Ending Inventory Calculator

The arithmetic

Ending inventory is beginning inventory plus purchases minus cost of goods sold. Rearranged, it is the figure that closes the period and opens the next one, and the same identity gives COGS when ending inventory is counted rather than derived.

The two directions matter. Deriving ending inventory from a COGS estimate produces a plug figure that absorbs every error; counting it physically and deriving COGS instead produces a COGS figure that includes shrinkage, which is usually the more honest arrangement.

How that looks in practice

Beginning inventory $3,200, purchases $37,800 over the year, and a physical count at year end valuing stock at $3,132.

COGS = 3,200 + 37,800 − 3,132 = $37,868. Against expected COGS of $37,440 from 2,080 units at $18, there is a $428 gap, about 24 units of shrinkage, or 1.1% of throughput.

Had ending inventory been derived from expected COGS instead, it would have been recorded at $3,560 and the shrinkage would have been invisible, sitting on the balance sheet as stock that does not exist.

Where this breaks down

The identity is arithmetic and always balances, which means any error simply moves into whichever term was calculated rather than measured. A business that derives ending inventory rather than counting it will never detect shrinkage, because the formula has nowhere to put it.

Goods in transit are the other common error. Stock paid for and shipped but not yet received belongs in inventory once title has passed, and whether it has depends on the incoterm, FOB origin means it is yours from the port, FOB destination means it is not.

What this changes

Count the stock and derive COGS, rather than the reverse. The difference between the two approaches is the entire visibility of shrinkage, damage and miscounting.

Then reconcile the counted figure against the system figure before adjusting. A large variance is more often a receiving or transaction error than genuine loss, and investigating it usually finds a correctable process rather than a thief.

Timing the count

A full physical count requires stopping movement, which is why it usually happens at the quietest point of the year. That timing is convenient and produces a figure least representative of normal operations, which matters if the number is used for anything other than the accounts.

Cycle counting, counting a rotating subset continuously, weighted so A items are counted most often, produces better accuracy with no shutdown and gives a running measure of how reliable the records are. Most businesses past a certain size move to it and keep an annual full count only where an auditor requires one.

Whichever method is used, the count has to be reconciled to a cut-off. Sales, receipts and transfers processed during the count are the largest single source of spurious variances, and freezing transactions for the counting window removes most of the investigation that follows.

It is worth reconciling the ending figure against the general ledger inventory account rather than only against the stock system. The two are maintained by different processes and they drift, usually because an adjustment was made in one and not the other.

Catching that at each period end keeps the gap to one period's worth. Catching it at a year end means investigating twelve months of movements to find where the two records parted company.