What is Cost of Goods Sold (COGS)?

Cost of Goods Sold is the total cost of materials, labour, and manufacturing overhead directly tied to producing goods that a business sells. It excludes indirect expenses like marketing and distribution, and is deducted from revenue to calculate gross profit.

Why It Matters

COGS is the headline number on every retailer and manufacturer's accounts. It directly determines gross profit and thus profitability. A retailer with 40% COGS leaves 60% for operating costs and profit - healthy. One with 80% COGS is fighting for survival.

For accountants advising manufacturing and retail clients, COGS is a key area of risk. Inventory valuation (how you count what's on hand), obsolescence (writing off stock that won't sell), and cost allocation (deciding whether a factory cost is COGS or overhead) are subjective. Get them wrong and profit is misstated and tax is wrong. HMRC audits COGS closely on manufacturing clients.

For business owners, COGS is the first lever to pull. If your gross margin is thin, your options are: raise prices, reduce material costs, or improve production efficiency. Understanding COGS month-to-month tells you whether the business is getting more or less efficient.

How It Works

COGS calculation depends on your inventory method and business model.

The standard formula:

Beginning Inventory + Purchases During Period - Ending Inventory = COGS

Inventory is valued at cost (what you paid), not selling price. If you bought fabric for £10 per metre and sell garments for £50, the COGS uses the £10 cost, not the £50 price.

Real example:

A clothing retailer starts January with £15,000 of stock on hand (at cost). During January they buy £8,000 of new stock. At the end of January they do a physical count and find £12,000 of stock remaining. COGS for January = £15,000 + £8,000 - £12,000 = £11,000. They sold £11,000 of goods (at cost). If their sales revenue was £28,000, their gross profit is £28,000 - £11,000 = £17,000 (a 60% gross margin).

Inventory valuation methods:

The tricky bit is valuing ending inventory. You could use:

  1. FIFO (First In, First Out): Assume the oldest stock is sold first. In inflationary periods, this results in lower COGS and higher profit.
  2. LIFO (Last In, First Out): Assume the newest stock is sold first. In inflationary periods, this results in higher COGS and lower profit.
  3. Weighted Average: Average the cost of all units. Middle ground between FIFO and LIFO.

Once you pick a method, you must use it consistently year to year (tax authorities require this). Changing methods requires formal approval.

Perpetual vs. periodic:

  • Periodic: You count inventory manually once a year (year-end). COGS is calculated using the formula above. Simpler for small businesses but you don't know real-time COGS.
  • Perpetual: Every sale is recorded and COGS is calculated instantly using a system (like accounting software). More accurate and gives you monthly COGS, but requires system integration.

Key Considerations

Common mistakes:

  • Including non-manufacturing costs. Sales salaries, marketing, office rent - these are NOT COGS. They're operating expenses. Only include direct production costs.
  • Ignoring obsolete inventory. If you have £5,000 of stock that won't sell, you must write it off as a loss, not leave it in the books at cost. HMRC will challenge it on audit.
  • Inconsistent inventory counts. If you don't count stock physically at year-end, your COGS is a guess. Do a proper stocktake and reconcile it to your records.
  • Including depreciation wrongly. Depreciation of manufacturing equipment is COGS (it's a manufacturing overhead). Depreciation of office furniture is not.
  • Not allocating shared costs. A factory manager's salary is manufacturing overhead and should be allocated to COGS. Don't put all of it as a separate operating expense.

Best practices:

  • Do a full physical inventory count at least once a year (ideally more for retail with high shrinkage).
  • Keep purchase invoices and stock records scrupulously - HMRC will ask for them on audit.
  • Use accounting software with inventory tracking to calculate COGS automatically and spot discrepancies.
  • Review gross margin monthly. If it drops unexpectedly, investigate immediately (theft, waste, price changes).
  • Decide your inventory valuation method upfront and document it. If audited, you need to explain your choice.

How Aarvo Helps

Aarvo integrates with your accounting software and bank feeds to pull purchase invoices and expense data. If you use inventory tracking (most modern accounting packages do), Aarvo can help you visualise COGS trends month-to-month and flag when gross margins shift.

You can also use Aarvo to model scenarios: "If I reduce material costs by 5%, what happens to profit?" This helps you focus on the biggest levers.

Sources & Further Reading

Frequently Asked Questions

What is Cost of Goods Sold?

COGS is the total cost of producing the goods a business sells in a given period. It includes raw materials, direct labour (wages of assembly line workers), and manufacturing overhead directly tied to production (factory electricity, machinery depreciation). It does NOT include indirect costs like sales salaries, marketing, rent on office space, or administrative staff. For a furniture maker, COGS is wood, nails, glue, and the carpenter's wages. It does NOT include the salary of the sales manager or the cost of a Google ad campaign. COGS is the cost of stuff you sold, not the cost of selling it.

Why does COGS matter for tax?

COGS is deducted from revenue to calculate gross profit, which is the first line of a profit and loss statement. Tax authorities in the UK and US require you to report COGS accurately because it directly affects taxable profit. Overstate COGS and your taxable profit shrinks (and so does your tax bill). Understate it and you've understated profit. HMRC and the IRS scrutinise COGS closely, especially for businesses with inventory. If you claim £50,000 in COGS but only spent £30,000, you'll face penalties if audited.

How do I calculate COGS?

Use the formula: Beginning Inventory + Purchases - Ending Inventory = COGS. For example: you start the year with £20,000 of raw materials on hand. During the year you buy £80,000 more. At year-end you have £15,000 left. COGS = £20,000 + £80,000 - £15,000 = £85,000. This method (called the periodic method) is simpler for small businesses. Larger businesses use perpetual inventory tracking where COGS is recorded every time a sale happens. The key is accuracy: inventory counts, purchase records, and material tracking must all be precise.

What's the difference between COGS and operating expenses?

COGS is the direct cost of goods sold. Operating expenses (also called SG&A - selling, general, and administrative) are everything else: marketing, office salaries, rent, utilities, insurance, accountancy fees. Both are deducted from revenue, but in order: revenue minus COGS = gross profit. Then gross profit minus operating expenses = operating profit. A business can have high revenue and high COGS but still be profitable if operating expenses are low. Conversely, low COGS with sky-high marketing spend can lead to losses.

What types of businesses report COGS?

Any business that sells goods (not services) reports COGS: manufacturers, retailers, wholesalers, and distributors. Service businesses (accountants, plumbers, consultants) do NOT report COGS. Instead they report cost of services or direct labour costs. A software company selling packaged software on USB drives might report some COGS (manufacturing the disks, packaging). A SaaS company with no physical goods does NOT report COGS - it reports customer acquisition costs as operating expenses instead.

What happens if I overstate COGS?

If you claim more COGS than you actually spent, your reported profit is lower and your tax bill is lower. This is tax fraud. If HMRC audits you and discovers you overstated COGS, you'll owe back tax plus interest (4-8% per year) plus penalties (30-70% depending on whether the error was careless or deliberate). If the overage is large, HMRC can open a criminal investigation. Keep accurate inventory records, purchase invoices, and stock count sheets - especially if you make physical goods. The cost is low; the risk of fraud charges is high.