Skip to main content
Metrics type: Key Metrics • Category: Ads

​
At a glance

Average cost to acquire one conversion via Google Ads. Rising CPA squeezes margin: if CPA exceeds gross profit per order, every paid sale loses money. The card is the single most important Google Ads health metric because it converts ad spend into the language of unit economics. Compare against contribution margin per order: a CPA below 30% of contribution is healthy; CPA above 50% is alert state.

​
Calculation

​
Worked example

A UK-based BC store, CPA reading on Wednesday 15 May 26. What the CPA reading is telling us:
  1. CPA at £45.58, well above the £40 alert threshold. Combined with the +27.2% rise vsP, this is a clear efficiency regression. The merchant’s Google Ads programme is becoming less efficient over time.
  2. Profitability still positive but compressing. At AOV £261 × 45% gross margin = £117 contribution per order. CPA £45 means £72 net contribution per acquired customer. Still profitable but margin is narrowing rapidly. At current trajectory, CPA could exceed contribution within 60-90 days.
  3. What’s driving the CPA rise:
    • Spend up 13.9%, conversions down 10.5%. Scaling spend without scaling conversions = inefficiency.
    • Likely causes: (a) audience saturation, campaigns reaching the bottom of the funnel, (b) creative fatigue, same ads losing CTR after weeks of impressions, (c) bid pressure, competitors raising bids, (d) landing-page regression, site speed or product issues reducing post-click conversion.
  4. Recommended response, in priority order:
    • Day 1: Cap spend at the prior level (£40K) until efficiency recovers. Don’t keep scaling unprofitable spend.
    • Day 1-2: Refresh creative, new ad copy, new images, new variations, to break creative fatigue.
    • Day 2-3: Audit landing pages for site-speed regression (cross-reference crux_lcp_p75 for the campaign’s landing pages specifically).
    • Day 3-5: Audit search-term reports; identify wasted spend on irrelevant queries; add negatives.
    • Day 7-14: Test new audience expansions or look-alike audiences if the issue is bottom-of-funnel saturation.
    • Day 30: Re-measure CPA; aim for return to £30-£35 range.
  5. The cross-platform context:
    • Cross-reference gads_roas: rising CPA and falling ROAS confirm the inefficiency is spend × conversion, not just spend × revenue.
    • Cross-reference gads_impression_share: rising impression share with rising CPA can mean the ad is winning auctions for clicks but losing on conversion.
    • Cross-reference psi_lab_lcp and landing-page CWV: site speed regressions on the campaign’s destination pages tank conversion.
The diagnostic flow:
  1. Read CPA. Above £40 alert; +25% vsP also alert.
  2. Compare to contribution per order; confirm profitability.
  3. Decompose: spend, conversions, CTR, conversion rate.
  4. Identify the root cause: creative fatigue, audience saturation, bid pressure, landing-page regression.
  5. Apply fix; re-measure at 30 days.
Rapid-response playbook:

​
Sibling cards merchants should reference together

​
Reconciling against the vendor’s own dashboard

Where to look in Google Ads: Reports → Performance → Cost / conv (CPA); per-campaign and per-account views. Why our number may differ: Quick rule: confirm attribution model and conversion goal first.

​
Known limitations / merchant FAQs

Q: Our CPA is £45 and AOV is £260, is that profitable? At 45% gross margin: contribution per order is £117; CPA £45 means £72 net. Still profitable, but margin is narrow. Most ecommerce stores should target CPA below 30% of contribution per order: £35 in this case. Current £45 is approaching unprofitable; further rise needs immediate action. Q: Our CPA jumped 25% in 30 days. What’s most likely the cause? In order of likelihood: (1) creative fatigue (same ads running too long; CTR drops, bids rise), (2) competitor entry or bid increase, (3) landing-page conversion regression (site speed, OOS bestsellers, payment issue), (4) audience saturation (bottom-funnel exhausted; remaining audience converts at lower rate), (5) seasonal demand softening. Q: Should I cut spend when CPA rises? Usually yes, temporarily. Capping spend at the prior efficiency level prevents losing money while you investigate. Resume scale once CPA recovers. Don’t accept rising CPA as the new normal without diagnosing the cause first. Q: How does CPA relate to ROAS? CPA = spend / conversions; ROAS = revenue / spend. Together: revenue per conversion = AOV. Rising CPA with stable AOV means falling ROAS. The two metrics tell the same efficiency story from different angles. Use CPA for unit-economics decisions; use ROAS for marketing-team performance reviews. Q: We have 5 campaigns; one has CPA £15, four have CPA £60. Should I just keep the £15? Often yes. The 80/20 rule applies to ad campaigns: a small number of campaigns drive most efficient conversions. The four high-CPA campaigns may be subsidising the one efficient one (broader reach, brand awareness). Audit each independently; pause campaigns where CPA exceeds breakeven contribution. Q: We don’t track gross margin per product. How do we set CPA threshold? Use a conservative blended estimate: AOV × 30-40% as the maximum CPA. If AOV is £100 and gross margin assumed 40%, max CPA is £30-£40. For accuracy, integrate per-product cost data (from BC’s product cost field) for true contribution-per-order CPA targets. Q: How does iOS 14.5+ ATT affect CPA reading? Significantly. ATT obscures app-install conversion paths and reduces conversion-tracking accuracy on iOS. Most stores see reported conversions drop 15-30% on iOS post-ATT, which inflates apparent CPA. Use Google’s enhanced conversions feature and server-side tagging to recover signal. Don’t compare pre-ATT and post-ATT CPA directly. Q: Branded vs non-branded CPA, should I track separately? Yes. Branded CPA (queries containing the brand name) is structurally low because brand-aware shoppers convert easily. Non-branded CPA is the real efficiency signal. Many merchants conflate the two and miss non-branded inefficiency. Most ad platforms support per-campaign CPA breakdown; segment manually if needed.

​
Tracked live in Vortex IQ Nerve Centre

Cost per Acquisition is one of hundreds of KPI pulses Vortex IQ tracks across Google Ads and 70+ other ecommerce connectors. Nerve Centre runs the detection layer; Vortex Mind investigates the cause when something moves; Ask Viq lets you interrogate any number in plain English. Start for free or book a demo to see this metric running on your own data.