There are multiple configurations for this scenario. Is this a one-time event, e.g, you're ordering for the holidays or is this something where you have constant demand and you just need to establish an efficient Re-Order point that takes into consideration service level agreements, lost opportunities, ordering costs, and liquidation costs?

Here is the basic formula for Excel: =SQRT((2*DEMAND*ORDERING_COSTS)/(CARRYING_COSTS))

Where Demand= Number you expect to sell
Ordering Cost = The cost of placing the order (this is NOT the unit cost)
Carrying Cost = The cost of carrying inventory-this includes things like shrinkage, warehousing, etc. This should be calculated as a percentage of the unit cost. For example, if it costs me $10 for a widget and 10% to maintain it, the inventory carrying cost is = (10 *.10) = $1.

For a more detailed explanation, please see this website: Economic Order Quantity- EOQ

Also, see this Spreadsheet with some examples of One-Time event ordering. One Time Event Ordering.

I'll post some additional examples as soon as I get more time.

(Disclaimer- this is material adapted from an Operations Class at the University of Utah and not my original material)