The short answer
Dividing a revenue target by an assumed ROAS gives a spend scenario. It does not establish that the market will deliver that revenue at that efficiency. Use the calculation to expose the assumptions behind a plan.
Build the basic scenario
Illustrative inputs: target attributed revenue $20,000 and assumed ROAS 4×. Modeled spend is $20,000 ÷ 4 = $5,000. If pre-ad contribution margin is assumed to be 35%, contribution after ads is $20,000 × 0.35 − $5,000 = $2,000 before fixed costs and other exclusions.
Show an alternative outcome
At 3× rather than 4×, reaching the same $20,000 target would require $20,000 ÷ 3 ≈ $6,666.67. On the same 35% margin assumption, modeled contribution after ads falls to $7,000 − $6,666.67 ≈ $333.33. These are scenarios, not probabilities or forecasts supported by campaign history.
Cross-check the traffic requirements
Assume $100 AOV and an orders-per-click rate of 2%. The $20,000 target implies $20,000 ÷ $100 = 200 orders and 200 ÷ 0.02 = 10,000 clicks. A $5,000 budget implies an average CPC of $5,000 ÷ 10,000 = $0.50. Check whether the assumptions are consistent with each other and with relevant observed data.
Allow for the limits of scale
A historical average on a smaller budget does not prove that additional traffic will have the same cost or conversion behavior. Product availability, fulfillment capacity, audience mix, and conversion lag may also constrain the plan. Evaluate the additional spend rather than assuming a straight-line projection.
Separate revenue goals from cash limits
Set the amount the business can afford to spend before returns arrive, considering inventory and payment timing. If the downside scenario is not affordable, revise the budget or test scope. Do not use an aspirational ROAS input to make an otherwise unaffordable plan appear safe.
A useful planning sheet records each assumption, its source period, the scenario calculation, and the condition under which the budget will be reconsidered. Keep actual results separate as they arrive.
Expose the budget assumptions
- Gather: State the revenue goal, assumed ROAS, margin, AOV and orders-per-click rate.
- Produce: Cross-check implied spend, orders and clicks against available capacity and cash.
Decision boundary: An algebraically consistent plan can still require demand the market does not provide.
Next reading
Examples are illustrative, not client results or benchmarks. The calculations use the inputs shown; real-world decisions depend on your records and assumptions. Report a correction.