CAC vs LTV vs ROAS: which should ecommerce track?

CAC, LTV and ROAS: which should you track in ecommerce?

Track all three, because each answers a different question. ROAS shows how much revenue ads return in the short term, CAC shows the total cost of winning a new customer, and LTV shows the value that customer leaves over time. Use ROAS for day-to-day campaign decisions and the LTV:CAC ratio for budget and growth decisions.

Problems start when decisions are made on just one of them. A brand that only looks at ROAS stops acquiring new customers; a brand that only trusts LTV can acquire customers so expensively that cash flow breaks. This guide shows how to read the three together.

What is ROAS, and what does it not show?

ROAS (return on ad spend) is revenue attributed to ads divided by ad spend: ROAS = ad revenue / ad spend. Automated bidding such as Google Ads' target ROAS strategy optimizes toward this ratio.

ROAS is fast: you can track it daily at campaign, ad set and creative level. Its weakness is that it hides three things:

• Profit: ROAS measures revenue; margin, shipping and returns are not included.
• New versus returning customers: Sales to existing customers can inflate ROAS.
• Future value: Customers with low first-order ROAS who go on to buy again and again stay invisible.

For formulas and common calculation errors, see our ROAS guide; for the profit threshold, see our break-even ROAS article.

What is CAC, and how do you calculate it?

CAC (customer acquisition cost) is the average amount you spend to win one new customer:

CAC = Total marketing and sales cost / Number of new customers acquired

Two distinctions matter:

• Ad spend only, or all costs? The narrow definition uses ad spend only. Fully loaded CAC also includes agency fees, creative production, tools and new-customer discounts.
• All orders, or new customers only? Dividing by total orders lets repeat orders make the cost look lower than it is. The denominator should be first-time customers only.

The "cost per purchase" platforms report is not CAC: platforms count orders from new and existing customers and apply their own attribution rules. Calculate CAC from your store data.

What is LTV, and how do you calculate it?

LTV (customer lifetime value) is the total value a customer leaves over their relationship with your brand. The basic formula, also used by Shopify:

LTV = Average order value x Average purchase frequency per year x Average customer lifespan (years)

That formula is revenue-based, though. For marketing decisions revenue LTV is misleading, because you pay for acquisition out of profit, not revenue. Multiply by contribution margin:

Contribution LTV = Revenue LTV x Contribution margin

Tracking LTV by monthly cohorts (customers who placed their first order in the same month) instead of across the whole customer base gives more reliable results, and shows which periods produced more valuable customers.

CAC vs LTV vs ROAS at a glance

Metric	Formula	Question it answers	Review cadence	Decision it informs
ROAS	Ad revenue / Ad spend	How much revenue did this ad return?	Daily, weekly	Campaign, creative and bid optimization
CAC	Marketing cost / New customers	What does a new customer cost?	Monthly	Channel and budget planning
LTV	AOV x Frequency x Lifespan x Margin	What is a customer worth over time?	Quarterly	How much CAC you can afford
LTV:CAC	LTV / CAC	Is acquisition profitable long term?	Quarterly	Growth pace and investment

How do you interpret the LTV:CAC ratio?

LTV:CAC shows how many times the value a customer leaves exceeds the cost of acquiring them. Shopify's LTV:CAC guide cites around 3:1 as a common reference, noting that ratios near 1:1 suggest overspending on acquisition while very high ratios may signal underinvestment in growth.

Two cautions when reading it:

1. LTV must be contribution-based. A 3:1 ratio on revenue LTV can drop below 1:1 on contribution.
2. Check payback period too. Even with a good ratio, if CAC takes too long to recover, cash flow suffers. Payback period = CAC / monthly contribution per customer.

Worked example

Example: a coffee brand.

• Average order value: $60
• Contribution margin: 40%, so $24 contribution per order
• A customer orders 4 times a year on average and stays for 2 years
• Last month: $12,000 marketing cost, 300 new customers

Calculation:

• CAC = 12,000 / 300 = $40
• Contribution LTV = 24 x 4 x 2 = $192
• LTV:CAC = 192 / 40 = 4.8
• First-order contribution ($24) is below CAC ($40), so the brand loses $16 per customer on the first order.
• Monthly contribution per customer is about 24 x 4 / 12 = $8; payback period is 40 / 8 = 5 months.

This brand loses money on the first order, but because customers come back it recovers acquisition cost in about five months. Someone looking only at first-order ROAS would cut these campaigns; the right call is to grow acquisition as far as cash flow allows.

What mistakes do people make with these metrics?

• Treating platform data as the single truth. Meta and Google can both claim the same order. Track blended ROAS (store revenue divided by total ad spend) alongside platform ROAS.
• Not separating new customers. You cannot calculate CAC without splitting orders into first and repeat purchases in your store data.
• Deciding on revenue LTV. Revenue-based LTV overstates the CAC you can afford, especially in low-margin categories.
• Ignoring returns and cancellations. Both ROAS and LTV are inflated when returned orders are not deducted.
• Changing definitions between periods. Including agency fees in CAC one month and not the next makes trends meaningless.

Which metric should drive which decision?

• Pausing a creative or ad set: ROAS or cost per purchase.
• Setting a campaign's target ROAS: Break-even ROAS and the share of new customers.
• Allocating budget across channels: Channel-level CAC and the repeat purchase rate of customers from each channel.
• Setting growth pace: LTV:CAC ratio and payback period.
• Investing in retention: Cohort LTV; increasing repeat purchases grows LTV and raises the CAC you can afford.

Key takeaways

• ROAS measures short-term revenue efficiency, CAC new-customer cost, LTV customer value.
• Calculate CAC on new customers only, from your store data.
• Calculate LTV on contribution margin and track it by cohort.
• Read LTV:CAC together with payback period.
• Use ROAS for daily optimization and LTV:CAC for growth decisions.

If you would like to calculate your CAC, LTV and target ROAS together using your own data, you can request a free growth analysis from the Performetic team.