Hi giovanny!

I think you need to restate your problem as it is vague.

It is not clear what information you want and what you have. Now you ask to calculate actual inventory levels with the information of sales per month and inventory levels... Now it seems as if you already have the inventory levels...??

Dear Giovanny,

The variation of inventory level depends much on the type of demands or sale. If they are deterministic, then the properties of reorders , that is, quantity, time and  lead time have to be considered. If all of these are fixed, then one can compute the inventory level at the start and end of each period exactly. In the case of stochastic demand and reorder lead time , you have to know their distributions (or means and variances for computing the average level). Some very good references for more information are:

1-Operations Research in Production Planning, Scheduling, and Inventory Control

Johnson, Lynwood A.; Montgomery, Douglas C.

Published by Wiley, 1974.

2-Analysis Of Inventory Systems: Prentice Hall International Series In Management And Quantitative Methods Series Hardcover – March 17, 2012

by George F. Hadley , Thomson M. Whitin.

3-PROBABILISTIC (Q,r) INVENTORY MODEL WITH PARTIALLY BACKORDERS

WHEN LEAD TIME DEMAND NORMALLY DISTRIBUTED UNDER HOLDING

COST RESTRICTION

Mona, F. El-Wakeel , and Hala A. Fergany,

Available online at  http://www.jgrma.info .

How about interpolation (and extrapolation for the future)?  Curve fitting may also work fine, but be careful, perfect fit polynomials have a way of dipping or rising steeply for a small distance,  Least squares is probably best to fit polynomials approximately to data points.

You had to use statistics data if you could, for instance to know if sales per month follow a normal Laplace-Gauss law (with an everage and a standard-deviation)... Have you these data (the best would be for 24 months [2014-2016])? I can help you if you want...

In all cases you will need to decide at a item level the amount of inventory you wish to hold, this is expressed as time or quantity. You also need to decide how much safety you want to protect against supply and demand variation. If the formulas you use fro either have a demand component then the inventory level now and in the future will vary as the demand varies. There are a wide range of on how these decisions are made. I explore some to the techniques in my book Inventory Management - by Kogan Page. There are also may other authors who cover this subject - Vollman and Whybark, Silver, Chambers etc...

If you have a more specific question by all means contact me directly.

Dr. Geoff