Definition
CPA is ad spend divided by conversions (purchase, signup, lead). It's a tactical metric; CAC usually refers to cost per new paying customer including broader costs.
Formula
CPA = ad spend / conversions
Common mistakes
- Calling lead CPA 'CAC' (different denominators).
- Comparing CPA across campaigns with different conversion definitions.
- Ignoring downstream quality (refunds, churn, LTV).
Measured as
CPA = ad spend / conversions
Misused when
- Calling lead CPA 'CAC' (different denominators).
- Comparing CPA across campaigns with different conversion definitions.
- Ignoring downstream quality (refunds, churn, LTV).
Operator takeaway
- Use CPA (Cost Per Acquisition) only inside a stable attribution rule, conversion definition, and time window so campaign comparisons stay honest.
- If performance changes, check whether the metric moved for a real business reason or because the measurement setup changed underneath you.
Next decision
- Quantify the impact with Target CPA from LTV Calculator if you need to turn the definition into an operating assumption.
- Read CAC vs CPA: definitions, formulas, and when to use each if the decision depends on interpretation, policy, or trade-offs beyond the raw formula.
Where to use this on MetricKit
Calculators
- Target CPA from LTV Calculator: Translate LTV and contribution margin into a target CPA (and break-even CPA) for paid acquisition.
- Paid Ads Funnel Calculator: Model CPM -> CTR -> CVR to estimate CPC, CPA, ROAS, and profit per 1,000 impressions (with margin and variable costs).
- Marginal ROAS Calculator: Estimate diminishing returns and find the profit-maximizing ad spend from a simple response curve.
- Max CPC Calculator: Compute break-even and target CPC (and optional CPM) from CVR, AOV, and contribution margin assumptions.
- CPL to CAC Calculator: Convert cost per lead (CPL) into CAC using lead-to-customer rate (and compute targets).
Guides
- CAC vs CPA: definitions, formulas, and when to use each: CAC vs CPA explained: what each metric measures, how to calculate them, and how to translate CPA into CAC for planning.
- Target CPA: how to set acquisition targets from LTV and margin: A practical guide to target CPA: connect acquisition cost to LTV, contribution margin, and payback constraints (and avoid common mismatches).
- Paid ads funnel: CPM, CTR, CVR -> CPC, CPA, ROAS (with profit): A practical guide to the paid ads funnel: how CPM, CTR, and CVR drive CPC, CPA, ROAS, and profit - with formulas and common pitfalls.
- CTR (Click-Through Rate): definition, formula, and how to improve: CTR explained: what click-through rate is, how to calculate it, and how to improve CTR without sacrificing conversion quality.
- CVR (Conversion Rate): definition, formula, and how to calculate: CVR explained: what conversion rate means, how to calculate it correctly, and how to improve it without sacrificing scale.