Ecommerce unit economics: CAC, LTV, and MER that add up.

When an ecommerce brand stalls, everyone looks at the ad account. In our experience the account is the crime scene, not the criminal. The actual failure usually happened earlier, in the unit economics: a CAC computed on the wrong denominator, an LTV inflated by revenue instead of margin, a bidding target that was never derived from either. Paid media executes the math you give it. If the math is wrong, excellent execution just gets you to the wrong place faster.

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<a href="https://adsrunner.com/insights/ecommerce-unit-economics-cac-ltv-mer"><img src="https://adsrunner.com/infographics/ecommerce-unit-economics-map.svg" alt="Ecommerce unit economics waterfall: an $85 average order value minus $32 COGS, $8 shipping, and $3 fees leaves $42 contribution margin — implying a breakeven ROAS of about 2.03, a breakeven CPA of $42, and a target ROAS of about 2.66 for $10 profit per order." width="1200" style="max-width:100%;height:auto;" /></a>
<p>Infographic by <a href="https://adsrunner.com/insights/ecommerce-unit-economics-cac-ltv-mer">ADSRUNNER</a></p>

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CAC: the denominator problem

Customer acquisition cost sounds simple — spend divided by customers — and is quietly ruined by both terms. The numerator should be fully loaded: platform spend plus agency or team cost plus creative production. The denominator should be new customers only. Blending returning customers into the denominator is the most common flattery we see; it can halve reported CAC while the true cost of acquiring a stranger climbs unwatched. Platform-side, new-customer goals and properly maintained customer lists exist precisely so campaigns can distinguish acquisition from re-engagement — use them, and mirror the split in your reporting.

LTV: margin or it is fiction

Lifetime value computed on revenue is a fiction that flatters. The only LTV that can justify an acquisition cost is contribution margin: revenue minus product cost, shipping, payment fees, and returns, accumulated per cohort over time. Two further disciplines keep it honest. First, use realized cohort curves — what the 6- and 12-month value of past cohorts actually was — rather than projections from your best year. Second, segment: LTV by first product purchased and by acquisition channel routinely varies two-to-three-fold, which means a single blended LTV target is wrong for every campaign you run.

Payback window: the cash constraint

LTV justifies CAC eventually; cash flow decides whether you survive until eventually. A brand with a 3x LTV-to-CAC ratio and an 18-month payback can still die scaling, because every new cohort consumes cash the business does not recover for a year and a half. Set an explicit payback ceiling that matches your balance sheet — bootstrapped brands often need first-order or 3-month payback; funded brands can stretch further — and let that ceiling, not the LTV ratio alone, cap how aggressively you bid.

MER: the blended guardrail

Marketing efficiency ratio — total revenue over total marketing spend — is the number that cannot be gamed by attribution, which is exactly why it belongs beside the platform metrics rather than instead of them. Platform ROAS tells you where to allocate; MER tells you whether the whole system is healthy. We covered the pairing in MER vs ROAS; the practical addition here is to set a MER floor derived from your blended margin and overhead — the level below which the business is unprofitable regardless of what any dashboard claims — and to watch MER by spend tier as you scale, because it falls predictably as prospecting share grows. A falling MER at rising spend is not failure; falling below the floor is.

Turning the numbers into targets

1. Compute contribution margin per order band, and set campaign-level ROAS targets from margin, not from habit — a 30%-margin catalog needs roughly 3.3x just to break even on first order. The free breakeven ROAS calculator does this per margin band in seconds.
2. Decide how much future-cohort value you are licensed to spend against (your payback ceiling answers this), and adjust acquisition targets accordingly.
3. Give every campaign a target derived from the margin and repeat behavior of what it sells — one blended account-wide target is the spreadsheet error that launches a thousand bad months.
4. Revisit quarterly with finance in the room. Margins move, shipping moves, product mix moves; targets that do not move with them rot silently.

None of this is glamorous, and all of it is upstream of every scaling decision you will make this year. Get the four numbers agreeing — honest CAC, margin LTV, explicit payback, MER floor — and the ad account becomes a machine for executing a strategy instead of a place to argue about one. This math is the entire operating system behind our DTC growth practice, and for how these numbers behave once you push spend, see what changes past $100k a month.