Your inventory costing method decides whether your profit calculation comes out right or wrong whenever your purchase price changes: FIFO deducts the cost of the earliest stock first, while weighted average uses the average price across all purchases. On the very same 100-unit stock movement, these two methods can produce a profit difference of several hundred dollars. Picking the wrong method can make a period you think was profitable look, in reality, like you sold at a loss.
Why Isn't Inventory Costing Just Your Accountant's Job?
Many business owners see the costing method as a technical detail that only concerns the accountant or the finance department; in reality, this choice directly affects pricing, promotion decisions, and which product gets shelf priority. A product costed incorrectly shows a profit margin that drifts from reality, which means pricing decisions (whether to discount a product, whether to run a sale) end up based on bad data. That's why understanding your costing method should be part of an owner's day-to-day operating decisions, not something left for year-end tax prep.
How Does FIFO Work?
FIFO (First In, First Out) assumes the stock that entered the warehouse first is the stock that gets sold first, and costs it accordingly. For products with expiry dates (food, cosmetics), the physical flow already works this way, which is exactly why FIFO makes sense both for accounting and for operations.
Here's a concrete example: a grocery store first buys 100 units at $4.00 (total $400), then prices rise and it buys another 100 units at $5.50 (total $550). Say 120 units get sold in this period. Under FIFO, the first 100-unit batch at $4.00 is deducted first, and the remaining 20 units come from the $5.50 batch: total cost = (100 x $4.00) + (20 x $5.50) = $400 + $110 = $510. If those 120 units sold at $7.00 each, revenue is $840, and FIFO profit comes out to $840 - $510 = $330.
How Is Weighted Average Cost Calculated?
The weighted average method divides the total cost of all purchases by the total quantity to produce a single unit cost, and applies that average to every unit sold. In the example above, total cost is $400 + $550 = $950, total quantity is 200; average unit cost is $950 / 200 = $4.75. Selling 120 units puts the cost at 120 x $4.75 = $570; profit comes to $840 - $570 = $270.
The same sale, at the same purchase prices, produces a $60 profit gap between the two methods — $330 versus $270. When prices are trending up, FIFO shows a higher profit because it uses the older (cheaper) cost; weighted average gives a more conservative, smoothed-out picture. This gap may look small on a single product, but across a business with hundreds of SKUs it can meaningfully shift the total profit figure at month end.
Why Does Profit Calculation Go Wrong When Purchase Prices Change?
The most common mistake is calculating profit by comparing your selling price only against the most recent purchase price. A shop owner might see the new purchase price jump to $5.50 while stock still sitting in the warehouse was bought at $4.00, raise the selling price to $6.50, and read that as a thin margin — when in fact most of the stock on hand is still costed at the older, cheaper price. The reverse happens too: when prices are falling, profit gets calculated against the current low price while old, higher-cost stock is still being sold, and real profit ends up lower than it looks.
This mistake is especially common in businesses selling imported goods tied to exchange rates (electronics, cosmetics, spare parts). When the exchange rate swings week to week, a business that doesn't track which batch was bought at which rate can look profitable on paper while actually sliding toward a cash-flow loss.
Which Method Fits Which Business?
The choice should be based on the nature of the product and how often its price fluctuates.
- Products with expiry dates (food, cosmetics, medicine): FIFO is preferred since it matches both the physical rotation and the accounting.
- Products with frequent, irregular price swings (currency-linked electronics, building materials): weighted average prevents sudden jumps from distorting the profit picture.
- Homogeneous, indistinguishable products (bulk food, fuel, hardware items): weighted average is a practical, workable solution.
- Products tracked by batch or serial number (electronic devices, spare parts): FIFO makes it clear which batch sold when.
Take a textile wholesaler: seasonal collections are usually bought once and sold through within that same season, so the gap between FIFO and weighted average is small; the real difference shows up in staple items that get restocked continuously year-round at prices that shift with the supplier (yarn, fabric by the meter). A hardware store might prefer weighted average for commodity-priced items like cement or steel, while choosing FIFO for certain branded power tools tracked by serial number with a warranty — meaning the same business can reasonably use different methods for different product categories.
How Should Costing Be Handled for Seasonal and Promotional Sales?
End-of-season sales and bulk promotions are another situation that puts your costing method to the test. A clothing store selling an item at a 40% discount at season's end might sell it below its purchase cost; knowing which batch (an early-season cheap buy, or a late-season expensive one) actually got sold is critical to seeing how big the real loss is. A store using FIFO, which deducts the cheap early-season batch first, might still show a slight profit even on a discounted sale, while a store using weighted average sees a more realistic — but lower — profit or loss figure from the blended cost.
Bulk promotions (buy 3, pay for 2, for example) make costing even more complex, since the effective selling price per unit drops. When planning this kind of promotion, knowing the real per-product cost ahead of time makes it clear whether the promotion is actually making or losing money. For businesses that want to base promotion decisions on data rather than guesswork, this is one of the most concrete benefits of costing discipline.
How Does Costing Change When Prices Are Falling?
The examples so far covered rising prices; the picture reverses in a period of falling prices. Say an electronics dealer first buys 100 units at $20.00 (total $2,000), then prices drop and it buys another 100 units at $16.00 (total $1,600). Selling 120 units, FIFO first deducts the 100-unit batch at $20.00, then 20 units from the $16.00 batch: cost = (100 x $20.00) + (20 x $16.00) = $2,000 + $320 = $2,320. Under weighted average, unit cost is ($2,000 + $1,600) / 200 = $18.00; cost for 120 units is 120 x $18.00 = $2,160.
In this example FIFO shows a higher cost and therefore a lower profit, while weighted average shows a lower cost and a higher profit — the exact reverse of the rising-price scenario. That's why it's wrong to treat either method as universally 'better'; the right move is to look at your product's price trend (rising, falling, or volatile) and see which method gives you a more stable picture.
How Does a Stock Count Affect Costing Accuracy?
No matter how correctly you choose your costing method, the calculation still comes out wrong if the underlying data — the actual physical stock count — is wrong. In a business that doesn't count stock regularly, a product showing 80 units in the system might actually have only 65 units on the shelf; this 15-unit gap (shrinkage, theft, or a recording error) goes unnoticed until a count is done, and it throws off your costing figures too. Count frequency should be set based on a product's value and how fast it moves; high-value, fast-moving products may need a monthly count, while low-value, slow-moving ones can get by with a count every quarter. We covered the steps of a proper count in our article on how to do a stock count.
Can You Change Your Costing Method Mid-Year?
It's technically possible, but a method change requires care both for accounting consistency and for tax purposes. If a business decides to switch to weighted average after, say, a long period of continuous price increases, it needs to document the reason for the change (a shift in product mix, a change in the supply chain) and apply it together with its accountant. Frequent, unjustified method changes both break the year-over-year comparability of financial statements and can raise extra questions in a tax audit. That's why choosing the right method once, at the outset, based on the nature of the product group, is far less costly than changing it later.
How Does Your Costing Method Affect Tax and Balance Sheet Value?
The costing method doesn't just change your profit figure — it also changes how your remaining inventory is valued on the balance sheet at period end. In a period of rising prices, FIFO values leftover stock at its current, higher cost, showing a higher inventory value on the balance sheet; weighted average values stock at the average cost, giving a more conservative figure. This gap matters in particular to anyone reading the balance sheet before a loan application or an investor conversation; a high inventory value can make a business look stronger on paper, even though it's an increase in value with no cash behind it.
On the tax side, the method choice affects reported profit too: in a period of rising prices, FIFO shows a higher profit and therefore a higher taxable base, while weighted average produces a lower, steadier base. Any method change should be made together with your accountant, respecting the principle of consistency (that a method, once chosen, shouldn't be changed arbitrarily and often); frequent changes both undermine accounting consistency and can raise flags in a tax audit.
How Do You Avoid Costing Mistakes?
The most practical fix is to hand the costing method over to inventory software rather than tracking it by hand. Keeping every purchase batch on a separate line in a hand-kept spreadsheet leads to both wasted time and calculation errors within a few months; our article on the limits of tracking inventory in Excel details the typical failure points.
The most common mistake is applying different methods to different product groups without documenting it consistently: if one employee costs a product with FIFO and another costs the same product with weighted average, the month-end reports won't match. The second mistake is counting the physical quantity correctly but forgetting which cost applies to it — counting 50 units isn't enough; you also need to know which purchase batch those 50 units came from. The third mistake is converting a foreign-currency purchase invoice using a delayed or average exchange rate instead of the actual daily rate, which throws the real cost off.
In a software-based stock system, every purchase batch is entered at its own cost, and the system automatically calculates the right figure at the point of sale based on the method you choose (FIFO or weighted average) — the owner never has to do the math by hand. Welda Stock records each purchase batch separately and automatically deducts the correct cost on every sale, so your month-end profit-and-loss report is based on real cost. You can find out how to read per-product profitability correctly in our profit-loss analysis guide, and see how stock cost integrates with the right POS system in our POS system guide. If you'd like help settling on the right costing method for your business, get in touch with us.