Inventory Costing and Why It Moves Your Margins

Two businesses can buy the same goods, sell them at the same price, and still report different gross margins. That is not accounting sleight of hand. It is inventory costing at work: the rules that decide which peso of purchase cost gets attached to the unit that just left the warehouse. Most owners meet that decision once, when the books are first set up, and never revisit it again — even though it shapes every margin report they read afterwards.

Costing is an assumption, not an observation

You buy the same item at different prices across the year. When one of those units is sold, the physical unit that left the shelf is anonymous — the box does not remember what you paid for it. So the books apply a cost flow assumption: a rule that says which cost is released to cost of goods sold and which cost stays on the balance sheet as inventory.

This is a split, not a creation. The total you paid your suppliers is fixed. Costing only decides how much of that total lands on this period's income statement and how much is carried forward as an asset. In a stretch of rising supplier prices, first-in-first-out releases the older, cheaper costs into cost of goods sold and leaves the recent, expensive ones sitting in stock — so the margin reads higher and the balance sheet reads richer. Weighted average blends them and smooths the swing.

Worth separating clearly: the assumption in your books is a different question from how stock physically moves in the warehouse. You can rotate goods strictly oldest-first for shelf-life reasons and still value them on a weighted average basis. One is operations, the other is measurement.

The methods a Philippine business chooses between

Philippine Accounting Standard 2, the local adoption of the international inventories standard, recognises specific identification, first-in-first-out, and weighted average. Last-in-first-out is not an available option here, so the practical choice for most companies is between FIFO and weighted average.

Specific identification means each unit carries its own actual cost. It is the right answer only when items are genuinely not interchangeable — serialised equipment, custom builds, high-value pieces you can trace individually. Applying it to fast-moving consumables creates a tracking burden with no benefit.

FIFO suits businesses whose stock is perishable, dated or fast-moving, and it keeps the balance sheet close to current replacement cost. Weighted average suits businesses buying the same commodity repeatedly at fluctuating prices, where a per-batch cost is more noise than information.

Whichever you pick, consistency matters more than cleverness. The method has to be applied the same way period after period, disclosed in your financial statements, and match what your books actually do. Switching methods because the other one produces a nicer-looking margin is a reporting decision with real consequences for both your auditors and the BIR, not a spreadsheet toggle.

Landed cost decides more than the invoice price

The cost of inventory is not the supplier's invoice price. It is the price plus everything you spent bringing that item to its present location and condition: freight, brokerage, import duties, insurance in transit, and handling into the store. Trade discounts and rebates come off. This is where a lot of quiet margin damage happens.

The common failure is procedural rather than conceptual. Freight and duties arrive as separate bills, often weeks after the goods, and get coded straight to a general expense account because nobody wants to reopen a closed receipt. The item's unit cost is then understated, gross margin on every sale of that item looks better than it is, and the shortfall reappears further down the income statement as an operating expense nobody can attribute to a product.

For imported stock the gap can be substantial enough to change a pricing decision. If you are setting a selling price off a unit cost that excludes duties and inland freight, you are marking up the wrong number. Recording landed cost against the receipt — while the receipt is still open — is the discipline that keeps the item-level margin meaningful.

Manufacturing turns costing into a chain

For a manufacturer the cost of a finished unit is not a purchase price at all. It is the costed value of the raw materials consumed plus the conversion cost you attach. That makes costing a chain rather than a lookup, and errors compound along it: a raw material valued wrongly propagates into every finished unit built from it, and from there into every margin report on that product line.

The bill of materials is the mechanism that makes the chain repeatable. It states what each finished unit consumes, so a production order can draw the right quantities, value them at whatever cost basis the books use, and post the result back as the cost of the output. ERPat's Production module works this way — manufacturing units, bills of materials and production orders that cost back into inventory — so the finished goods valuation is derived from actual consumption rather than re-estimated by hand each cycle.

The payoff is that you can finally answer a question most manufacturers guess at: whether a specific product line is carrying the business or being carried by it.

The count is what makes the number true

Costing is arithmetic performed on a quantity. If the quantity is wrong, the method does not matter — a beautifully applied weighted average over a phantom fifty units is a wrong number, precisely calculated. Shrinkage, breakage, samples given away, and branch transfers recorded on one side only are the usual sources of drift.

This is why stock movement has to be recorded as movement. Transfers between locations, write-offs and adjustments should each be dated, attributable and visible in the ledger rather than silently absorbed into the next count. ERPat's Inventory module tracks stock levels, transfers and adjustments across every location for exactly that reason: the valuation is only as defensible as the movement history behind it.

Getting a margin number you can defend

The final step is reconciliation. Your stock ledger and your general ledger describe the same asset from two directions, and they have to agree — when they do not, the difference is either an unrecorded movement or a costing error, and both are worth finding before an auditor finds them for you.

That reconciliation gets much shorter when inventory movement and accounting are not separate systems joined by a monthly spreadsheet. ERPat's Finance module handles accounts, expenses, payments and reconciliation alongside the Inventory and Production modules on the same platform, so a receipt, a transfer or a production run carries its cost through without being re-keyed.

None of this makes costing exciting. It does make your gross margin a number you can act on — set prices from, judge product lines by, and explain to a lender or an auditor without a caveat. That is worth the hour it takes to decide your method properly and the discipline it takes to keep applying it.