Days of supply is a term used to quantify the number of days a given quantity will last under certain conditions. It can be applied to manufacturers (calculates the time between acquisition of materials and sale of finished products) and retailers (how long inventory will last without replenishing under predicted demand), among others. The calculation can also be applied to medication to determine the length of time medication will last with recommended usage.

### Ascertain the frequency of use of a medicine per day

Calculate the number of doses being consumed on any given day. For instance, if the medicine needs to be taken every 12 hours, the number is 2.

### Assess the inventory on hand

Determine the number of available doses. For a new bottle that contains 100 pills, the number would be 100.

### Calculate days supply

Divide the inventory figure by the number of doses used daily. In this case, the result would be 100/2 = 50 days.