To calculate purchases, use this formula: (Ending Inventory − Beginning Inventory) + Cost of Goods Sold = Purchases. This is the standard accounting method for figuring out how much inventory a business bought during a given period when that number isn’t directly available in the records. The formula works by reverse-engineering what must have been purchased based on what was on hand, what was sold, and what remained.
The Core Purchases Formula
The calculation has three inputs you’ll need before you start:
- Beginning inventory: the dollar value of inventory on hand at the start of the period
- Ending inventory: the dollar value of inventory on hand at the end of the period
- Cost of goods sold (COGS): the total cost of inventory that was actually sold to customers during the period
With those three numbers, the steps are straightforward. First, subtract beginning inventory from ending inventory. This tells you whether inventory levels went up or down over the period. Then add cost of goods sold to that result. The final number represents your total inventory purchases for the period.
Here’s a quick example. Say your beginning inventory was $50,000, your ending inventory was $65,000, and your cost of goods sold was $200,000. The math looks like this: ($65,000 − $50,000) + $200,000 = $215,000 in purchases. That makes intuitive sense: you sold $200,000 worth of goods and your shelves grew by $15,000, so you must have bought $215,000 worth of inventory to make both of those things possible.
Why This Formula Works
Think of it as a bathtub. Beginning inventory is the water already in the tub. Purchases are the water flowing in. Cost of goods sold is the water draining out. Ending inventory is what’s left. If you know three of those four numbers, you can always solve for the missing one. In most accounting systems, COGS, beginning inventory, and ending inventory are recorded directly, while purchases sometimes need to be derived.
This formula gives you net purchases, meaning the total cost of goods brought into inventory. If you need gross purchases (before returns or allowances), you’d add back any purchase returns, purchase discounts, or allowances that were netted out during the period.
Calculating Cash Actually Paid to Suppliers
The purchases formula tells you how much inventory was acquired, but not necessarily how much cash left the bank. If your business buys inventory on credit, there’s a second step to figure out what you actually paid suppliers during the period. This is especially important when preparing a cash flow statement using the direct method.
Start with the purchases figure you just calculated. Add your beginning accounts payable balance (the amount you owed suppliers at the start of the period). This gives you the total amount you were on the hook to pay. Then subtract your ending accounts payable balance (what you still owe suppliers at the end of the period). The result is the cash you actually paid for inventory during the period.
Using the earlier example: if you purchased $215,000 in inventory, started the period owing suppliers $30,000, and ended the period owing them $25,000, you paid $215,000 + $30,000 − $25,000 = $220,000 in cash. You paid off some old bills on top of covering most of your new purchases, which is why the cash number is higher than the purchases number.
When to Use Gross vs. Net Purchases
In financial statements and internal reporting, purchases can appear in slightly different forms depending on context. Gross purchases represent the full invoice price of everything ordered. Net purchases subtract out any returns to suppliers, trade discounts taken at the time of purchase, and allowances received for damaged or incorrect goods. The inventory purchases formula described above typically yields net purchases, since COGS and inventory balances already reflect those adjustments.
If you’re reconciling supplier invoices or building a detailed cost of goods sold schedule, you may want to track gross purchases and the deductions separately. The breakdown looks like this: Gross Purchases − Purchase Returns − Purchase Discounts − Purchase Allowances = Net Purchases.
Calculating Total Cost of Purchased Goods
For businesses that import products or pay significant shipping costs, the invoice price alone doesn’t capture the real cost of a purchase. The total landed cost includes every expense required to get goods from the supplier to your warehouse. The main components beyond the base purchase price are:
- Freight charges: shipping costs by sea, air, rail, or truck
- Customs duties and taxes: tariffs or levies imposed when goods cross international borders
- Insurance premiums: coverage for damage or loss during transit
- Handling and documentation: packing, compliance paperwork, certificates of origin, and customs clearance fees
- Currency exchange costs: the impact of fluctuating exchange rates between when you order and when you pay
Adding these to the base purchase price gives you the true per-unit cost of inventory, which leads to more accurate pricing decisions and profit margin calculations. Many businesses that only look at the supplier invoice underestimate their real cost by 20% or more once freight, duties, and handling are factored in.
A Practical Example Tying It All Together
Suppose you run a retail business and want to understand your purchasing activity for the quarter. Your records show beginning inventory of $80,000, ending inventory of $95,000, and cost of goods sold of $310,000. Plugging into the formula: ($95,000 − $80,000) + $310,000 = $325,000 in net purchases.
Your accounts payable started the quarter at $40,000 and ended at $55,000. That means you paid $325,000 + $40,000 − $55,000 = $310,000 in cash to suppliers. The fact that your ending payables grew by $15,000 tells you that you bought more on credit than you paid off, conserving cash during the quarter.
If $25,000 of that $325,000 was imported goods with an additional $8,000 in freight and $3,000 in duties, the landed cost of those imported items was $36,000, not $25,000. Your effective cost per unit on those goods is 44% higher than the supplier invoice suggests, which matters when you set retail prices.