Kardex and Cost of Sales
Kardex and Cost of Sales
Many small business owners believe a kardex is just a list of what enters and leaves the warehouse. In reality, it is much more: the kardex is the tool that turns physical inventory movement into accounting figures. One of those figures is the cost of sales, one of the numbers with the greatest impact on a company's profit. If the kardex is kept properly, knowing how much the month's sales actually cost takes minutes. If it is not, that figure remains a mystery until year end, when it is too late to fix decisions.
What cost of sales means
Cost of sales is the value of the merchandise the business delivered to customers during a period. It is not what customers paid: that is revenue. It is what that merchandise cost the business to acquire or produce. The difference between revenue and cost of sales is gross profit, which then covers operating expenses, taxes, and any final gain.
A simple example: if a hardware store sells fasteners for $150 but those fasteners cost $90, revenue was $150 and cost of sales was $90, leaving a gross profit of $60. Anyone who ignores cost of sales cannot know whether the business is really earning, because a company can invoice heavily and still lose money if what it sells costs almost as much as it receives.
How the kardex feeds cost of sales
This is where the kardex comes in. A well-organized kardex records every merchandise entry with its quantity and cost, and every exit with its quantity and valuation cost. When the business sells and issues an invoice, merchandise leaves inventory; that exit, valued at cost, is exactly the cost of sales for that transaction. In other words, cost of sales is not a number calculated separately from inventory: it is the sum of all valued exits during the period.
This is easy to see with Kardex Tauro: the software records entries and exits with their costs, and when a purchase is entered it updates the reference's average cost. Exits invoiced to customers are valued automatically in the kardex, so at month end the business can add those exits and obtain cost of sales without redoing calculations or reviewing invoices one by one. The Kardex Tauro record becomes the direct support for the income statement.
The classic cost of sales formula
In accounting, the cost of sales for a period is calculated with a well-known formula:
Cost of sales = Beginning inventory + Purchases during the period − Ending inventory
If the business started the month with $2,000,000 in merchandise, bought $1,200,000, and ended with $746,667 left in the warehouse, cost of sales was $2,453,333. The formula is useful, but it hides a delicate assumption: that ending inventory is correctly valued. And ending inventory is only known with confidence when the kardex was kept up to date throughout the period. Without a reliable kardex, ending inventory is a guess, and an error in that figure distorts cost of sales and the entire period's profit.
Valuation methods and their effect
The cost of exits can be assigned in different ways, and each produces a different cost of sales. The three best-known methods are:
- Weighted average: every purchase recalculates the average unit cost of available merchandise, and exits are valued with that average. It is the most common method in small businesses because it smooths price variations and is simple to apply. Kardex Tauro works this way: when a purchase is recorded, the average cost is updated, and subsequent exits are valued with that updated figure.
- FIFO (first in, first out): assumes the oldest merchandise is sold first. When prices rise, cost of sales reflects older, lower values, so profit appears higher. It requires tracking inventory by lots or cost layers.
- LIFO (last in, first out): assumes the most recent merchandise is sold first. With rising prices, cost of sales reflects recent, higher values, which reduces accounting profit. Many accounting standards do not accept LIFO, but it is worth understanding its effects.
Choosing one method over another does not change how much merchandise left the warehouse: it changes the value assigned to each exit. Two identical businesses can therefore report different costs of sales if they use different valuation methods. The key is to apply the same method consistently, because comparing periods only makes sense when figures are calculated with the same criteria.
Practical example: valued exits in the kardex
Imagine a clothing business starting May with 200 units of a shirt at a unit cost of $10,000. During the month it buys 100 more units at $12,000 each. The average cost becomes $10,666.67 per unit, dividing the total available value of $3,200,000 by the 300 units. The month's exits, as they would appear valued in the kardex, are shown below:
| Date | Exits (units) | Unit cost ($) | Cost of sales ($) | Ending balance ($) |
|---|---|---|---|---|
| May 6 | 50 | 10,666.67 | 533,333 | 2,666,667 |
| May 12 | 40 | 10,666.67 | 426,667 | 2,240,000 |
| May 19 | 80 | 10,666.67 | 853,333 | 1,386,667 |
| May 27 | 60 | 10,666.67 | 640,000 | 746,667 |
The cost of sales column shows how each exit becomes an accounting value. Adding the month's exits: 50 + 40 + 80 + 60 = 230 units, and total cost of sales was $2,453,333, which matches the formula: beginning inventory of $2,000,000 plus purchases of $1,200,000 minus ending inventory of $746,667. This consistency is not a coincidence: the $746,667 ending balance (70 units at $10,666.67) is the balance the kardex itself recorded after the last exit.
Why cost of sales matters for profit
Cost of sales is usually the largest expense of a trading business. A valuation error of a few percentage points in inventory can turn real profit into an accounting loss, or the opposite: showing profits that do not exist and leading the owner to spend money the business never generated. Taxes are also calculated on profit, so understating ending inventory is not a minor oversight: it distorts filings and exposes the business to adjustments.
Keeping the kardex current also helps detect problems before they grow. If a month's valued exits do not match what sales show, there may be merchandise that left without an invoice, losses from damage, or differences between what the system says and what is physically in the warehouse. That comparison is only possible when the kardex records exits with cost, not merely with quantities.
Tips so the kardex supports cost of sales
- Record every entry with its real cost, invoice in hand, with no approximations. A misstated entry cost carries into every later exit.
- Record exits at the moment of the sale. Valued exits left by Kardex Tauro when invoicing avoid month-end backlogs.
- Review the average cost after each purchase to confirm it was updated and that the kardex balance in pesos matches the inventory value.
- Perform periodic physical counts and compare them with the kardex balance; differences should be adjusted and documented.
- Do not mix valuation methods between products or periods without a clear accounting reason.
In short, the kardex and cost of sales are two sides of the same coin. The kardex shows how much merchandise exists and what it cost; cost of sales shows how much of that merchandise was delivered to customers and what value left inventory. With Kardex Tauro, exits are valued from the moment of the invoice, and cost of sales stops being a year-end calculation and becomes a figure available every month. For a small business that wants to know whether it is truly making money, having that number at hand is not a luxury: it is the foundation of every pricing, purchasing, and growth decision.