These three metrics get used interchangeably in casual conversation and that's exactly why so many marketing budget decisions are made on the wrong number. Each answers a genuinely different question, uses a different denominator, and is appropriate for a different kind of decision.

ROI vs. ROAS vs. LTV:CAC

Metric	Formula	What it answers	Best used for
ROAS	Revenue ÷ Ad Spend	How much revenue came back per dollar spent	Quick, channel-level comparison of top-line efficiency
ROI	(Profit − Cost) ÷ Cost	How much profit was generated per dollar invested, after costs	Budget allocation decisions — the one finance actually cares about
LTV:CAC	Customer Lifetime Value ÷ Customer Acquisition Cost	Whether an acquired customer is worth more than it cost to acquire, over their full relationship	Subscription/repeat-purchase businesses evaluating long-term acquisition economics

Why ROAS overstates performance

ROAS uses revenue, not profit, in the numerator — so a 4:1 ROAS ("$4 back for every $1 spent") sounds great until you remember that only your gross margin percentage of that $4 is actual profit. At a 40% gross margin, $4 of revenue yields $1.60 of gross profit against a $1 spend — a real ROI of 60%, not the 300% the headline ROAS implies.

ROI = (Revenue x Gross Margin % − Marketing Spend) ÷ Marketing Spend

This is the conversion every marketer should be able to do in their head before presenting a ROAS number to finance.

roi_vs_roas_walkthrough.txt

text

Campaign spend:                $20,000
Revenue generated:              $80,000
Gross margin:                       40%

ROAS = Revenue / Spend
     = $80,000 / $20,000
     = 4.0  ("4:1 ROAS" — the number ad platforms report)

Gross profit from that revenue = $80,000 x 40% = $32,000

ROI = (Gross Profit - Spend) / Spend
    = ($32,000 - $20,000) / $20,000
    = $12,000 / $20,000
    = 60%  ("every $1 spent returned $1.60 in profit, a 60% ROI")

A 4:1 ROAS sounds 5x better than a 60% ROI — but they describe
the exact same campaign.

The most common ROI miscalculation: ignoring fully-loaded cost

Marketing spend reported to finance should include more than media cost — agency fees, creative production, the marketing platform's allocated cost, and in some models a share of headcount time. A campaign that looks like a 3:1 ROAS win on media spend alone can turn negative once agency fees and tooling costs are fully loaded in. Always ask whether a reported ROI number includes fully-loaded cost or media cost only.

Payback period: the time dimension ROI ignores

ROI and ROAS are usually calculated over a fixed window (a campaign, a month, a quarter) and say nothing about *when* the business gets its money back. Payback period answers that: how many months of a customer's margin contribution does it take to recover the acquisition cost?

Payback Period (months) = CAC ÷ Monthly Gross Margin per Customer

A business with great LTV:CAC but an 18-month payback period has a real cash-flow problem even though the long-run math works — it needs to fund 18 months of acquisition cost before any single cohort turns cash-positive, which matters enormously for how fast a business can scale its acquisition spend without running out of cash.

LTV:CAC in practice, and its most common distortion

LTV:CAC = Customer Lifetime Value ÷ Customer Acquisition Cost. A widely cited rule of thumb is that a healthy ratio is roughly 3:1 or higher, though the right number depends heavily on payback period and the cost of capital. The most common distortion: calculating LTV using *undiscounted* future revenue over an optimistic assumed lifespan, while calculating CAC using only paid media cost and excluding sales commissions, onboarding cost, or marketing headcount. That mismatch — generous numerator, stingy denominator — is how a genuinely mediocre acquisition channel gets dressed up as a great one in a board deck.

Match the metric to the decision, not the other way around

Use ROAS for a fast same-week channel comparison where everyone already understands it's a top-line number. Use ROI for any decision about whether to keep funding a program. Use LTV:CAC and payback period together for any decision about how aggressively to scale acquisition spend in a subscription or repeat-purchase business — LTV:CAC tells you if it's worth doing, payback period tells you if you can afford to do it at scale.

What's next

ROI measurement tells you whether past spend was worth it. The next lesson covers the planning side — how to set a marketing budget in the first place and allocate it across channels before the money is spent.

Next: Marketing Financial Planning & Budgeting →

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