The max allowable
CAC formula, one page

Every DTC operator eventually has the same argument, usually in a meeting about whether to keep scaling spend. "Can we afford a $50 CAC?" And the room splits between the person who feels it is too high and the person who feels it is fine. Both are guessing.

You do not have to guess. The maximum you can pay to acquire a customer is a number you can calculate from two inputs you already have: your contribution margin per order and how long you are willing to wait to earn it back. Then you sanity-check that number against a lifetime-value ceiling so you do not talk yourself into spending your whole margin. That is the whole framework, and it fits on one page.

This builds directly on contribution margin and feeds your payback period, so if those are fuzzy, start there. If they are solid, here is the formula.

A spending ceiling
turns CAC from a
fight into a number.

Max allowable CAC is the highest amount you can pay to acquire a customer while still hitting your economics. It is a ceiling, not a target. You do not have to spend up to it, but you should never spend past it without a deliberate, financed reason.

The value of having the number is that it ends the argument. Instead of debating whether a CAC "feels" sustainable, you compare it to a line you calculated. Under the line, scale. At the line, hold. Over the line, you are knowingly buying growth with cash you will need to finance, and that is a board decision, not a media-buyer decision.

Contribution per order
times the months
of payback you'll accept.

Here is the core formula. Your max allowable CAC equals your contribution margin per order multiplied by the number of months of contribution you are willing to spend, which is just your target payback period.

The intuition is clean. If a customer throws off a known amount of contribution each month, and you are willing to wait, say, six months to get your acquisition cost back, then the most you can spend is six months of that contribution. Want a faster payback? Your ceiling drops. Willing to wait longer? It rises, but so does your cash risk. The target payback you choose is the dial, and it should be set by your category and your cash position, which is exactly the conversation in payback by vertical.

Then cap it against
lifetime value so
you don't go broke.

The payback formula tells you what you can spend to hit a timing target. But you also need a guardrail against the long game, and that is where the LTV:CAC ratio comes in. The common rule of thumb is to keep CAC at or under one third of your gross-margin lifetime value, which is the same as targeting an LTV:CAC ratio of roughly 3:1 or better.

So your true ceiling is the lower of the two: the figure the payback formula produces, and the figure the LTV:CAC cap allows. If payback says you can spend $60 but a 3:1 cap on your gross-margin LTV says $45, your real ceiling is $45. The two numbers protect against different failure modes, payback protects your cash timing, the LTV cap protects your long-run profitability, and you want both. One caution: use a defensible LTV, not the inflated one in your pitch deck, which is the whole point of the LTV math piece.

Two numbers in,
one ceiling out.
Take the lower.

Let me run it with clean, illustrative numbers so you can copy the structure. Say a customer generates $20 of contribution margin per month and you are willing to accept a six-month payback. And say your gross-margin lifetime value works out to $150.

Read what happened. The payback math alone would let you spend $120, six months of $20 contribution. But the LTV cap says one third of $150 is $50. The binding constraint is the LTV ceiling, so your real max allowable CAC is $50, not $120. If you had only run the payback formula, you would have set a ceiling more than twice as high as your lifetime value can actually sustain.

Flip the inputs and the binding constraint flips too. A subscription brand with high LTV but a cash-tight balance sheet might find payback is the tighter limit. That is the point of computing both: you never know in advance which one binds, and the one that binds is the one that matters.

Post the number.
Spend up to it.
Revisit it monthly.

Once you have your ceiling, make it operational. Put the number where your growth team can see it and make it the line every channel is measured against. A channel coming in under the ceiling earns more budget. A channel running over it gets fixed or cut. The decision stops being a debate and becomes a comparison.

Then revisit it on a cadence, monthly is plenty, because the inputs move. Contribution margin shifts as COGS, shipping, and returns change. LTV moves as retention improves or erodes. Your target payback should tighten when cash is scarce and can loosen when it is plentiful. The formula does not change, but the number it produces should breathe with the business. Recompute it from a current per-order P&L, never a stale one, which is the whole reason contribution margin has to be live, not annual.

That is the one page. Contribution per order times your target payback, capped by one third of a defensible LTV, take the lower number, post it, spend to it, revisit it. It is the difference between a growth team that argues about CAC and one that knows its line. To keep the inputs honest, pair this with a real CM3 and payback benchmarked to your category.