What Is a Kardex and How Do You Automate It?

It is the third week of January and the firm that audits your company's financial statements asks for the kardex, the item-by-item inventory ledger, of the twenty highest-value SKUs. The warehouse hands over one spreadsheet per product, fed by hand, and on the first SKU the balance matches neither the December physical count nor the trial balance. On the second, a negative balance shows up in October, because an issue was entered before the receipt that made it possible, and those units were valued at the wrong average cost. By Friday the auditor has widened the sample, the year-end close has stalled and the cost of sales your company expected to deduct has to be rebuilt movement by movement.

The kardex is the chronological record of every receipt and issue of a product, with its quantity, its cost and the resulting balance. Automating it means every movement updates it instantly, valued and reconciled with accounting.

The record behind every peso of your cost of sales

The kardex is the complete history of a product inside your company. Every time a unit enters or leaves a warehouse, it adds a line with the date, the source document, the quantity, the cost and the resulting balance. Its name comes from the stock control cards that used to be filled out by hand, one per item. In Mexico the term is still used for the inventory subsidiary ledger, the equivalent of a perpetual inventory record, whether it lives on paper, in a spreadsheet or in an ERP.

Its importance reaches well beyond the warehouse. The valued balance of the kardex is the inventory that appears on your balance sheet, and the sum of the valued issues is the cost of goods sold. When the kardex fails, three things fail at once: the margin management sees, the figure the auditor reviews and the deduction your company files with the SAT, Mexico's tax authority.

Receipts, issues and balance: the anatomy of a line

A well-kept kardex answers five questions for every movement: when it happened, which document supports it, how many units came in or went out, at what cost and how many remained. Receipts come from purchases, customer returns, incoming transfers and finished production; issues come from picked orders, returns to suppliers, outgoing transfers, consumption and shrinkage. Adjustments from physical counts are movements too, and they must be recorded with their reason and their authorization. The balance is kept in units and in value, and both columns have to reconcile at all times.

The valuation method decides what each issue costs

Units of the same product are bought at different prices over the year, so every issue needs a rule to assign it a cost. The Ley del ISR, Mexico's income tax law, allows several inventory valuation methods, among them first in, first out (FIFO, known in Mexico as PEPS), specific identification and average cost, and it requires the chosen method to be applied consistently. In distribution and manufacturing, average cost is one of the most widely used because it smooths out swings in purchase prices.

The calculation is simple and very sensitive to sequence. If you have 100 units valued at 5,000 pesos and receive 50 more at 56 pesos each, the balance becomes 150 units valued at 7,800 pesos and the new average cost is 52 pesos. An issue of 30 units recorded after that receipt is valued at 52; the same issue recorded before it is valued at 50. Multiplied across hundreds of SKUs and thousands of movements, that difference explains a good share of the discrepancies between the kardex, the trial balance and the real margin.

Why a manual kardex stops reconciling when you need it most

With few SKUs and a single shift, a kardex on cards or in a spreadsheet holds up through discipline. The problem arrives with volume, because every movement depends on someone entering it completely, on time and in sequence.

Movements entered late and out of sequence

In real operations, goods are received in the morning and the delivery note is entered the next day, once the clerk finishes reviewing invoices. Meanwhile, the warehouse has already picked orders with those units. The kardex records issues with no stock on hand, shows negative balances and values those issues at an average cost that does not yet include the latest purchase. Every late entry is corrected with another adjustment, and every adjustment is one more line the auditor will want to understand.

Undocumented adjustments and transfers that exist on only one side

Transfers between warehouses are a classic source of discrepancies. The sending branch records the issue and the receiving branch takes its time to record the receipt, or records a different quantity because a box arrived damaged; in the meantime the inventory disappears from the kardex, even while it travels on one of your own trucks. Shrinkage and samples that leave without a document are worse: the difference shows up months later in the physical count, with no date and no one responsible.

Three versions of the same balance

With a manual kardex, your company ends up with three figures for the same product: the warehouse figure, in units; the accounting figure, in value; and the sales figure, taken from a morning report. In earlier articles we covered how that gap produces ghost inventory and why a spreadsheet stops describing the warehouse as volume grows. In the kardex, the effect concentrates at month-end close, when reconciling the valued balance with the inventory account takes several days.

What an automated kardex looks like inside the ERP

Automating the kardex changes where each line comes from. In a well-implemented ERP, the kardex is a query over the movements the system recorded at the moment they happened, using the same documents your company uses to buy, pick, transfer and produce.

Every operational document generates its line

Receiving a purchase order generates the receipt; picking an order, the issue; a transfer, the issue at one warehouse and the in-transit quantity toward the other until receiving confirms it. Production consumption and finished production orders move raw materials and finished goods, and adjustments can only be recorded with a reason from the catalog and authorization from the corresponding user profile. With radio frequency terminals, the line is created when the operator scans the location and the product, and deferred data entry disappears.

Cost is recalculated with every receipt

The system applies the valuation method your company defined and recalculates cost in the order the movements occur. If the invoice arrives with a price different from the purchase order, or if freight and import costs are added, the ERP adjusts the receipt and the cost of subsequent issues. The margin on each sale is calculated with the cost in effect at the moment of picking.

The journal entry comes from the same movement

On an integrated platform, every inventory movement carries quantity and cost, and with those two data points it generates its journal entry. Receiving affects inventory and accounts payable; picking, inventory and cost of sales; authorized shrinkage, its expense account. The valued balance of the kardex and the inventory account in the trial balance come from the same lines, so they match by design, and the electronic accounting records your company submits to the SAT reflect what happened in the warehouse.

Detail by warehouse, lot, serial number and location

An automated kardex can be queried by company, by warehouse, by lot, by serial number and, when there is a WMS, by location. From any line you can reach the document that originated it, the user who recorded it and the exact time, and for products with expiration dates you can rebuild in minutes which lot shipped in which order.

What changes at close, in the audit and in the physical count

The first change shows at month-end close, because accounting and the warehouse query the same movements and neither area waits for figures from the other. The second shows in the audit: the auditor picks a SKU and follows the chain back to the source document of any line. The third shows in cycle counts: every difference is recorded as an adjustment with a reason, and the kardex shows since when the system drifted from the physical stock, so the cause can be investigated instead of simply adjusted away.

Signs your kardex has outgrown manual entry

Your kardex needs to be automated when the close is delayed by inventory reconciliation, when negative balances appear on products that do exist, when transfers take days to show up on the receiving side and when cost of sales is corrected after the close. With two of these signs present, more data-entry discipline is no longer enough.

The kardex on a platform that integrates warehouse and accounting

At Oasys we have spent more than 30 years developing management systems for companies in Mexico, built on one premise: the inventory the warehouse sees, the one sales checks and the one accounting records must be the same data. Our platform integrates ERP, WMS, TMS and Production, and every warehouse movement is recorded with quantity and cost and generates its journal entry automatically.

The warehouse runs on radio frequency, with document traceability and tracking by lot, serial number and expiration date, covering both owned and consignment inventory. Physical, cycle and full inventories are recorded in the same system, and reports can be viewed on screen or exported to Excel for the audit firm. Everything runs on our own servers, in a data center.

Frequently asked questions

Is the kardex mandatory for tax purposes in Mexico?

Mexican tax rules require companies that handle goods to keep an inventory control showing the items, their quantities and the valuation method used, applied consistently. Each company chooses the format, and the kardex is the most widespread way to meet that requirement.

Which is better, FIFO or average cost?

It depends on the product and on how volatile your purchase prices are. FIFO more closely mirrors the physical flow of products with expiration dates and leaves inventory valued at recent costs; average cost smooths out price swings. Choose the method with your accountant and apply it the same way across all warehouses, because changing methods has tax implications.

Can I automate the kardex without a WMS?

Yes. The inventory module of an ERP generates the kardex by product and by warehouse from purchases, sales and transfers. The WMS adds detail by location and real-time capture with radio frequency, which is what eliminates movements recorded late and out of sequence.

If your company's kardex is rebuilt at every close from spreadsheets and delivery notes, the cost of sales you report depends on the memory of whoever enters the data. At Oasys we integrate ERP, WMS and TMS on a single database, on our own servers, so every warehouse movement generates its kardex line and its journal entry at the same instant. Find out how at https://www.oasys.com.mx/en