A business can be busy, pay its bills and still be selling work at an unsustainable margin. That is why understanding how to calculate gross margin matters. It shows how much income remains after the direct cost of delivering what you sell, before rent, software, salaries, tax and the other costs of running the business enter the picture.
For a growing business, gross margin is not an accounting exercise to revisit once a year. It is a commercial measure. It helps you decide whether a price rise is needed, whether a product line is worth keeping, whether a new client is genuinely profitable and whether rising supplier costs are quietly eroding performance.
The gross margin formula
The calculation is straightforward:
Gross margin = (Revenue - Cost of sales) ÷ Revenue × 100
The answer is expressed as a percentage. Revenue less cost of sales is your gross profit, measured in pounds. Gross margin tells you what proportion of each pound of sales is left after direct costs.
For example, if your business generates £100,000 in sales and incurs £35,000 in direct costs:
Gross profit = £100,000 - £35,000 = £65,000
Gross margin = £65,000 ÷ £100,000 × 100 = 65%
In plain English, the business retains 65p from every £1 of revenue to cover overheads, finance costs, tax and profit for the owner.
That percentage is usually more useful than the gross profit figure alone. A £65,000 gross profit may sound healthy, but it means very little without knowing the scale of revenue, the overhead base and whether the margin is improving or declining.
Start with the right numbers
The formula is easy. The judgement lies in deciding what belongs in revenue and cost of sales. This is where many small business reports become misleading.
Revenue should normally be sales earned in the period, excluding VAT. If you invoice a client £1,200 including VAT, and £200 is VAT, the revenue is £1,000. VAT collected on behalf of HMRC is not your income and should not inflate your margin calculation.
Cost of sales, sometimes called cost of goods sold or direct costs, includes costs that arise because you made a sale or delivered a service. For a retailer, this is usually the cost of stock sold, delivery charges and potentially transaction fees. For a manufacturer, it may include materials, production labour and subcontracted production.
For a service business, direct costs can be less obvious. A consultant who personally delivers all client work may have very few direct costs. But if work is delivered by freelancers, associates or specialist subcontractors, those costs are generally directly linked to client revenue and should be included in cost of sales.
A marketing agency, for instance, may invoice a client £10,000 for a campaign and pay £4,000 to freelance designers and copywriters. Assuming those freelancers worked specifically on that campaign, the gross profit is £6,000 and the gross margin is 60%.
The key question is simple: would this cost exist if the sale had not happened? If yes, it is likely to be a direct cost. If it would exist regardless, it is more likely to be an overhead.
Do not confuse direct costs with overheads
Overheads are necessary costs of operating the business, but they do not usually belong in gross margin. These may include office or home-working costs, accounting fees, insurance, general software subscriptions, director salaries, marketing, bank charges and administrative staff.
This distinction matters because putting every expense into cost of sales can make gross margin look artificially weak. Equally, treating all labour as an overhead can make a service business appear more profitable at gross margin level than it really is.
There is no universal answer for every cost. A salaried project manager who spends all their time delivering client projects may reasonably be treated as a direct delivery cost. A manager who splits time between delivery, sales and operations may need an informed allocation. Consistency matters more than false precision. Choose a sensible approach, document it and apply it in the same way each month.
For owner-managed businesses, the same principle applies to the owner’s time. A sole trader may not pay themselves a salary, but their labour is still economically important. Your statutory accounts may show a high gross margin because no wage is recorded for your own work. For pricing and planning, however, it is often wise to model a commercial cost for the time required to deliver the service. Otherwise, the business may only be profitable because the owner is underpaying themselves.
Gross margin versus markup
Gross margin and markup are often used interchangeably. They are not the same measure.
Gross margin is calculated as a percentage of the selling price. Markup is calculated as a percentage of the cost.
If an item costs £60 and sells for £100, the gross profit is £40. The gross margin is 40% because £40 is 40% of the £100 selling price. The markup is 66.7% because £40 is 66.7% of the £60 cost.
This difference can cause costly pricing errors. If you want a 40% gross margin, you cannot simply add 40% to the cost. A 40% markup on a £60 cost gives a selling price of £84, which produces a gross margin of only 28.6%.
To set a selling price for a target gross margin, use this calculation:
Selling price = Direct cost ÷ (1 - target gross margin)
If direct cost is £60 and the target gross margin is 40%, the price is £60 ÷ 0.60 = £100.
Use gross margin to test pricing decisions
A gross margin calculation becomes useful when it is applied to real decisions rather than treated as a historical number.
If supplier prices rise by 10%, look at the effect on margin before deciding whether to absorb the increase. A business with a generous margin may have room to do so temporarily. A business operating on a thin margin may need to raise prices promptly, change suppliers or reduce the scope of what it delivers.
The same is true when a client asks for a discount. A 10% price reduction does not simply reduce profit by 10%. If your margin was already 30%, it can remove a far greater proportion of the profit from that sale. The lower the existing margin, the more damaging indiscriminate discounting becomes.
It is also worth calculating gross margin by product, service line, customer type or project where the information is available. A business-wide average can hide an unprofitable service behind a stronger one. One large client may generate substantial revenue but consume so much subcontractor time, delivery support or rework that its margin falls well below the rest of the business.
That does not automatically mean you should stop working with that client. A lower-margin contract may provide reliable volume, introductions, strategic experience or work for otherwise quiet capacity. But the decision should be conscious. Revenue is not the same thing as value.
Watch the trend, not just one month
One month’s gross margin can be distorted by timing. You may buy stock before selling it, incur a large subcontractor cost before invoicing the corresponding work, or recognise annual income in a particular period. For project-based businesses, a simple monthly calculation can be particularly misleading if income and costs are recorded in different months.
Look at the trend over several months and compare like with like. If gross margin has moved from 62% to 55% over two quarters, ask why. Possible reasons include supplier price increases, weaker pricing discipline, a shift towards lower-margin work, higher use of contractors, stock wastage or unrecorded direct costs that have finally been captured.
Management accounts are especially valuable here. They turn the calculation into a regular conversation about performance: what changed, whether it is temporary and what action is needed before the issue reaches year-end accounts.
A practical monthly routine
Calculate gross margin once a month using reliable, VAT-exclusive revenue and consistently classified direct costs. Compare the result with the prior month, the same period last year and your target. If the figure changes materially, investigate the driver rather than assuming it will correct itself.
For businesses with several services or products, keep the analysis proportionate. You do not need a finance department or complicated spreadsheet to begin. A clear view of your main income streams and the direct costs attached to them is often enough to expose the decisions that deserve attention.
Gross margin will not tell you everything. It does not account for fixed overheads, loan repayments, tax liabilities or cash collection. A business can have an excellent gross margin and still struggle with cash flow. But it is one of the clearest early indicators of whether your core offer is priced and delivered in a way that can support a sustainable business.
Every number tells a story. Gross margin tells you whether the work you are winning is creating enough room to build the business you actually want to run.