CAC payback (months) = CAC ÷ monthly contribution per customer. Back to GlowKart. CAC is ₹800 and contribution ≈ ₹100/month. That comes from three orders a year × CM2 of ₹425 ≈ ₹1,275/yr ≈ ₹106/month, which you can call ₹100. So payback ≈ 8 months. For sales-led SaaS, the aggregate version of the same idea is S&M spend of the prior period ÷ (new ARR added × gross margin), in months. Its reciprocal cousin, the "magic number", turns up in SaaS metrics.
How do investors judge CAC payback?
Of all the metrics in this family, payback is the one professional investors trust most, and there are three reasons.
It's nearly assumption-free. There's no lifetime forecast and no discount rate to argue about. You're only asking when the acquisition loan gets repaid.
It's the cash metric. Every month of payback is a month the company finances its own growth. An identity follows from that, and it killed a generation of 2021-vintage startups. A company growing fast holds an inventory of not-yet-repaid CAC roughly equal to monthly acquisition spend × payback months ÷ 2, because its cohorts are at different stages of repayment. Spend ₹0.8 cr/month acquiring at an 8-month payback and you permanently float ~₹3.2 cr of unrecovered CAC. Double growth and the float doubles. Stretch payback to 24 months and it triples. That's great LTV/CAC on paper and financing dependence in fact, and it turns fatal the year capital markets close. In 2022, companies with identical LTV/CAC but 8- vs 30-month paybacks met very different fates.
It self-corrects for churn risk. Money back in 8 months barely cares whether year-5 retention materialises. A 36-month payback is a leveraged bet on a retention curve nobody has observed yet.
Archetype thresholds are rules of thumb, so calibrate them to the sub-sector and the cycle. For SaaS, < 12 months is great, 12–24 is workable, and > 24 needs cheap capital and elite retention. Consumer subscription wants < 6–12 months. Churn is higher there, so the repayment window has to be shorter. In e-commerce the gold standard is first-order profitability, meaning CM2 of order #1 ≥ CAC, so payback ≈ 0. Payback within 2–3 orders is workable if, and only if, the cohort curve proves those orders happen.
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What does CAC payback miss?
What can't payback see? Everything after repayment. A 6-month-payback customer who churns at month 9 built you nothing. Payback looked great, and LTV/CAC was ~1×. An 18-month-payback enterprise customer who stays a decade is a fortune. Payback was mediocre, and LTV/CAC was 6×. So use the pair, always. Payback tells you about cash reality and survival, and LTV/CAC tells you about wealth creation. One metric of each, properly computed, beats any four metrics from a pitch deck.