What Is Cost of Sales? Formula, Calculation & Why It Matters (2026)

August 17, 2026 4 Min Read

Cost of sales is the direct cost of the goods a business actually sold during a period — the stock itself, not the rent, marketing, or wages that keep the business running. It’s the number gross profit gets calculated from, and it shows up on almost every Australian business tax return.

Get it wrong, then, and both your gross profit margin and your taxable income end up wrong too.

You might also find our Gross Profit Margin Explained: Formula, Industry Benchmarks & How to Improve It (2026) guide useful.

Quick Answer

Cost of sales equals opening stock plus purchases, minus closing stock. It covers only the direct cost of goods actually sold in the period, not overhead like rent or admin wages. Businesses report it on their tax return to work out taxable income, and it’s the figure gross profit margin is built from.

The Cost of Sales Formula

The standard formula:

Cost of Sales = Opening Stock + Purchases − Closing Stock

Say a business starts the year with $20,000 in stock, buys $80,000 more stock during the year, and ends the year with $15,000 in stock still on the shelf. That works out to $20,000 + $80,000 − $15,000 = $85,000. It’s the direct cost tied to whatever actually sold, then, regardless of how much stock is sitting unsold at year end.

What’s Included in Cost of Sales

Cost of sales covers what goes directly into the product sold:

Stock purchased for resale
Raw materials used in production
Direct labour tied to making the product
Freight and delivery costs on goods bought in

It does not include rent, office wages, marketing, insurance, or general admin costs. Those are operating expenses instead, deducted separately when working out net profit.

Cost of Sales vs. Cost of Goods Sold

These two terms cause more confusion than they should, mostly because they’re usually the same thing. “Cost of goods sold” (COGS) is the more common term in the US and in general accounting software. “Cost of sales” is the term used on Australian tax return forms and by many local accountants.

In practice, treat them as interchangeable unless a specific report or return defines one more narrowly. If in doubt, check exactly what a form or software field is asking for before entering a figure.

How Cost of Sales Appears on an Australian Tax Return

Cost of sales gets reported under the Financial Information section of the Company Tax Return, at Item 8, alongside opening and closing stock values. The opening stock figure has to match the closing stock value reported the previous year — a mismatch there is one of the more common review triggers.

Most accounting software calculates this on its own from stock records. Still, the manual formula matters for checking the number makes sense, not for doing the maths by hand every quarter.

Why Cost of Sales Matters

Cost of sales drives two things that really matter: taxable income and gross profit margin. A higher figure here means lower taxable profit, which then affects tax owed. It also means a thinner gross margin, which affects how healthy the pricing model looks to a lender or investor.

Getting the stock count right at year end is what most of this comes down to. An inflated closing stock figure understates the cost and overstates profit. An understated one does the reverse instead. Either way, it throws off the numbers everyone downstream relies on.

Common Mistakes

A handful of errors show up often:

Skipping a proper stocktake. Estimating closing stock instead of counting it leads to a cost of sales figure that’s simply wrong.
Including overhead costs. Rent or admin wages don’t belong there, even though they still feel related to running the business.
Mismatched opening and closing figures. The prior year’s closing stock has to match this year’s opening stock exactly.
Ignoring freight and delivery costs. These direct costs belong in cost of sales, and skipping them understates the true cost of what was sold.

FAQ

What is the formula for cost of sales?
Opening stock plus purchases, minus closing stock. It gives the direct cost of goods actually sold during the period.

Is cost of sales the same as cost of goods sold?
Generally, yes. “Cost of sales” is the term used on Australian tax returns, while “cost of goods sold” (COGS) is more common in everyday accounting software and US-style reporting.

Where does cost of sales appear on an Australian tax return?
Under the Financial Information section, at Item 8 of the Company Tax Return, alongside opening and closing stock figures.

Does cost of sales include wages?
Only direct labour tied to producing what was sold. Office wages, admin, and other overhead are operating expenses, not cost of sales.

What happens if closing stock is counted wrong?
It distorts cost of sales, gross profit, and taxable income all at once, since all three are calculated from the same stock figures.

Why does opening stock need to match last year’s closing stock?
Because they’re the same figure, just at different points in time. A mismatch usually signals a stocktake or reporting error, and it’s a common trigger for ATO review.