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COGS formula: understand what the formula measures
The basic COGS formula is beginning inventory plus purchases and relevant additional costs, minus ending inventory. It estimates the cost of inventory sold during a period, not the revenue those sales produced. Cost of goods sold is useful for understanding gross profit, but the calculation depends on correct inventory records and cost classification. Consult IRS Publication 334 and an accountant for the rules that apply to your business.
Work through a labeled example
Suppose an illustrative retailer starts the month with $2,000 of inventory, purchases $5,000 more and ends with $1,500. Ignoring other adjustments in this simplified example, COGS equals $2,000 + $5,000 – $1,500 = $5,500. The unsold $1,500 remains ending inventory rather than becoming part of the cost sold. These figures demonstrate arithmetic; they are not a real retailer’s financial results.
Keep revenue separate from cost
If that same example earns $9,000 in sales, gross profit is $9,000 minus $5,500, or $3,500. Operating expenses and other items still need to be considered before determining later profit subtotals. Do not call gross profit take-home income. A strong sales month may coincide with substantial inventory purchasing, and the cash impact can differ from the statement’s cost of goods sold.
Classify the inputs carefully
Purchases are not the only possible cost component. Freight, materials, labor and other costs may require consideration depending on the business and applicable method. Returns, discounts and inventory changes also need appropriate treatment. Keep ordinary overhead distinct where required. Ask your accountant to document which accounts feed the calculation so the process is repeatable rather than an improvised total assembled at year-end.
Check the inventory evidence
An inaccurate ending count changes the result. Record purchases, goods received, returns and items removed for non-sale purposes. Investigate missing or damaged stock and follow the appropriate valuation rules. Physical counts and reconciliations should match the reporting period. Do not simply use a supplier invoice total as proof that all purchased goods were sold. The formula is only as reliable as the inventory information behind it.
Use the calculation responsibly
Review COGS alongside sales, gross margin, stock levels and cash needs. A change can reflect product mix, input costs or counting errors—not necessarily improved efficiency. Keep the accounting method consistent and discuss exceptions for eligible small businesses with a professional. Build the first calculation from documented records, confirm it with your accountant, then preserve the process so future comparisons use the same definitions.
Common questions
Does every purchase become COGS immediately? Not in the basic inventory example. Goods still held at the end of the period remain in ending inventory, subject to the applicable accounting rules. Keep purchase records and stock information aligned instead of assuming supplier payments equal the cost of goods sold.
Is this formula a complete tax calculation? No. The example explains a common inventory relationship, not every adjustment or permitted method. Tax rules, cost classification and small-business exceptions can affect the calculation. Use current IRS guidance and professional review before applying it to a filing.
Sources and further reading
Related reading
An Income Statement Example You Can Read Line by Line
Cash vs. Accrual Accounting: Timing Matters for Small Businesses