There is no universal good CAC
A CAC of $80 can be excellent.
It can also be catastrophic.
The number has no economic meaning until it is compared with:
- order value;
- gross margin;
- fulfillment;
- payment fees;
- refunds;
- variable service cost;
- repeat contribution;
- cash-payback requirement.
The same is true for ROAS.
A 2.0x campaign can be highly profitable for a high-margin product.
A 5.0x campaign can destroy contribution in a low-margin business.
This calculator is designed to connect media metrics to those business constraints.
It includes:
- Inputs;
- Outputs;
- Scenarios;
- Marginal Model;
- Dashboard.
The workbook is intentionally transparent. Every major output is derived from visible inputs rather than from a hidden benchmark database.
Begin with net revenue, not sticker price
The Inputs sheet starts with AOV and Expected Refund %.
If a business sells $120 orders but expects 5% of revenue to be refunded, the planning model should not treat the full $120 as durable revenue.
The example workbook produces $114 of net revenue after expected refunds.
That number then flows through gross margin and variable costs.
This creates a more realistic media ceiling.
A platform can optimize revenue before the return window closes.
Finance cannot.
Pre-ad contribution is the budget available to acquire the order
The calculator then subtracts:
- product cost through gross margin;
- fulfillment;
- payment fees;
- variable service cost.
The remainder is pre-ad contribution.
That is the economic pool available to fund acquisition and the required post-ad contribution.
If the order creates $49 of pre-ad contribution and the business requires $10 after media, the first-order allowable CAC is about $39.
That number is not a target.
It is a boundary created by the stated assumptions.
The business can decide to accept less first-order contribution in exchange for growth.
It should make that decision explicitly.
Break-even ROAS and target ROAS are different
The calculator produces two useful ROAS values.
Break-even ROAS describes the revenue efficiency at which first-order contribution after advertising reaches zero.
Target ROAS for target contribution describes the efficiency required to preserve the business’s desired post-ad contribution.
That distinction matters.
A team may describe a campaign as profitable because it is above break-even while finance expected a meaningful contribution margin.
The campaign can be profitable and still miss the plan.
Repeat contribution changes allowable CAC
The workbook includes 30-, 60-, 90- and 365-day repeat contribution.
This is a safer framing than simply adding “LTV.”
Lifetime value often becomes an abstract future number used to justify current spend.
Time-bounded contribution asks a better question:
How much contribution is expected to return within a specific period?
In the example model:
- first-order allowable CAC is one number;
- 60-day allowable CAC is higher because expected repeat contribution has arrived;
- 365-day allowable CAC is higher again.
This creates a direct relationship between acquisition aggression and cash timing.
Payback can be more restrictive than LTV
Suppose Customer A produces $300 of contribution over two years.
Customer B produces $220 over one year.
Customer A has higher long-term value.
If most of that value arrives after month twelve, a cash-constrained business may prefer Customer B.
Paid media scale consumes cash before future contribution arrives.
That is why the calculator includes a Cash Payback Target.
The current workbook does not attempt to simulate a full balance sheet.
It forces the operator to keep the constraint visible.
A more advanced implementation can connect the same logic to cohort cash curves.
Incrementality belongs in the economics model
Reported revenue and incremental revenue are not always equal.
The calculator includes an Incrementality Factor for the marginal model.
A 2.5x platform ROAS at 0.70 incrementality does not create the same economic value as a 2.5x return at 1.0.
This is especially relevant when comparing:
- branded Search;
- retargeting;
- prospecting;
- upper-funnel video;
- mature versus new channels.
The factor should come from evidence.
Do not use incrementality as a convenient penalty for channels you dislike.
The Marginal Model answers the scaling question
The core economics outputs answer:
What can we afford?
The Marginal Model answers:
How far should we scale under the current return assumptions?
It contains spend bands with predicted marginal ROAS.
For each band, the workbook estimates:
- incremental revenue;
- incremental contribution before ads;
- marginal contribution after ads;
- invest or stop/test.
As spend rises, the sample marginal ROAS declines.
This represents diminishing returns.
The model identifies where additional spend stops creating positive contribution under the stated incrementality and margin assumptions.
That is the economic frontier.
The economic frontier is not always the operating frontier
A business may stop before the model reaches zero marginal contribution because:
- cash is constrained;
- inventory is limited;
- creative is exhausted;
- uncertainty is high;
- the company needs diversification;
- management requires a higher hurdle rate.
A business may also spend beyond first-order break-even when:
- repeat contribution is strong;
- payback is acceptable;
- customer acquisition has strategic value;
- capital is available.
The model should reflect the chosen objective.
Common Thread Collective’s public material describes similar trade-offs between maximizing first-order contribution, revenue and lifetime contribution. CTC is a commercial agency; the workbook here uses the underlying capital-allocation logic without reproducing its proprietary models. citeturn728515search0turn728515search4
Google bidding values should reflect the same economics where possible
Google’s conversion value rules allow advertisers to adjust reported conversion values for factors such as location, device and audiences, and those adjusted values can feed Target ROAS or Maximize Conversion Value bidding. Google explicitly describes uses such as profit, offline value and lifetime value. citeturn396883search5
That creates an important connection.
The finance model and the bidding model should not describe completely different businesses.
If the calculator says one customer segment is twice as valuable, but Google receives identical values for both, the bidding system cannot act on the distinction.
The calculator is therefore useful upstream of conversion-value design.
Scenario planning should include downside assumptions
The Scenarios sheet includes Base, Growth and Cash-Constrained examples.
Do not use scenarios as decorative sensitivity analysis.
Use them to define action.
For each scenario, decide:
- allowable CAC;
- target ROAS;
- total budget;
- payback tolerance;
- trigger for moving to another scenario.
For example:
If gross margin falls below 55% for two weeks, move from Growth to Base acquisition thresholds.
That turns finance sensitivity into an operating rule.
Failure mode: using historical average LTV as guaranteed value
The customer acquired today may not behave like the historical average.
Channel mix changes customer quality.
Promotions change cohorts.
Product changes retention.
Predicted repeat contribution should have a downside case.
A useful operating model discounts uncertain future value instead of treating it as cash in the bank.
Failure mode: calculating CAC from all customers
For acquisition decisions, new-customer CAC usually matters more than total-order CPA.
If repeat buyers convert cheaply, a blended CPA can make prospecting look healthier than it is.
Use the economic object that matches the decision.
If the team is deciding how much to pay for a new customer, use new-customer economics.
Failure mode: lowering the required contribution to make the campaign “work”
The calculator is a model, which makes it easy to manipulate.
If the media team owns every input, pressure can turn assumptions into targets.
Finance should approve margin and contribution definitions.
Growth leadership should approve the risk horizon.
Media should supply spend-response assumptions.
Measurement should supply incrementality evidence.
Shared ownership protects the model from becoming a justification tool.
Download the calculator
Use this resource before increasing a major paid-media budget, changing bidding values, setting an acquisition target or evaluating whether a high CAC is actually a problem.
The most important output is not one target ROAS.
It is the ability to explain the chain:
customer value → contribution → allowable acquisition cost → payback → marginal opportunity → budget decision.
That is paid media economics.
