How to calculate client profitability for fixed-fee bookkeeping services

Why revenue alone gives the wrong answer

A $500 monthly client is not automatically better than a $300 client. If the first requires ten hours and the second requires three, their value to your practice is very different. Fixed pricing hides this difference because the invoice stays the same even when the workload grows.

Client profitability should answer three separate questions:

• How much does the engagement cost you to deliver?
• What effective hourly rate does it produce?
• Is the remaining margin high enough for the engagement to be worth keeping?

Step 1: calculate your internal hourly cost

Start with the cost of one hour of delivery—not the rate you want to charge. For a solo bookkeeper, use the annual income the practice must generate for you, plus business overhead, divided by realistic billable hours.

Do not divide by every working hour in the year. Sales, administration, training and time off are necessary but not billable. Using an unrealistically high number of billable hours makes weak engagements look healthy.

Step 2: track the client's real monthly hours

Record all time attributable to the client for at least one representative month. Include routine production as well as the work most likely to disappear from memory:

• Transaction processing and reconciliations
• Client emails, calls and follow-ups
• Fixing incomplete or late records
• Reports, explanations and “quick questions”
• Review, quality control and month-end corrections

If a month is unusual, use a three-month average. The objective is not perfect timekeeping; it is a defensible picture of what the engagement normally consumes.

For a review across several months, the Client Margin Guard Excel + Google Sheets toolkit ($29, one-time) includes monthly, YTD and custom-period views.

Step 3: calculate delivery cost, profit and margin

Example

A client pays $600 per month. Delivery takes eight hours, your internal hourly cost is $40, and the engagement has $20 of direct software expense.

The percentage is useful, but it is not a universal pass-or-fail score. Your target must reflect overhead, capacity, service complexity and the return you need from the practice.

Step 4: measure the cost of scope creep

Scope creep is work delivered beyond the assumptions behind the fixed fee. Track each extra task with its date, category and time. Then convert those hours into cost.

This creates evidence for a fee review. Instead of saying “the client has become demanding,” you can show that the engagement now requires specific recurring work that was not included in the original price.

If these extra hours are already included in your total monthly hours, do not add their cost again. The scope log documents the reason for the extra work.

Step 5: turn the numbers into a decision

One month of data should start a conversation, not trigger an automatic dismissal. Look for repeated patterns and consider strategic value, payment reliability and working relationship alongside the financial result.

How often should you review profitability?

Run a lightweight check monthly and a full fee review quarterly. Also recalculate when transaction volume changes, payroll headcount grows, reporting requirements expand or the client repeatedly needs work outside the agreed scope.

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