has a concentration of $250$ mg per $5$ mL, the patient takes $375$ mg twice daily, and $150$ mL is dispensed. The volume per dose is $375\text{ mg} \times \left(\frac{5\text{ mL}}{250\text{ mg}}\right) = 7.5\text{ mL}$. Daily use is $7.5\text{ mL} \times 2 = 15\text{ mL}$ per day, so the days supply is $150\text{ mL} \div 15\text{ mL} = 10$ days.

For a reconstituted medicine, calculate using the final volume after reconstitution. A shorter beyond-use or stability period may limit how long that volume can be used.

Inhalers

For inhalers, calculate from the labeled number of usable actuations in the specific inhaler. Do not use the inhaler’s weight or the number of containers as the quantity. Divide total actuations by the number of puffs used per day.

For example, an inhaler with $200$ actuations prescribed as $2$ puffs every $6$ hours as needed permits a maximum of $4$ doses per day. Maximum daily use is $4 \times 2 = 8$ puffs, so the supply is $200 \div 8 = 25$ days.

Check the product labeling for the available actuations and device-specific instructions, including when to discard the inhaler. Check priming requirements and the dose counter; do not assume all inhalers have the same number of actuations or priming instructions.

Topical products

For creams and ointments, divide the dispensed grams by the grams expected to be used each day. An exact calculation requires a known amount per application and the application frequency.

For example, if a $30$ g tube is prescribed at $1$ g per application twice daily, daily use is $1\text{ g} \times 2 = 2\text{ g}$ per day. The days supply is $30\text{ g} \div 2\text{ g} = 15$ days.

If directions do not specify an amount, days supply cannot be determined exactly from the tube size alone. The amount used depends on the treated area and application instructions. A rough “rule of hand” estimate is that $1$ g may cover an area of about four handprints. Clarify complex or extensive topical use with the prescriber.

Checks and changing directions

Before finalizing a calculation, check that the quantities being divided use the same units—for example, milliliters with milliliters or puffs with puffs. For variable or as-needed directions, use the maximum permitted use when calculating maximum daily use.

For a changing schedule, calculate how long the quantity lasts across each step rather than treating the entire course as one fixed daily dose. Keep fractional results during the calculation. Follow applicable pharmacy and payer rules for reporting days supply; do not automatically round every result up.

If directions, package instructions, or an amount-per-application estimate are unclear, seek clarification rather than guessing.