How to Do a Weighted Average Kardex: Step by Step

If you run a business with inventory —a hardware store, a small grocery, a distribution company, a workshop that keeps spare parts— you know that buying merchandise is the easy part. The hard part begins when you have to answer three questions: how many units do I have, how much did they cost me, and how much is what I sell really worth? Without a reliable answer to the third question, every sale is a lottery: you may be making money… or you may be giving your merchandise away without knowing it. The kardex is the classic tool that answers those questions. It is the chronological record of every inflow and outflow of a product, with its quantities and values. And the weighted average cost method is, for most small businesses, the fairest and simplest way to value that record. In this article we explain what a kardex is, how to do weighted average costing step by step with a complete numerical example, and how a program like Kardex Tauro handles the entire calculation for you.

What a kardex is and why your business needs one

In simple words, the kardex is the life story of each product in your warehouse. You write down every time merchandise comes in (purchase, customer return, surplus) and every time it goes out (sale, shrinkage, internal use), noting the date, the quantity, the unit value and the balance left after the movement. At the end of the period, that record tells you not only how many units remain, but also how much money you have invested in them and how much what you already sold actually cost you. For a small business this is not an administrative luxury: it is the foundation for knowing whether the business makes a profit. If you do not know the real cost of what you sell, you cannot calculate your margin, you do not know how much you can discount in a promotion, and you cannot reconcile your warehouse with your accounting.

What weighted average cost is

There are several methods for valuing inventory. The most widely used in small businesses is the weighted average, and for good reason: it is easy to understand and it smooths out changes in market prices. The central idea is that all units of the same product are worth the same, no matter which batch they came from or what price you paid for them. That single value is the weighted average cost, and it is calculated like this: Weighted average cost = Total value of merchandise on hand ÷ Total units on hand Every time merchandise enters at a different price, the average is recalculated. Every time merchandise goes out, the outflow is valued at the average in effect at that moment. It is called weighted because larger purchases weigh more in the calculation than small ones.

What you need before you start

Before you open your first kardex card, gather these items:

One card per product: a sheet or chart dedicated to each item; never mix two products in the same record, because the balance loses all meaning.
Dates in chronological order: every movement must be recorded on the actual date it happened; a movement out of place unbalances everything that follows.
Quantity and unit value of every inflow: use the total cost you really paid per unit, tax included, so you do not undervalue your inventory.
Be clear about outflows: the quantity leaving is charged at the average cost of the moment, not at the price it was sold for and not at the cost of the last purchase.
A calculator or supporting tool: the recalculation repeats at every inflow; it pays to rely on something that does not make mistakes.

Step-by-step example: weighted average kardex

Let us record a simple product —say 1 kg powder detergent— over a month in a neighborhood store. Three movements occur: an opening purchase, a second purchase at a higher price, and a sale. Notice how the average cost is recalculated at each inflow and how the sale is charged at the average in effect.

Date	Description	Received			Issued			Balance
		Qty	Unit cost	Total	Qty	Unit cost	Total	Qty	Unit cost	Total
Jun 1	Opening purchase	50	$6,000	$300,000	—	—	—	50	$6,000	$300,000
Jun 8	Second purchase (new price)	30	$6,500	$195,000	—	—	—	80	$6,187.50	$495,000
Jun 15	Sale	—	—	—	20	$6,187.50	$123,750	60	$6,187.50	$371,250

Let us look at each line. On the first one, June 1, the opening purchase of 50 units at $6,000 each comes in; since there were no previous units on hand, the average cost is simply $6,000 and the balance is $300,000, which is the real money invested. When the second batch arrives on June 8, you can no longer average by eye. The new average cost is calculated by adding up the total value on hand ($300,000 + $195,000 = $495,000) and dividing it by the 80 units available, which gives $6,187.50 per unit. Notice that the average went up because the second purchase was more expensive: 30 units at a higher price pushed the cost of the previous 50 units upward, but not all at once. When you sell 20 units on June 15, the outflow is valued neither at $6,000 nor at $6,500, but at the average cost in effect of $6,187.50 per unit. The cost of that sale is 20 × $6,187.50 = $123,750, and the balance is left at 60 units worth $371,250. That $123,750, subtracted from the sale value, is the product's real profit for the period.

Why weighted average works well for a small business

It is stable: one isolated purchase at a high or low price does not distort the cost of the whole inventory overnight; the change is spread across all units.
It is easy to audit: you only need to review the sequence of movements and the recalculation at each inflow to validate any balance.
It is accepted by accounting: it is one of the recognized methods for valuing inventory and it simplifies each month-end close.

Common mistakes when keeping a kardex by hand

Keeping the kardex on paper or in a spreadsheet without well-built formulas has its traps. These are the most frequent mistakes:

Not recalculating the average at every purchase: keeping the old cost to value outflows after a batch has already come in at a different price.
Averaging prices instead of weighting them: doing (6,000 + 6,500) ÷ 2 = 6,250 ignores that you bought 50 units at one price and only 30 at the other. The correct average was $6,187.50.
Recording movements out of order: a movement with an earlier date entered afterwards unbalances the balance from that point on.
Mixing values with and without tax: if some inflows are recorded with tax and others without it, the unit cost becomes inconsistent and the reported profit is wrong.
Not reconciling with the physical count: only the periodic physical inventory confirms that the kardex balances are real.

How Kardex Tauro keeps the weighted average up to date

Doing this record by hand works, but it costs time and exposes your business to errors: a single imbalance forces you to review the whole card from the beginning. Kardex Tauro, an inventory program made in Colombia for small businesses of up to 50 employees, removes that problem at its root: the system generates the kardex movement automatically when you record purchases and updates the weighted average cost without you typing a single formula. When you record a purchase or receive a purchase order, the program creates the inflow, adds to the stock and recalculates the average instantly. When you invoice a sale, the outflow is charged automatically at the average cost in effect and the balance comes out error-free. In the Kardex window of the Warehouse module you will find the complete history of each product with date, third party and concept, and that record cannot be deleted or edited. If you are still keeping your inventory in notebooks or spreadsheets that nobody updates, this is the moment to make the move: try Kardex Tauro and see how the weighted average kardex keeps itself up to date while you focus on selling and growing your business. Your accountant —and your wallet— will thank you.

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#kardex #weighted average cost #inventory control