The Retention Math Every Founder Should Know: LTV, Churn, and Repeat Rate
You cannot manage what you model wrong. The four numbers that determine whether a loyalty investment pays back — and the traps in each of them.
Loyalty conversations go wrong when they run on vibes — "engagement," "delight," "community." Getting retention right comes down to four numbers. None of them are complicated; all of them are easy to compute wrong.
1. Repeat purchase rate (RPR)
The share of customers who buy a second time. Repeat purchase rate varies enormously by category, so the number that matters is the direction of your own, not a borrowed benchmark. It is the single most honest indicator of product-market fit for retention, because no incentive program can rescue a product nobody wants twice.
2. Churn — measured on a cohort, not a blend
Blended churn hides everything. If you acquired heavily last month, your "average" churn looks great while every cohort is quietly leaking. Always read churn as: of customers acquired in month X, how many were still active in month X+n? Plot three cohorts and you will learn more than from a year of blended dashboards.
3. LTV — with margin, not revenue
The first LTV inflation to check: using revenue instead of contribution margin. A $300 revenue LTV at 25% margin is a $75 customer. If acquisition costs $60, you are running a very tight boat while your dashboard celebrates. Loyalty rewards come out of that margin too — a 2% earn rate on a 25%-margin business consumes 8% of your profit pool.
4. Payback window
How long until a cohort's cumulative margin covers its acquisition cost. Under 6 months and you can reinvest aggressively; over 18 and growth is financed on hope. Retention is a direct lever on this number, because every extra order lands inside an already-paid-for relationship.
The uncomfortable conclusion
A loyalty program is a margin reallocation: you tax every transaction to change future behavior. It pays back only if the incremental orders it creates exceed the discounts it hands to customers who would have returned anyway. Estimating that incrementality — not the point balance, not the signup count — is the entire game, and it is why every serious program needs a holdout group from day one.
If you want to run these numbers against your own figures, the retention calculator does the arithmetic above, including what one percentage point of retention is actually worth to you.
The four numbers on one business
Take a store with 1,000 customers acquired in January, an average order value of $80, and a 25% contribution margin after payment fees, fulfilment and returns.
Repeat purchase rate: 240 of them buy a second time, so RPR is 24%. That sits inside the normal band — nothing here is broken, and nothing is exciting either.
Cohort churn: by month six, 180 of the original 1,000 are still buying. Read that as a cohort retained at 18% after six months, not as "82% churn", which sounds like a crisis and describes nothing you can act on.
LTV: those repeat customers average 3.2 orders at $80. Revenue LTV reads $256. Contribution LTV is $64. If acquisition cost $45, the business works — barely, and only if that margin number is honest.
Payback: at $20 of contribution per order, $45 of acquisition cost clears after roughly two and a quarter orders. On a six-week buying interval that lands in month four, which is inside the range where reinvesting is defensible.
Now add a loyalty program earning 2% of spend. That is $1.60 per $80 order against $20 of contribution — 8% of the margin pool, charged on every repeat order including the ones that needed no incentive at all. The program has to create enough additional orders to cover that, which is the entire reason the holdout exists.
Frequently asked questions
How many cohorts do I need before cohort churn means anything?
The constraint is cohort size, not cohort count. A cohort of forty customers moves several points of retention when two people leave, so the noise is larger than the signal you are looking for. Plot three consecutive cohorts and read the shape rather than the decimal. If your monthly volume is small enough that single-digit churn swings the line, widen the cohort to a quarter instead of reading noise as a trend.
Gross margin or contribution margin in the LTV calculation?
Contribution: gross margin minus the costs that scale with an order — payment fees, fulfilment, shipping subsidy, returns, and the reward itself. Gross margin flatters LTV precisely because it hides the costs that grow with every extra order a loyalty program buys you. If contribution margin is not available yet, run the number both ways and treat the gap between them as the size of your uncertainty rather than picking the friendlier one.
How large does the holdout need to be?
Large enough to detect the effect you would act on, which depends on your base rate and that effect — not on a fixed percentage. Work backwards: if repeat rate is 25% and you would only keep the program for a three-point lift, the holdout has to be big enough for three points to be distinguishable from ordinary variation. If that sample exceeds your monthly volume, run the holdout for longer rather than shrinking it, and accept that you are measuring a quarter rather than a month.