Blended CAC vs New-Customer CAC (aMER)
Blended CAC vs new-customer CAC (aMER) for US DTC operators. Formulas, why repeats hide expensive acquisition, and why cheap CAC fails if retention is weak.
Mark Cijo
Founder, GOSH Digital
Blended CAC is your marketing spend divided by every customer who ordered in the period. New-customer CAC is that same spend divided by first-time buyers only. aMER is the revenue twin of the second number: new-customer revenue divided by the same spend.
If you only watch the blended number, returning buyers make acquisition look cheap. If you only watch new-customer CAC, a first order that never repeats looks like a win. US DTC operators need both, plus a retention system that actually collects the second sale.
This is the scoreboard I use with brands that already spend real money on paid media and still cannot tell whether growth is profitable.
What is blended CAC vs new-customer CAC?
Two different questions get stuffed into the word "CAC."
Blended CAC answers: what did we spend per customer who bought this period, counting new and returning people together?
New-customer CAC answers: what did we spend per first-time buyer?
aMER answers the revenue version of that second question: how much first-time revenue did each media dollar produce?
Human Marketing treats MER as blended business-level efficiency (all sales divided by total media spend) and aMER as the new-customer growth test (new-customer sales divided by that same spend). Their point is blunt: blended efficiency can look fine while new-customer volume stalls. Read Human's MER vs ROAS vs aMER page.
Bloom Analytics makes the same split. Blended MER uses all revenue. aMER uses new-customer revenue only, against the same spend, so repeats cannot hide a weak acquisition engine. Read Bloom's MER guide and Bloom's aMER explainer.
One language trap: some operators say "blended CAC" when they mean all-channel spend divided by new customers, as opposed to Meta's cost per purchase. That is a better number than a platform dashboard. It is still not aMER. And it still lies if those new customers do not come back.
How do you calculate blended CAC and new-customer CAC?
Lock the period first. Same month, same spend, same customer file. Then run both formulas.
Blended CAC = Total marketing spend / Total customers who ordered
New-customer CAC = Total marketing spend / First-time buyers
aMER = New-customer revenue / Total media spend
MER (the blended twin) = Total revenue / Total media spend
Human keeps the MER and aMER denominator as media spend only. Salaries, contractors, and design sit outside that ratio. Human's definition is total sales (or new-customer sales) divided by total media spend in the same window.
Bloom's marketing metrics docs define CAC as total marketing spend divided by the number of new customer orders, and aMER as net revenue from new customers divided by total marketing spend. Bloom's metric definitions.
Shopify's CAC formula is wider: (total ad spend + sales expenses) / new customers acquired. Shopify on LTV to CAC.
Pick one denominator and do not mix them mid-month. For the weekly media meeting, I use media spend in both aMER and new-customer CAC so the ratios sit on the same spend. For the monthly finance review, I add agency fees, creative, and tools, because that is the cash that left the account.
Worked example (illustrative)
These are round numbers so you can see the split. They are not a benchmark.
| Line | Amount |
|---|---|
| Media spend | $40,000 |
| Agency, creative, tools | $8,000 |
| Total marketing spend | $48,000 |
| Total revenue | $160,000 |
| New-customer revenue | $56,000 |
| Customers who ordered | 1,600 |
| First-time buyers | 400 |
Blended CAC (all buyers) = $48,000 / 1,600 = $30
New-customer CAC = $48,000 / 400 = $120
MER (media only) = $160,000 / $40,000 = 4.0
aMER (media only) = $56,000 / $40,000 = 1.4
The $30 blended number is what a founder quotes when they want the story to feel cheap. The $120 number is what it cost to put a new buyer on the file. The 1.4 aMER is the growth test: each media dollar returned $1.40 in first-time revenue.
Bloom's published example uses $10,000 spend, $40,000 total revenue, and $24,000 new-customer revenue: blended MER 4.0, aMER 2.4. Bloom aMER. Human's published example uses $10,000 spend, $40,000 total revenue, and $11,000 new-customer revenue: MER 4.0, aMER 1.1. Human MER vs aMER. Same blended headline. Opposite acquisition story.
If you want a second set of eyes on your own split, book a strategy call. We start with new vs returning, not with a platform ROAS screenshot.
What is aMER and how does it pair with new-customer CAC?
New-customer CAC is a cost per person. aMER is revenue per dollar. You need both.
New-customer CAC can look fine when first-order AOV is high and volume is low. aMER tells you whether the spend that produced those people is still buying growth. Human's framing: aMER shows whether acquisition is paying, because repeats cannot inflate it. Human.
Bloom watches aMER divided by MER as one split. In their $10,000 / $40,000 / $24,000 example, 2.4 / 4.0 = 0.6, so 60% of blended efficiency came from new buyers. If that ratio drifts down while spend climbs, growth is tilting toward the existing base. Bloom aMER.
That is the media half. The other half is whether those new buyers become the returning revenue that later props up MER. That is email and SMS, not another prospecting campaign.
Why can blended CAC look healthy while acquisition is failing?
Because returning customers do not need new ads to buy again.
Human's example is the cleanest version I have seen in public: $40,000 revenue and $10,000 media spend is MER 4.0. New-customer revenue of $11,000 in the same period is aMER 1.1. Leadership sees "efficiency is fine." The new-customer pipeline is not. Human writes that new-customer spend is typically profitable around 1.7x and up, with 2.4x described as strong, as their own stated range, not a law of nature. Human.
Bloom's spend-scale table shows the same lie in motion. At $50,000 spend they publish blended MER 4.20 against aMER 1.52. By $90,000 spend, blended MER is still 3.62 while aMER falls to 0.98, which means you are paying more for new-customer revenue than that first-order revenue brings in. Bloom MER guide.
If your "CAC" includes returning buyers in the denominator, you get the same disguise in dollar form. The file looks cheap. New logos are expensive. You scale paid media into a hole.
Why does new-customer CAC lie if retention is weak?
Because CAC only prices the first purchase. Profit lives in the second, third, and fourth.
Shopify's LTV to CAC guide treats about 3:1 as a common reference point. The same page says ecommerce brands often range between 2:1 and 4:1, and later that they typically see 3:1 to 4:1. It treats 2:1 or less as close to break-even, because CAC does not include product cost or overhead. Shopify: What is a good LTV to CAC ratio?
That ratio is useless if the LTV is a slide, not a cohort. If the first-time cohort never repeats, your 3:1 is a forecast, not a result.
This is why I refuse to treat new-customer CAC as a standalone green light. Example: a $70 new-customer CAC against a $90 first order can still be a loss after COGS, shipping, and discounts. The only way that math works is if email, SMS, and post-purchase actually create the next order.
We are a Klaviyo Gold Partner. We have built 100+ flows. The point of that work is not "more email." It is making the second purchase cheap enough that you can afford the first one.
The Phoenix is the public proof we are allowed to name: $22.3M in email revenue over 31 months. Paid media put people on the file. Retention made the file worth the CAC. If your flows are a welcome discount and a ghosted post-purchase, new-customer CAC is a vanity metric with extra steps.
What should US DTC operators track every month?
Same period. Same spend. No platform-attributed mashup.
- New vs returning revenue. Shopify reports new vs returning customers. Start there.
- New-customer CAC. Spend / first-time buyers. Use media spend weekly, fully loaded spend monthly.
- Blended CAC. Spend / all buyers. Keep it on the sheet so you can see the gap.
- MER and aMER. Total revenue / spend, and new-customer revenue / spend. Human and Bloom both put these on the same scoreboard for a reason.
- Repeat purchase rate on the last 90-day new-customer cohort. If this is weak, stop scaling before you "fix CAC."
- Payback. How many days of gross profit until new-customer CAC is recouped. Bloom defines payback as CAC divided by gross margin per new customer. Bloom metrics.
If blended MER is stable, aMER is falling, and new-customer count is flat, you do not have an efficiency win. You have a retention-funded disguise.
If new-customer CAC looks cheap and 90-day repeat is weak, you do not have a CAC win. You have a leaky bucket.
The fix is not a new attribution tool. It is a retention program that turns first-time buyers into the returning revenue you are already counting in MER. Paid media and email have to share one number: contribution after the customer, not credit inside an ad account.
What is a good blended CAC or aMER?
There is no universal good CAC, and Human says there is no universal good MER either. Break-even depends on contribution margin before ads: about 1 divided by that margin. Human's examples: 40% before ads implies break-even MER around 2.5, 25% implies about 4.0. Human. Bloom: a store keeping 50% before ads breaks even at MER 2.0, a 30% margin store needs roughly 3.3. Bloom.
Human's own stated range for when new-customer spend is typically profitable is about 1.7x aMER and up, with 2.4x called strong. That is their language on their page, not a law.
Shopify's LTV to CAC reference is about 3:1. The same page says ecommerce often ranges between 2:1 and 4:1, and later 3:1 to 4:1. Shopify. Use it as a conversation starter, then replace it with your cohort LTV and fully loaded new-customer CAC.
What to do this week
Pull last month from Shopify and your ad accounts.
Write down one spend number. Write down first-time buyers, all buyers, new-customer revenue, and total revenue.
Run blended CAC, new-customer CAC, MER, and aMER. If the blended story and the new-customer story disagree, believe the new-customer story for growth, then look at 90-day repeat before you raise budget.
If you want help doing that on your numbers, book a strategy call. GOSH Digital is a Klaviyo Gold Partner. 150+ clients. $70M+ revenue. $23M+ ad spend. Pricing starts from $1,500/mo. We will tell you whether you have a CAC problem, a retention problem, or both.
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Written by Mark Cijo
Founder of GOSH Digital. Klaviyo Gold Partner. Helping eCommerce brands grow revenue through data-driven marketing.
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