What Is CAC? Formula, Cost Allocation, and Customer Counting
Shusaku Yosa
Customer acquisition cost is included acquisition spending divided by unique new customers. The formula is simple, but the result changes when you include sales labor, production, or other allocated costs rather than media alone. Define cost scope, customer eligibility, and timing before calculating CAC.
Fix the included cost scope
Media-only CAC describes new customers relative to media spending. A CAC including allocated sales and marketing costs evaluates a broader acquisition scope. Neither should silently replace the other. Including every sales expense without examining its purpose can mix new acquisition with retention and expansion activity.
Stripe's CAC guidance discusses matching acquisition costs with new customers. Your allocation policy should be documented as a reproducible management rule, rather than presented as a single universal accounting standard.
Cost | Treatment | Supporting record |
|---|---|---|
Media and agency fees | Identify new-acquisition activity; separate retention campaigns | Invoices, purpose, and period |
Sales and production labor | Allocate effort supporting new acquisition | Time records and allocation basis |
Content, tools, contractors | Assign to relevant acquisition work | Project, contract, and shared-cost rules |
Referrals and events | Include applicable costs rather than assuming “free” acquisition | Reward conditions and eligible customers |
Retention and shared costs | Explain exclusion or allocation | Activity classification and change history |
In a separate fictional allocation example, ¥500,000 of shared labor is 60% attributable to new acquisition. Include ¥300,000 and assign the remaining ¥200,000 to other activity. The 60% is an invented time-record assumption, not a recommended standard. It is not an extra amount to add automatically to the channel example below.
Count unique new customers
If the measure concerns paying customers, a signup, trial, or qualified opportunity is not yet a customer. Decide how multiple contracts under one company, reactivations, refunds, and cancellations are counted. With zero eligible customers, CAC is undefined—not zero cost.
Repeat purchases can make CAC look artificially low when purchase events are mistaken for new customers. Use the guide to CAC versus CPA to reconcile actions, leads, and unique customers.
A fictional example: aggregate costs and customers
For one mature acquisition period, suppose paid activity has ¥500,000 of assigned costs and 20 new customers. Nonpaid activity, such as organic search and referrals, has ¥300,000 and 30. Attribution is nonoverlapping, and both use the same allocation policy.
Scope | Acquisition costs | New customers | CAC |
|---|---|---|---|
Paid | ¥500,000 | 20 | ¥25,000 |
Organic and other nonpaid | ¥300,000 | 30 | ¥10,000 |
Blended total | ¥800,000 | 50 | ¥16,000 |
Blended CAC is ¥800,000 / 50 = ¥16,000. The unweighted average of the two channel CACs is ¥17,500 and is incorrect because customer counts differ. Sum compatible costs and unique customers first. If platforms claim overlapping customers, reconcile them to the overall unique count before adding channel totals.
Organic acquisition is not inherently cheaper. Labor, content, and referral rewards can change its cost. Moving budget away from advertising does not automatically reduce CAC; review additional spending and incremental customers.
Match the acquisition lag
Customers closing this month may originate from activity several months earlier. Conversely, this month's spending may not have produced its eventual customers yet. A short reporting window can therefore misalign numerator and denominator.
- Distinguish cost dates from lead and customer acquisition dates.
- Use a mature acquisition cohort or an explicitly documented period policy.
- Label still-maturing results as provisional and record the observation cutoff.
- Document allocation of long-lived content and shared costs, including changes.
Quarterly or annual averaging can reduce volatility, but a longer period does not automatically establish correct cost matching or causality. Note definition and allocation changes when comparing historical results.
Interpret changes without confusing metrics
Investigate whether spending increased, customer count fell, or the channel mix changed. Increasing LTV alone does not lower CAC; it changes the relationship between customer value and acquisition cost.
When using customer lifetime value, specify revenue versus gross profit and whether acquisition cost has already been deducted. Continue with LTV:CAC, payback, and cash-flow decisions rather than inferring company-wide profit or optimal investment from CAC alone.