You quote them in every status call: ROAS, CPA, conversion value. A client asks whether a 3.2 is good, and you find yourself nodding rather than explaining. This guide takes the three numbers apart so you know exactly what each one measures, how it is built, and how to set a target you can defend. If you are new to the platform itself, start with our Google Ads reporting for beginners guide and come back here.
Start with conversion value
Everything else is built on conversion value, so it goes first. A conversion is simply a desired action: a purchase, a form submission, a phone call, a signup. Conversions as a metric is just the count of those actions. Conversion value is the revenue or worth you attach to them.
That value comes from one of two places. For ecommerce, it is the actual transaction revenue passed back from the website or store (the order total). For lead generation, there is no transaction, so you assign a fixed value per action: maybe $40 for a quote request, $5 for a newsletter signup. You decide that number based on how much a lead is worth to the business.
Garbage value in means garbage ROAS out. If your assigned lead value is a guess, every ROAS figure downstream is a guess too. Pin the value to something real, such as average deal size multiplied by lead-to-sale close rate, before you report on it.
ROAS: return on ad spend
ROAS answers one question: for every dollar spent, how many dollars of value came back?
- ROAS = conversion value ÷ cost
It is a ratio, not a currency figure. A ROAS of 4.0 means $4 of value for every $1 of spend. Some platforms show the same thing as a percentage, so 4.0 becomes 400%. ROAS is the right lens when conversions carry real, variable revenue, which is why ecommerce reporting leans on it. To get accurate revenue flowing into the metric, your Google Ads and GA4 connection needs to be set up so transactions are tracked cleanly.
CPA: cost per acquisition
CPA (cost per acquisition, also called cost per conversion) answers a different question: what did it cost to get one conversion?
- CPA = cost ÷ conversions
CPA is a currency figure, and unlike ROAS, lower is better. A $22 CPA beats a $35 CPA. It is the natural lens for lead generation, where each conversion is roughly worth the same and there is no per-order revenue to compare against. If every lead is worth about the same to you, the only thing that varies is how much you paid to get it.
How the two relate
ROAS and CPA are two lenses on the same efficiency, not rival metrics. When you know the average order value (AOV), you can derive one from the other:
- ROAS = AOV ÷ CPA
If your AOV is $80 and your CPA is $20, your ROAS is 4.0. Push CPA down to $16 and ROAS climbs to 5.0. They move together because they are both built from cost, value and conversion count, just arranged differently.
ROAS and CPA are the same story told in two grammars: one as a ratio, one as a price.
Setting sensible targets
A number means nothing without a target, and targets come from margin, not from a gut feeling.
For ROAS, start at break-even:
- Break-even ROAS = 1 ÷ gross margin
If your gross margin is 50%, break-even ROAS is 2.0 (1 ÷ 0.5). Below 2.0 you lose money on ads; above it you profit. You would then set a target above break-even to leave room for overheads and profit, say 3.5.
For lead gen, work backward from value and the margin you want to keep:
- Target CPA = conversion value × target margin
If a lead is worth $100 and you want to keep 40% as margin after ad cost, your target CPA is $40. Spend more than $40 to win a lead and the math stops working.
The weighted-total trap
This is the one that quietly breaks reports. When you roll campaigns up to an account total, never average the campaign-level ratios. Always recompute from the underlying totals:
- Account ROAS = total value ÷ total cost
- Account CPA = total cost ÷ total conversions
A tiny campaign with a 12.0 ROAS and a large campaign with a 2.0 ROAS do not average to 7.0. Weighted by their actual spend, the real account ROAS sits close to the large campaign. Averaging ratios overstates performance every time, sometimes badly.
The three at a glance
| Metric | Formula | What good looks like / when to use |
|---|---|---|
| Conversion value | Transaction revenue, or a fixed value you assign per action | The foundation. Must be real, not a guess. Drives ROAS. |
| Conversions | Count of desired actions | The volume number. Pairs with cost to give CPA. |
| ROAS | conversion value ÷ cost | Higher is better. Best for ecommerce and revenue. Target above break-even (1 ÷ margin). |
| CPA | cost ÷ conversions | Lower is better. Best for lead gen with similar-value conversions. |
In Clearly
When you build a client report, the useful move is to show ROAS or CPA next to its target, not in isolation. A scorecard that reads 3.1 against a 3.5 goal tells the client more in one glance than a bare 3.1 ever could, because it says whether you are ahead or behind, not just where you landed.
Roll-ups are computed the right way: account-level ROAS and CPA are derived from total value, total cost and total conversions, so the weighted total is correct by default and you are not averaging ratios across campaigns. That means the headline number in the report matches the number you would get by hand from the raw spend and value.
The short version
Conversion value is what an action is worth. ROAS (value ÷ cost) is the ratio lens, best for revenue. CPA (cost ÷ conversions) is the price lens, best for leads, and lower wins. Targets come from margin, and totals come from summing the inputs, never from averaging the ratios. Get those four habits right and the next status call gets a lot easier.