Most ten-person firms run capacity planning on a whiteboard and a gut feeling. That works in May. It does not work in the week of 24 January, when six clients have not sent records, two staff are off sick, and the senior manager is quietly updating their CV. A working capacity model is not a spreadsheet trophy. It is the one document that tells you, twelve weeks out, that you are about to overpromise.
Start with productive hours, not headcount
Headcount is a vanity number. A ten-person firm with two trainees, one part-timer, and a partner who spends three days a week in sales does not have ten capacity units. Begin by listing every fee earner and their realistic productive hours per week. The honest figure for a UK accountant is rarely the 37.5 contracted. Strip out holidays (typically 28 days plus bank holidays), CPD (40 hours per year), team meetings, internal admin, and a 15% buffer for illness and slippage.
A senior with a 37.5 hour contract realistically delivers about 1,350 productive hours per year. A trainee studying ACA is closer to 1,000. A part-time bookkeeper at three days might give you 950. Add these up across the team. A typical ten-person firm has 11,500-12,500 productive hours per year, not the 19,500 the contracts imply.
Map demand by job type, not by client
The mistake firms make is treating capacity as a single bucket. In practice you have at least four demand streams: recurring compliance (year-end accounts, VAT, payroll), one-off advisory (incorporations, restructures, R&D claims), seasonal personal tax, and ad-hoc client queries that nobody books in. Each consumes different skill levels and clusters in different months.
Build a demand calendar by month. For each job type, record average hours, the partner-to-junior split, and the realistic completion window. A standard small-company year-end set might be 14 hours: 2 partner, 4 senior, 8 trainee. A personal tax return averages 1.5 hours but compresses 70% of demand into November-January.
Find your January cliff
Overlay demand against productive hours by month. Almost every ten-person UK firm has a visible cliff in January when self-assessment, December year-ends, and Q3 VAT collide. The cliff is not the problem itself. The problem is discovering it on 5 January.
A working model surfaces the cliff in October. If your demand chart shows 1,400 hours needed in January but you only have 1,050 available, you have three levers: pull work forward into November, push non-statutory work into February, or buy contractor capacity at a known cost. Make the call in October and the cliff is a manageable hill.
Build in skill-mix constraints
Hours are not interchangeable. A partner cannot file 80 personal tax returns because there is no partner-time bottleneck - it is a partner-quality bottleneck. Conversely, a trainee cannot independently review a corporation tax computation on a £4m turnover client. Your model needs to track hours by grade.
A simple way to do this: split productive hours into three pools (partner/manager, senior, trainee/admin) and demand into the same three categories per job. Many firms find their senior-grade pool is the actual constraint, not partner time. That changes the hiring conversation.
Make capacity visible to the team
A capacity model that lives in the managing partner's spreadsheet is worthless. The seniors making day-to-day allocation decisions need to see the picture. This is where practice-management tooling earns its keep. Accupe's capacity views and Smart Boards roll up assigned jobs by staff member and surface red flags when someone is booked past their realistic productive hours for the period.
When the senior taking on a new audit can see her January load is already at 110%, she pushes back in real time rather than discovering the overload at month-end. The conversation moves from blame to triage.
Review the model fortnightly
Capacity is not annual. Clients miss deadlines, staff leave, a referral brings in three new jobs. A fortnightly capacity review - 30 minutes, the partner and the senior responsible for allocation - keeps the model honest. The review answers three questions: are we ahead or behind on planned hours, where is slack we can redeploy, and what jobs are at risk of slipping out of their deadline window?
Firms that run this cadence consistently report 15-20% fewer missed deadlines and meaningfully lower partner stress in busy season. The model itself is not the win - the discipline of looking at it together is.
Hire against the constraint, not the symptom
When the model shows a recurring shortage in a particular skill grade or month, you have data to recruit against. "We need a senior who can run audits independently by Q3" is a hiring brief. "We are too busy" is not. Use the capacity model to build the case for hires, set start dates that align with demand, and avoid the classic mistake of hiring trainees when the bottleneck is senior review time.
Closing
A capacity model for a ten-person firm is not complex. A single spreadsheet, refreshed fortnightly, with productive hours and demand mapped by month and grade, will tell you 80% of what you need to know. The discipline is in actually maintaining it, and in trusting the numbers when they tell you to say no to work in October so you can deliver in January.