A founder I know runs a twelve-person consultancy. Her gross margin sits at 68%, which looks healthy on any benchmark you care to use. She is not, in any meaningful sense, making money. Each month finishes roughly where it started, and she cannot work out why.
The answer turned out to be straightforward. Her accountant’s definition of cost of goods sold was too narrow, so gross margin was measuring something that did not reflect the economics of her actual work. The metric she needed was contribution margin, and once calculated properly by service line, the picture changed entirely.
What is contribution margin?
Contribution margin is what remains from a piece of work once you subtract everything you spent to deliver it. The formula is straightforward: revenue minus variable delivery costs. Variable costs are the ones that change depending on whether you take on a job: billable staff time at fully-loaded rates, subcontractors, usage-based software charges, direct travel. If you did not do the work, you would not incur those costs.
This distinction matters because services firms often have blurry cost structures. In a typical owner-managed professional services firm, 60 to 80% of total costs are people-related, covering salaries, employer National Insurance, pension contributions, and benefits. Where those people costs vary with workload, including contractors hired for specific engagements or billable staff whose time is tracked per job, they count as variable. They belong in your contribution margin calculation, even if your profit and loss sheet has classified them differently.
The FRC’s thematic review of staff cost reporting noted that how services firms classify people costs can materially affect the performance indicators they produce, and that presentation should reflect economic substance rather than accounting convention. That is a polite way of saying that many firms are, in practice, reading the wrong number.
Why does gross profit give the wrong picture in services?
Gross profit answers a different question: how much revenue remains after cost of goods sold. In a professional services firm, COGS is often defined narrowly, perhaps a bit of software and some direct subcontractor spend. That leaves a gross margin of 60 or 70% looking healthy while the business barely covers its overheads. Contribution margin brings labour and delivery back into the calculation, where they belong.
The UK Insolvency Service’s commentary on company failures in 2022-23 pointed to a recurring theme: rising input costs not reflected in selling prices. That is, at root, a contribution margin problem. The firm is spending more to deliver each unit of work, but pricing has not caught up.
For owner-operated businesses, the gap is often wider because founder time is treated as effectively free. If a founder spends 40% of their week on delivery, that time has a cost, even if it does not appear on the payroll as a separate line. A genuine contribution margin calculation includes a market-rate allocation for founder hours spent on each engagement. Many founders who run this calculation for the first time find that certain service lines they assumed were profitable are running at a thin or negative contribution once all real costs are counted.
Where does contribution margin show up in your pricing decisions?
When you price a new engagement or decide whether to accept work at a discounted rate, contribution margin is the number that tells you whether it is viable. Take a project quoted at £2,000. If your variable delivery costs, covering staff hours at fully-loaded cost, any contractor involvement, and AI tool usage, come to £1,600, your contribution is £400. Whether that £400 is enough depends entirely on how much fixed overhead each project needs to carry.
To work out whether it is enough, you need your required contribution per unit. If fixed monthly overheads run to £40,000, covering non-billable staff salaries, rent, insurance, compliance costs, and core marketing, and you complete 80 projects a month, you need £500 of contribution per project just to cover those overheads before any profit. At £400, you are running at a shortfall on every engagement, even though each one appears to generate revenue.
Deloitte’s professional services maturity model identifies project-level margin, which is effectively contribution margin at project level, as the primary performance indicator for smaller consulting and IT firms. Grant Thornton’s professional services insight reports for law and accounting firms make a similar point: matter-level and client-level profitability analysis is what separates firms that understand their economics from those surprised at year end.
Comparing contribution margin across service lines is also where the real decisions live. A service line at 30% contribution margin pays down fixed overhead more slowly than one at 50%, even if its headline revenue is higher. The higher-margin line funds your fixed costs faster once you fill capacity.
When does contribution margin matter, and when can you keep it simple?
Contribution margin earns its place when you have more than one service line, more than one team member, or meaningful variability in how much each job costs to deliver. If you are a sole trader whose only real variable cost is your own time, a simple income-versus-expenses view is probably sufficient. But once you have employees or contractors, the contribution calculation is worth doing.
There are genuine limits. If your prices are constrained by a regulator or a government framework contract, knowing your contribution margin tells you where the pressure is but gives you limited room to act on it. The priority in those situations shifts to cost structure and operational efficiency rather than repricing.
A second limit: the calculation depends on decent cost data. If your timesheets are unreliable or job costing has not been done consistently, the resulting numbers can create a false sense of precision.
For FCA-regulated firms, contribution margin has a specific additional function. The Consumer Duty’s price and value outcome, set out in PS22/9, requires firms to demonstrate that pricing bears a reasonable relationship to costs and benefits. Per-engagement contribution analysis is part of the evidence trail for that relationship.
How does contribution margin connect to break-even, capacity, and hiring?
Contribution margin feeds into three related calculations. Break-even analysis: fixed costs divided by contribution per unit tells you how many jobs the firm needs monthly to cover overheads. Capacity planning: what happens to margins when billable utilisation slips from 80% to 65%? Hiring decisions: does average contribution per fee-earner justify adding another head? Each of these becomes easier to answer once you have the contribution figure in place.
SPI Research’s professional services benchmarks put typical target billable utilisation at 70 to 80% of available hours. Many owner-operators set their fees assuming something closer to 90%, and then find that holidays, sickness, non-billable internal work, and time spent on sales erode real utilisation to 60 or 65%. At that level, contribution per head falls below what is needed to cover the fixed costs each person must carry. Pricing should account for the realistic number, not the optimistic one.
Where AI tools are part of your delivery workflow, they introduce a new variable into both calculations. If an AI-assisted process cuts staff hours per job by 20 to 30%, variable cost per job falls and contribution improves. But if per-token charges for that AI usage have not been tracked as delivery costs, you are not capturing the real picture accurately. These costs belong in your contribution margin, not buried in an IT overhead line, and should be treated the same way as contractor time.
The ICO’s guidance on AI and data protection is worth reading here as a practical discipline. If you are using AI to model client profitability or allocate work based on predicted engagement value, those systems are processing personal data. That requires a lawful basis, transparency, and human review in the decision loop. It is manageable, but it belongs in the planning conversation rather than as an afterthought.
The consultancy I mentioned at the start has not taken on new clients or restructured its team. What changed was the analysis. Once contribution margin was calculated properly by service line, it became clear which work was paying for itself and which was not. Two service lines were repriced. One was discontinued. The margin picture improved within a quarter.
Getting the number right is the work. The decisions tend to follow from there.
If you would like to work through how this applies to your firm, book a conversation.
