Most brands optimise to a metric the platform hands them. ROAS, MER, cost per purchase. All of them sit inside the ad account and none of them reconcile with the P&L. The number we actually run accounts against lives in the business, not the platform.
It is 30-day LTGP : CAC. Gross profit generated from a customer's first purchase within 30 days, divided by the cost to acquire that customer. One ratio that tells you, in absolute terms, whether acquisition is making or losing money. Not whether it improved on last week. Whether it works.
Why gross profit, not revenue.
Revenue is the wrong numerator because revenue is not yours to keep. Before a dollar of acquired revenue can pay for marketing, it has to clear the cost of actually delivering the order. That is the part most CAC conversations skip, and it is the part that decides whether the spend is solvent.
Gross profit on a first order is what survives after the costs that scale with each sale: cost of goods, payment processing, and the shipping and fulfilment to get the product out the door. Strip those out and most eCommerce brands are left with somewhere in the region of 60 to 70 percent of revenue as gross margin. That is the pool that has to fund acquisition, every other cost, and profit. Use revenue instead of gross profit and you will overstate what you can afford to pay for a customer, often badly.
If you build your CAC ceiling off revenue instead of gross profit, you are acquiring customers you cannot actually afford to acquire.
Why 30 days.
Because a window you can see is worth more than an LTV you have to believe. Lifetime value models are useful, but they are a forecast, and forecasts are where wishful acquisition hides. The 30-day window forces the question into the open: is the first order, plus whatever repeats inside a month, enough to justify the cost of acquisition?
If the answer only works once you extend the horizon to twelve months, that is a legitimate model, but it is a different and more demanding one. It needs strong, proven repeat economics and the cash to fund the gap. Most brands assume they are in that model when they are not. The 30-day read stops them finding out the expensive way.
The ranges.
The ratio is only useful if you know what good looks like. These are the bands we run against.
First-order economics are negative. The spend is borrowing against an LTV assumption that may never arrive. Usually a margin or offer problem, not a media one.
Marginally profitable on first order, but constrained. Pricing, creative, or structure is leaving room. Find the constraint before you scale spend.
The right place for most acquisition-led brands. Profitable on first order with room to invest. The job is to hold here while spend climbs.
Acquisition is over-efficient, which usually means money is being left on the table. The brand could likely absorb more spend without breaking the economics.
Note the top band, because it is the one operators misread. A number above 3 feels like winning. More often it is a brand under-spending into a profitable position out of caution, leaving growth on the table that the economics would happily support.
How to actually use it.
The ratio is a diagnostic, not a dashboard ornament. It does two jobs. It tells you whether to scale, and when it breaks, it tells you where to look.
- It sets the scaling decision. In the healthy band, the question is how hard you can push while holding the ratio, not whether to push at all. Below it, more spend just loses money faster.
- It points at the real constraint. A ratio under 1 is rarely a media problem. It is usually margin, price, or offer. A ratio that is healthy but cannot scale is usually a structure or creative problem. The number tells you which conversation to have.
- It avoids the blended trap. Judge acquisition on new-customer economics. Let returning-customer revenue into the numerator and you will flatter the read until the spend ladder breaks something.
It is the metric that reconciles the ad account with the P&L.
Platform metrics tell you how the account is doing on its own terms. LTGP : CAC tells you whether the business is actually better off. When the two disagree, the business wins.
So if you take one thing away from this
Stop running acquisition off a number the platform invented to make itself look good. Build the ratio that ties spend to gross profit, judge it against the ranges, and use it to decide whether the constraint in front of you is media, margin, structure, or offer.
Less than 1 is losing money. 1 to 2 is weak. 2 to 3 is the target. Above 3 and you are probably leaving growth on the table. That is the whole framework. The discipline is in actually running the account against it.