Formulas
Break-even CPA = contribution per order Target CPA = AOV × (contribution margin − target profit %) Max CAC = target CPA × orders per customer Break-even CPC = break-even CPA × conversion rate Max CPC = target CPA × conversion rate
Worked example
With a $60 average order, $27.90 contribution per order (46.5%) and a 15% profit target, target CPA = $60 × (0.465 − 0.15) = $18.90. At a 2% conversion rate, max CPC = $18.90 × 0.02 = $0.38; the break-even CPC is $27.90 × 0.02 = $0.56. If customers order twice over their lifetime, you can spend up to $37.80 to win one.
Checking a campaign's return instead? Use the break-even ROAS calculator.
Questions
What is the maximum CPC I can afford?
Max CPC = target CPA × conversion rate. If each order can carry $18.90 of ad cost and 2% of clicks buy, you can pay up to $0.378 per click.
What is a target CPA?
The ad cost per order that still leaves your target profit. Target CPA = AOV × (contribution margin − target profit). Break-even CPA is the whole contribution per order.
How is CAC different from CPA?
CPA is the cost per order. CAC is the cost to win a customer, who may order more than once. Max CAC = target CPA × orders per customer over their lifetime.
What conversion rate should I use?
Your own, from your store or ads platform: orders divided by ad clicks over the last month or two. Small online shops often see 1% to 3%, but yours is what counts.