Marketplace Unit Economics Calculator: An Excel Walkthrough

A marketplace can process massive gross merchandise value (GMV) and still lack the cash to cover acquisition and overhead. Assuming a high gross margin on software automatically translates to a high margin on physical or service transactions is a mistake. GMV is a necessary measure of scale, but it does not establish contribution or profit by itself.

Download the Marketplace Unit Economics Calculator to separate your underlying transaction contribution from the cost of acquiring a defined group of customers. The Excel workbook calculates contribution amounts and simple payback from the numbers you input. The worked example below demonstrates exactly what belongs in each field.

Keep in mind that transaction value is not platform revenue. GMV measures the total dollars passing between users. Platform revenue consists of the commissions and fees your business actually earns—a distinction well documented in a16z's marketplace metrics guide. Contribution is what remains after you deduct fee credits and direct costs, but before you pay for acquisition or fixed overhead.

When we design and build custom marketplaces for our clients at Directorism, the underlying payout and fee ledgers dictate the build decisions. Without reliable transaction records, you are just guessing at your contribution. Those actual records form the foundation of a useful model.

Structuring Your Data

The workbook contains four tabs: Read Me, Inputs, Model and Model Choice. Start with the Read Me tab, then apply a single, consistent currency on the Inputs tab. Blue input cells start blank. Enter a numeric zero for a category that genuinely does not apply to your model, and leave completely unknown values blank.

The Inputs sheet deliberately separates your platform-wide revenue period data from your specific acquisition cohort. A cohort is a defined group of buyers or suppliers who first paid during a specific window. Because period totals and cohort metrics measure different things, the workbook structures them in separate fields, but it is your responsibility to keep the concepts distinct.

To demonstrate the calculator's mechanics, we use a purely hypothetical financial example.

Revenue-Period Data and Direct Costs (Hypothetical)

The fields for direct costs require total period costs for the stated revenue window. Do not enter fee percentages or per-transaction amounts here, though you can use clearly labeled period estimates for hypothetical scenarios.

Acquisition Cohort (Hypothetical)

This sample cohort observes the 20 buyers who first paid in September 2026. Note that the $5 monthly contribution per buyer and the 12-month lifetime are teaching assumptions, not measured historical results. Monthly cohort contribution is a manual input; it is never automatically derived by dividing total platform contribution by newly acquired users.

Understanding the Outputs

Switch to the Model tab to review the calculations generated from the hypothetical inputs above:

• B2 GMV: $20,000 (Inputs B4 × B5)
• B3 Gross commission: $2,000 (GMV × B6)
• B4 Gross platform revenue: $2,000 (commission + subscriptions + other revenue)
• B5 Revenue after platform fee credits: $1,900 (gross revenue - credits)
• B6 Direct costs: $500 (processing + support + other direct costs)
• B7 Contribution before acquisition/overhead: $1,400 (net platform revenue - direct costs)
• B8 Cohort customer acquisition cost (CAC): $30 (acquisition spend / new paying customers)
• B9 Monthly cohort contribution per payer: $5 (the separate user input)
• B10 Assumption-based contribution lifetime value: $60 (monthly cohort contribution × assumed paying months)
• B11 Simple constant-contribution payback months: 6 (CAC / positive monthly cohort contribution)

Crucially, that $1,400 contribution still has to cover your acquisition footprint and all fixed overhead. The $600 acquisition spend belongs exclusively to the defined cohort; it is not a complete record of platform acquisition costs. Subtracting it from the platform contribution does not equal profit.

If you need to model supplier acquisition, define a separate cohort. When establishing an overall platform acquisition budget, count each shared transaction's contribution only once, but account for the acquisition costs of both sides. Our unit economics modeling guide details how to reconcile these figures.

The workbook handles edge cases strictly. Missing or invalid required inputs leave dependent outputs blank. A zero or negative monthly contribution leaves payback blank (negative contribution is not artificially zeroed out). If acquisition spend is zero but monthly contribution is positive, simple payback correctly reports as zero.

Testing Sensitivities and Limitations

Adjusting inputs allows you to test the fragility of your assumptions. Using our hypothetical scenario, if the monthly cohort contribution per payer drops from $5 down to $3 while holding all other inputs flat:

• Assumption-based contribution lifetime value falls to $36.
• Simple constant-contribution payback stretches to 10 months.

Remember that payback assumes a constant positive monthly contribution—it is not a cash-flow forecast. This workbook is deliberately narrow. It contains no measured retention algorithms, no fixed overhead fields, and no automated break-even outputs. If you want to evaluate how different commission tiers impact supplier proceeds and platform margins, switch to the marketplace commission scenario guide.

Finally, use the Model Choice tab to log the selected business model, an alternative, the chargeable event, operating risk, evidence, owner, next test and review date. If you find yourself guessing at too many inputs, the marketplace validation checklist will help you prioritize which risks to test first.

Next Steps

Download a fresh copy of the workbook. Enter one recorded revenue period and define one acquisition cohort, clearly labeling your assumptions. Save a separate file for each scenario you test. Your goal is not to hit the hypothetical numbers in this article, but to systematically replace your riskiest assumptions with recorded evidence.