What beginning inventory means
Beginning inventory is the dollar value of all inventory a business holds at the start of an accounting period, whether that period is a month, a quarter, or a fiscal year. It is not a separate count from your ending inventory; it is the same number carried forward, since the value on hand when one period closes is by definition the value on hand when the next one opens.
The reason calculating beginning inventory comes up as its own question is that businesses do not always have a clean, dated physical count sitting at the exact start of a period. When that number is missing or needs to be reconstructed, the formula below rebuilds it from the accounting records instead of requiring a new count.
The beginning inventory formula
The formula is: Beginning Inventory = Cost of Goods Sold (COGS) + Ending Inventory minus Purchases, all measured over the same period. It is a rearrangement of the standard COGS formula (COGS = Beginning Inventory + Purchases minus Ending Inventory), solved for beginning inventory instead.
All three inputs need to come from the same accounting period and use the same costing method (FIFO, LIFO, or weighted average), since mixing methods between the inputs will produce a beginning inventory figure that does not reconcile with either period.
- Beginning Inventory = COGS + Ending Inventory − Purchases
- COGS: total cost of inventory sold during the period, from the income statement
- Ending Inventory: value of inventory on hand at period close, from a physical count or perpetual records
- Purchases: total cost of inventory bought or produced during the period
Worked example
A retailer closes out March with an ending inventory of $18,000. During March, cost of goods sold was $22,000, and the retailer purchased $15,000 worth of new stock. Plugging into the formula: Beginning Inventory = $22,000 (COGS) + $18,000 (Ending Inventory) − $15,000 (Purchases) = $25,000.
That $25,000 should match the ending inventory value recorded at the close of February. If it does not, the discrepancy points to an error in one of the three inputs, most often a miscounted physical inventory or a purchase recorded in the wrong period.
Common mistakes when calculating beginning inventory
Most errors come from mismatched periods or mismatched costing methods rather than the arithmetic itself, since the formula has only three inputs and one subtraction.
- Using purchases or COGS from a different date range than the ending inventory figure
- Including sales returns or shipping costs inconsistently between periods
- Switching costing method (FIFO to weighted average, for example) between periods without adjusting for the change
- Treating the result as a unit count rather than a dollar value; the formula only works in cost terms
- Not reconciling the calculated figure against the prior period actual ending inventory as a sanity check
Beginning inventory vs. average inventory
Beginning inventory is a single point-in-time value: what you held at the start of the period. Average inventory takes beginning and ending inventory together, usually (Beginning Inventory + Ending Inventory) ÷ 2, to smooth out the swing between the two and better represent typical stock levels across the period.
Beginning inventory feeds directly into the COGS formula; average inventory feeds into ratios like inventory turnover and days sales of inventory, which are more useful for judging how efficiently stock is moving than a single snapshot value would be.
Keep beginning and ending inventory values accurate automatically
StockFlow tracks inventory value in real time by costing method, so period-end figures are already there when you need them, without reconstructing them by hand.
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