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CAC and MER Calculator for Ecommerce Ad Spend
Enter your total marketing spend for a period, the new clients it won, the store's total revenue and your gross margin. You get your acquisition cost (CAC), the marketing efficiency ratio (MER, also called blended ROAS), revenue per new client, the contribution left after marketing and the break-even MER your margin sets. Nothing you type leaves your browser.
Calculator
Calculate Your CAC, MER and Break-Even MER
The form holds a worked example: $12,000 of marketing spend in a month that won 150 new clients, with $60,000 of total revenue at a 40% gross margin. Replace it with your numbers; the results update as you type.
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The idea
MER vs Platform ROAS
The marketing efficiency ratio (MER) is total store revenue divided by total marketing spend, every channel and every sale included: $60,000 ÷ $12,000 = 5.00x. Some call it blended ROAS. Platform ROAS is narrower: the revenue Google Ads or Meta attributes to its own clicks, divided by that platform's spend.
The two disagree by design. Each platform claims the sales it touched, so the attributed totals across platforms add up to more than the store took in, and brand searches from people who already knew you count as ad revenue. MER cannot be inflated that way, because the only inputs are the bank statement and the order count.
Acquisition cost (CAC) is the other half: spend divided by new clients, $12,000 ÷ 150 = $80.00. Together they answer the two questions platform reports avoid: did the whole marketing spend pay off this period, and what did a new client cost? Our ROAS vs ROI guide and the ROAS calculator cover the platform side of the same month.
marketing spend ÷ new clients$12,000 ÷ 150 = $80.00total revenue ÷ marketing spend$60,000 ÷ $12,000 = 5.00xtotal revenue ÷ new clients$60,000 ÷ 150 = $400.00total revenue × gross margin - marketing spend$60,000 × 0.40 - $12,000 = $12,000.001 ÷ gross margin1 ÷ 0.40 = 2.50xYour inputs
How to Find the Four Inputs
All four come from the same period, usually a calendar month. A quarter smooths out a slow week; a single week is too short for the client count to mean much.
Marketing spend
Ad spend from every platform's billing page, plus agency or freelancer fees, marketing software and creative production for the period. Leave the fees out and CAC looks cheaper than it is.
New clients
People whose first order fell inside the period. Your store's order export flags first and repeat orders, and a CRM shows the created date. Never use platform conversions here; they count repeat orders and sometimes the same sale twice.
Total revenue
Every sale the store took in the period, every channel, after refunds and before sales tax. Organic, email and direct sales belong in it: MER measures the whole business against the whole spend.
Gross margin
The share of revenue left after product cost, the shipping you pay, packaging, card fees and returns. Price minus product cost overstates it. The Shopify profit margin calculator works it out from one order.
By MER
Contribution at Four MER Levels
Same $12,000 of marketing spend and 40% gross margin. Only the revenue moves.
| MER | Total revenue | Contribution after marketing (revenue × 40% - $12,000) |
|---|---|---|
| 2.00x | $24,000.00 | -$2,400.00 |
| 3.00x | $36,000.00 | $2,400.00 |
| 5.00x (example) | $60,000.00 | $12,000.00 |
| 8.00x | $96,000.00 | $26,400.00 |
Break-even MER comes from the margin alone. At a 40% margin every dollar of revenue leaves 40 cents to pay for marketing, so the spend is covered at $2.50 of revenue per dollar: 1 ÷ 0.40 = 2.50x. A 30% margin needs 3.33x; a 50% margin needs 2.00x. The line between the 2.00x row and the 3.00x row is where the month stops losing money, and no bidding setting moves it. Only the margin does.
Which number when
CAC, CPA, MER and ROAS: Which to Use
Blended figures judge the month; platform figures steer the campaigns. Trouble starts when one is used for the other's job.
CAC is not CPA
CPA is spend ÷ every conversion a platform counted: repeat orders, leads that never ordered, and with loose settings the same sale twice. CAC is spend ÷ new clients from your own records. A $40 CPA and an $80 CAC in the same month is normal. Compare the $80 with lifetime value in the LTV calculator, not the $40.
When blended metrics decide
Use MER and CAC to set the budget and to judge the month. They are the only figures that survive attribution changes, consent gaps and a tracking tag that broke in week two. If platform ROAS climbs while MER falls, the platform is taking credit for sales it did not cause, often brand searches. Our conversion tracking guide covers the tag side.
When platform metrics decide
Use platform ROAS and CPA to move money between campaigns, because MER cannot say which campaign earned the month. Set each campaign's target from the break-even that the margin gives, then let the blended figures confirm, a month later, that the targets were right. Our Google Ads for online stores page shows the structure we run.
Common mistakes
Where CAC and MER Go Wrong
Six habits that make the blended numbers as flattering as the platform ones.
- Platform conversions used as new clients. 300 Google Ads conversions and 150 first orders give a CAC of $40 or $80. Only the second is a cost per new client.
- Fees left out of the spend. A $2,000 agency fee on $10,000 of ads is 20% of the real cost. CAC and MER both move when it goes in.
- Revenue from one channel only. MER divides all revenue by all spend. Counting only tracked ad revenue turns it back into platform ROAS with a new name.
- Sale months compared with normal months. A 30% off week lifts MER and cuts the margin at the same time. Compare like with like, and recompute the break-even for the sale margin.
- Margin taken as price minus product cost. Shipping, packaging, card fees and returns come off first. A margin ten points too high puts the break-even half a turn too low.
- Blended numbers used to pick campaigns. MER cannot see inside the account. Pausing a campaign because MER fell punishes the campaign that happened to be visible.
What is MER, and how is it different from ROAS?
MER, the marketing efficiency ratio, is total store revenue ÷ total marketing spend across every channel: $60,000 ÷ $12,000 = 5.00x. Some call it blended ROAS. Platform ROAS is the revenue one platform attributes to its own clicks ÷ that platform's spend. MER counts every sale and every channel once, so attribution settings cannot inflate it; ROAS tells you which campaign to change.
How do I calculate CAC?
Marketing spend ÷ new clients won in the same period: $12,000 ÷ 150 = $80.00. Count first orders only; a repeat order is not an acquisition. Put agency fees, software and creative costs into the spend if you want the full cost of a new client.
What is the difference between CAC and CPA?
CPA is spend ÷ every conversion a platform counted, which includes repeat orders, leads that never ordered and, with loose settings, the same sale counted twice. CAC is spend ÷ new clients, taken from your store or CRM. A $40 CPA in Google Ads and an $80 CAC for the same month is normal, and the $80 is the one to compare with lifetime value.
What is a good MER?
One above your break-even MER with room for the profit you want. Break-even MER is 1 ÷ gross margin: at 40% it is 2.50x. At 5.00x the example keeps $12,000.00 after marketing; at 2.00x it loses $2,400.00 on the same spend. A figure from another store says nothing about yours, because its margin is different.
When should I use blended metrics instead of platform numbers?
For the budget and the monthly judgment. MER and CAC say whether the whole marketing spend paid off and what a new client cost. For moving money between campaigns, use platform ROAS and CPA, which split the result by campaign. When platform ROAS rises while MER falls, the platform is claiming sales it did not cause.
Does the calculator store my numbers?
No. The math runs in your browser, and nothing you type is sent to us or saved anywhere.
Free audit
Is the Account Behind This CAC Paying Too Much?
Send us the account and the CAC and MER you got here. We tell you whether we found what is pushing the cost per new client up and name one or two issues before any work starts.
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Sources
Sources and Related Pages
- Logical Gecko, ROAS vs ROI: Formulas, Examples and Which to Use.
- Logical Gecko, Google Ads Conversion Tracking: 2026 Setup and Testing Guide.
- Logical Gecko, Free Google Ads audit.