Analytics
Free LTV to CAC ratio calculator with a capped horizon
By Charles Summers · Updated · Free, no signup
Short answer
Lifetime value is average revenue per account multiplied by gross margin, divided by monthly churn; the ratio is that figure over acquisition cost. This calculator reports it three more ways: capped at 12, 24 and 36 months so you can see how much of the answer is extrapolation, and as the share of lifetime gross profit you are spending to acquire, which is the claim the 3:1 rule of thumb is really making. The rule itself is folklore, not evidence.
Use the ltv to cac ratio calculator
What does this tool actually do?
Lifetime value is average revenue per account multiplied by gross margin, divided by monthly churn; the ratio is that figure over acquisition cost. This calculator reports it three more ways: capped at 12, 24 and 36 months so you can see how much of the answer is extrapolation, and as the share of lifetime gross profit you are spending to acquire, which is the claim the 3:1 rule of thumb is really making.
It runs entirely in your browser. Nothing you type is sent to a server, no account is required, and there is no usage limit, because there is no cost per run to control.
Nobody has ever proved that three to one is the right number
The 3:1 target circulates in every fundraising deck and originates in early software-investing blog writing from the late 2000s onwards, where it was offered as a rule of thumb by people who were explicit that it was a rule of thumb. There is no study establishing three as a threshold, no dataset where companies above it survive and companies below it fail, and no mechanism that would make three special rather than 2.6 or 3.4. It survives because it is memorable and because it is roughly right for a particular kind of business, which is not the same as being true.
What it is actually asserting becomes clearer when you invert it. A ratio of three means you are spending one third of a customer's entire lifetime gross profit to acquire them, leaving two thirds for research and development, general and administrative overhead, and profit. That framing is testable against your own profit and loss in an afternoon: add up everything that is not sales and marketing, express it as a share of gross profit, and see whether a third is genuinely what is left over. For a company running heavy engineering costs it is not, and their ratio target should be higher than three.
The ratio is also comparing two quantities that are not measured in the same units of time. Acquisition cost is money you spend today. Lifetime value is money that arrives over the following three or four years, undiscounted, in a currency of unrealised future subscription payments. At a 10 percent annual cost of capital, gross profit arriving 40 months out is worth about 72 percent of its face value today, so an undiscounted ratio of three is closer to two and a half in the money you actually hold. The discount is rarely applied because it makes the number worse, which is the wrong reason to skip a step.
None of this means the ratio is useless. It means it is a summary statistic with wide error bars and a heavy dependence on assumptions you chose, so it belongs in the same category as a valuation multiple: useful for noticing that something has changed, dangerous as the basis for a decision on its own.
The churn window is the assumption doing most of the work
One divided by monthly churn is only an average lifetime if the churn rate stays where it is for the entire lifetime. Real retention curves do not behave that way. Cancellations concentrate heavily in the first weeks after signup, when people who never activated give up, and the rate then declines as the surviving population becomes progressively more committed. What looks like a constant 4 percent might be 9 percent in month one falling towards 2 percent by month twelve.
That shape breaks the formula in both directions at once. Applied to a book of business dominated by recent signups, a blended rate overstates future churn and understates the value of the customers who will stick. Applied to a mature base it does the opposite. The version of the error that costs real money is measuring churn over a short recent window, three months say, and then projecting it forward for 33. You are extrapolating a rate observed over one quarter across nearly three years, and the confidence interval on that extrapolation is enormous even when the arithmetic is flawless.
The practical repair is a capped horizon, which this tool shows alongside the standard figure. Instead of summing gross profit to infinity, sum it over a fixed number of months: monthly gross profit times one minus survival to the power of the horizon, divided by churn. Capped at 24 months, a business churning 5 percent a month has already banked about 71 percent of its uncapped lifetime value, while one churning 2 percent has banked only 38 percent. That is the counter-intuitive part: the better your retention, the more of your quoted lifetime value depends on years nobody has lived through yet. Cap at whatever your oldest meaningful cohort has actually reached, and you have replaced an assertion about the future with a measurement of the past.
There is one more distortion worth naming, because it is invisible in the arithmetic. If churn is measured only on customers who have passed some minimum tenure, or the earliest cohorts have been dropped as unrepresentative, the surviving sample is biased towards people who were always going to stay. The churn rate falls, lifetime value rises, and nothing about the business has changed except which customers were allowed into the calculation.
Two definitional choices that swing the answer more than the business does
The first is whether gross margin is in the numerator. Lifetime value means lifetime gross profit, not lifetime revenue, because revenue arrives already owing money to hosting, support and payment processing. Omitting the margin term inflates the ratio by the reciprocal of your margin, so a genuine 2.4 shows up as a comfortable 3.1 at 78 percent margin. This is not a subtle modelling difference; it is the single most common reason two people looking at the same company quote different ratios.
The second is which acquisition cost you divide by, and it matters more than most teams expect. Blended acquisition cost spreads total sales and marketing spend across every new customer including the ones who arrived through word of mouth, organic search and existing-customer referral. Paid acquisition cost divides paid spend by paid customers only. If 40 percent of your customers arrive without paid touch, blended cost is 60 percent of paid cost and your blended ratio is 1.67 times your paid ratio. Both numbers are honest; they answer different questions.
Blended has a further property that flatters growing companies. As brand and word of mouth compound, the free share of new customers rises, so blended acquisition cost falls and the ratio improves even if every paid channel is getting steadily worse. A team watching only the blended ratio can spend a year with degrading paid efficiency and rising reported quality of unit economics, and the two facts never meet in the same report.
For allocation decisions neither average is really the right input. What you want is marginal acquisition cost: the cost of the next thousand customers, not the average of the last ten thousand. Channels saturate, audiences get exhausted, and the marginal cost rises well before the average one moves. If your average is 900 dollars and the last increment of spend brought customers in at 1,600, the ratio that governs your next budget decision is the one built on 1,600.
Reading the number, including when it is too high
Below one you are destroying value on every sale and the only question is how fast. Between one and two you are covering acquisition and little else, which is survivable only if the ratio is improving and you can name the reason it is improving. Around three is the conventional comfortable zone. Above five, most experienced operators would tell you to spend more, because a very high ratio usually means demand is going uncaptured while you optimise efficiency, and efficiency is not a goal, it is a constraint on growth.
The asymmetry is worth sitting with. A ratio that is too low kills the company slowly and visibly. A ratio that is too high kills it invisibly, by letting a competitor with a ratio of two and triple the spend take the market while your dashboard shows excellent numbers. The healthiest reading of the metric is as a budget signal in both directions rather than a score to maximise.
It also belongs in a family rather than on its own. Payback in months tells you about cash, which the ratio cannot, because a business with a ratio of four and a 30-month payback will still run out of money. Net revenue retention tells you whether the lifetime figure is likely to grow or shrink from expansion. Burn multiple, net cash burned per unit of net new annual recurring revenue, checks the whole story against the bank statement without requiring anyone to agree on how churn is defined. Four numbers, read together, are much harder to fool than one.
Finally, compute it per segment before you compute it in aggregate. An overall ratio of three built from a self-serve segment at seven and an enterprise segment at 1.2 is a company doing one thing well and one thing badly, and the aggregate recommends more of both. Segment-level ratios usually reverse at least one decision that the blended figure had made look obvious.
Numbers worth knowing
| Metric | Typical | What it means |
|---|---|---|
| The 3:1 rule of thumb | folklore, not evidence | Popularised by early software-investing blog writing as a heuristic. No published dataset establishes three as a threshold, and it is only right for a particular cost structure. |
| What a ratio of 3 actually claims | 33% of lifetime gross profit spent on acquisition | Check it against your own overheads. If engineering and general costs consume more than two thirds of gross profit, your target should be above three. |
| Value-destroying threshold | below 1.0 | Acquisition costs more than the customer will ever contribute in gross profit. Growth accelerates the loss, and reported revenue rises the whole time it is happening. |
| Where a high ratio becomes a warning | above 5.0 | Usually underinvestment rather than excellence. Demand is going uncaptured while a competitor spending at 2:1 takes the market you were being efficient in. |
| Discount on undiscounted lifetime value | about 28% at 40 months, 10% cost of capital | The ratio compares today's cash to money arriving over years. Applying a discount rate is skipped mostly because it makes the number smaller. |
Mistakes that quietly cost you results
- Putting lifetime revenue in the numerator instead of lifetime gross profit
- Revenue is committed to cost of goods before it reaches you. Multiply by gross margin and the ratio drops by the reciprocal of that margin, which turns a comfortable-looking 3.1 into a real 2.4 at 78 percent.
- Projecting a churn rate measured over one quarter across a 30-month lifetime
- Cap the horizon at the age of your oldest meaningful cohort and sum gross profit over that many months instead. A capped figure is a measurement; an infinite sum on three months of data is a guess with a decimal point.
- Reporting the blended ratio as though it described your paid channels
- Blended cost includes customers who arrived free, so it improves automatically as word of mouth compounds. Paid efficiency can degrade for a year while the blended ratio rises, and nobody sees the two facts side by side.
- Budgeting off average acquisition cost when channels are saturating
- The next increment of spend does not buy customers at the historic average. Track marginal cost, the price of the most recent tranche of customers, because it turns upward long before the average notices.
- Treating a ratio of 8 as a success story
- It usually means you are underspending. Efficiency is a constraint, not an objective, and a rival at 2:1 with triple your budget will take the segment while your dashboard looks excellent.
What does the output look like?
This is the exact output the tool produces from the example inputs. It is generated by the same code that runs when you click the button, so what you see here is what you get.
Frequently asked questions
Is the 3:1 rule actually based on anything?
It is a rule of thumb from early software-investing commentary, repeated until it acquired the authority of a finding. No published dataset shows companies above three surviving and companies below it failing. What it implicitly claims is that spending a third of a customer's lifetime gross profit on acquisition leaves enough for everything else, which is a statement about your cost structure, not a universal constant. Check it against your own overheads rather than adopting it.
Why does the tool show LTV capped at 12, 24 and 36 months?
Because the standard formula sums gross profit to infinity, and most companies quoting it have not existed long enough to observe the lifetime they are claiming. The capped figures use the same maths over a fixed window: monthly gross profit times one minus survival to the power of the horizon, divided by churn. Comparing them to the uncapped number tells you exactly how much of your ratio is measurement and how much is extrapolation.
Should I use blended CAC or paid CAC in the ratio?
Both, reported separately. Blended divides all sales and marketing cost by all new customers and answers whether the company as a whole is fundable. Paid divides paid spend by paid customers and answers whether your channels are working. They diverge by the share of customers who arrive without paid touch, so at 40 percent organic the blended ratio is about 1.67 times the paid one, and only the paid one should drive a budget decision.
What if my ratio comes out very high, like 9 to 1?
Treat it as a signal to spend more, not as a win. A very high ratio nearly always means you are capturing far less demand than exists, or that your churn figure is being measured on too favourable a sample. Verify the churn input first, then look at whether increasing acquisition spend by half would still leave you comfortably above three. Underinvestment is the failure mode that does not show up on any dashboard.
Why does gross margin matter so much to the answer?
Because it scales the entire numerator. Lifetime value is lifetime gross profit, and a customer paying 240 dollars a month at a 76 percent margin contributes about 182 dollars of it. Leave the margin term out and every ratio you report is inflated by roughly a third at typical software margins, and considerably more in businesses carrying services delivery or hardware in cost of goods.
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