If my biggest customer leaves, what happens to my margin?
Every business has a biggest customer. Some businesses depend on that customer more than they realize. Here is how to calculate exactly what would happen to your revenue and margin if they walked away.
The short answer
What happens if your top client leaves? Find what percentage of revenue they represent. Subtract that revenue from your total. Then check whether the remaining revenue covers your expenses. If it does not, you have a concentration problem.
Why customer concentration is a hidden survival risk
Your biggest customer probably feels like your best asset. They pay on time, the work is steady, and they make up a big chunk of your revenue. But dependence and stability are not the same thing.
When one client represents 25%, 30%, or 40% of your revenue, you do not just have a customer. You have a single point of failure. They get acquired and the new owners bring their own vendor. They have a bad quarter and cut your contract. Their payment person leaves and suddenly invoices that used to clear in 15 days are stuck at 60.
The businesses that survive client losses are the ones that already knew the impact. They had the math done. They knew their margin without that client. And they had a plan, even if the plan was just “build a bigger cash cushion.”
How to measure customer concentration and margin impact
Customer concentration is expressed as a percentage. The margin impact requires a bit more math:
That example is stark: a profitable business becomes deeply unprofitable the moment one client walks away. The question is whether you could cut expenses fast enough to survive while you replace the revenue.
How to check customer concentration in QuickBooks Online
QuickBooks has a built-in report that makes the first part easy. The margin calculation requires a second report.
- 1Pull Sales by Customer Summary
Go to Reportsin the left sidebar. Search for “Sales by Customer Summary.” Set the date range to the last 3 months for a more stable picture. Sort by amount descending.
- 2Calculate your top client's percentage
Take your top customer's total and divide by Total Sales at the bottom. Multiply by 100. If it is above 20%, you have meaningful concentration. Above 30% is a significant risk. Above 40% is a dependency.
- 3Pull your Profit and Loss
Go to Reports→ “Profit and Loss” for the same period. Note Total Income, Total Expenses, and Net Income.
- 4Model the loss
Subtract your top client's revenue from Total Income. Keep Total Expenses the same (in the short term, most expenses are fixed). The new Net Income shows what your business looks like without them.
- 5Calculate the new margin
Divide the new Net Income by the new Total Income. If the result is negative, losing this client puts you in a loss position immediately. Then ask: how many months could your cash cover that loss?
Total time: about 15 minutes. The Sales by Customer report and the P&L are easy to pull. The scenario math is straightforward but sobering.
How to check customer concentration in Xero
Xero does not have a built-in Sales by Customer report like QuickBooks. You need to work a bit harder.
- 1Export your invoices
Go to Business → Invoices. Filter by the last 3 months. Click Export to download as CSV.
- 2Sort by customer in a spreadsheet
Open the CSV. Create a pivot table or sort by customer name. Sum the invoice amounts per customer. Identify your top customer and calculate their percentage of total invoiced revenue.
- 3Pull your P&L
Go to Accounting → Reports→ “Profit and Loss” for the same 3-month period. Note Total Revenue, Total Operating Expenses, and Net Profit.
- 4Model the client loss
Subtract the top client's revenue from Total Revenue. Recalculate margin. If you go from profitable to unprofitable, you know the risk you are carrying.
Total time: 20-30 minutes. The invoice export and spreadsheet sort for customer concentration adds significant time compared to QuickBooks.
Why customer concentration shifts faster than you think
Your customer mix changes every month. A client that was 15% of revenue six months ago might be 30% today if they ramped up while other clients shrank. Here is why tracking this matters:
- Concentration creeps up silently. You do not decide to become dependent on one client. It happens gradually as they grow and others do not. By the time you notice, you are already exposed.
- The export-and-spreadsheet method is tedious. Especially in Xero, checking concentration takes 15-20 minutes of data wrangling. That is 15-20 minutes nobody does regularly.
- You need the margin impact, not just the percentage. Knowing your top client is 28% of revenue is useful. Knowing that losing them flips you from profitable to a $22K monthly loss is actionable. The second number requires combining customer data with your P&L.
Or get customer concentration risk calculated automatically
Bottomline calculates your customer concentration and models the impact of losing your top client automatically every month. It pulls customer revenue data from your accounting software and cross-references it with your P&L.
No invoice exports. No spreadsheet pivots. No manual margin math. Bottomline also tracks concentration over time so you can see if you are becoming more or less dependent on any single client.