Periodic Review System Calculator
Periodic Inventory System Formulas
The formulae for a periodic review system are as follows:
RP = EOQ / μ
TIL = SS + μ(RP + LT)
SS = zrσ√(RP + LT)
RP is your periodic review period, EOQ is your economic order quantity and μ is your average demand for the item
TIL is your target inventory level, which is established by adding your SS (safety stock) to your average daily demand multiplied by your RP and your LT (lead time)
In this formula, your SS (safety stock) is a bit more tricky because you have an established service level, which is determined by setting a z-score value (see below), you multiply your service level by the standard deviation for your daily demand. This figure is then multiplied by the square root of your RP and LT value.
z = 1.645 when service level (confidence level) is established at 90%
z = 1.96 when service level is established at 95%
z = 2.58 when service level is established at 99%
What is a Period Inventory System?
A periodic inventory system is generally applied when there are several items that need to be ordered at the same time, in an effort to consoliate purchasing efforts.