CPA = Ad Spend / Conversions
Contribution Margin = AOV x Margin%
Maximum CPA = Contribution Margin (cannot exceed this)
Margin Left = Contribution Margin - Your CPA
CPA is what you paid on average per customer. It should never exceed your contribution margin (the profit after variable costs). If it does, you are losing money on each acquisition.
A worked example
$10,000 spend, 50 conversions, $500 AOV, 40% margin:
- CPA = 10,000 / 50 = $200 per customer
- Contribution margin = 500 x 0.40 = $200
- Maximum CPA = $200
- At your current CPA, you have $0 left per customer. You are at break-even. Scale carefully and look for lower-cost channels.
If margin were 60% ($300):
- Maximum CPA = $300
- Margin left = 300 - 200 = $100 per customer
- You can afford to scale until CPA climbs to $300.
CPA varies by channel and vertical
Expected CPA ranges vary widely:
- High-AOV B2B: $50 to $500+ CPA is common because you are targeting small, high-value audiences.
- Mid-market SaaS: $30 to $150 CPA depending on product and deal size.
- E-commerce: $5 to $100 CPA depending on product margin and repeat purchase rate.
- Apps: $1 to $10 CPI (cost per install) on average, then divided by install-to-paid rate for true CPA. A 4% conversion (install to paying user) on a $2 CPI is $50 CPA.
Your vertical sets the baseline. Competing on CPA alone within your vertical is common; comparing across verticals is not meaningful.
When to scale and when to cut
Scale when:
- CPA is 30% to 50% below your maximum. You have room to rise as audiences get exhausted.
- Marginal CPA (the cost of the last tranche of spend) is still below maximum.
- You have tested the next tier of budget and CPA held steady.
Cut or pause when:
- CPA exceeds your maximum. Every customer acquired costs more than the margin, so you are destroying profit.
- CPA has climbed 20%+ in two consecutive weeks. Your best audiences are exhausted.
- The cost to acquire equals or exceeds LTV. For first-purchase acquisition, this is always tight; test repeat purchase data if you have it.
Questions people ask
What is a good CPA?
One below your maximum (contribution margin). The difference is your profit per customer. If maximum is $300 and CPA is $200, you have $100 profit per customer. Scale when CPA is 30% to 50% below maximum.
Should I include customer service cost in margin?
Yes. Variable cost includes cost of goods, payment fees, and average support cost per customer. Fixed cost (salaries, rent, software) goes in a P and L, not in CPA.
What if I do not know my exact AOV?
Use the average first purchase or first-month revenue. If customers buy multiple times, you can calculate a lifetime value separately and use a higher AOV once you have repeat purchase data.
Can my CPA be negative?
No. CPA cannot go below zero. If your CPA is zero, you got conversions for free. More likely, you are not tracking all sources of acquisition cost.
How do I know if my CPA is too high?
Compare it to your maximum. If CPA is $300 and margin is $200, you are above maximum and losing money. If CPA is $200 with a $300 margin, you have $100 profit and can scale.
Sources
Last reviewed 11 September 2026. Formulas and benchmarks are published on the page so you can check them. This tool gives estimates, not quotes.
