Know what you can afford to spend on paid traffic

A £150 sale with £20 of hosting, payment processing and delivery costs attached is not a £150 sale for acquisition purposes, it is a £130 one. Fulfilment cost is money that has already left the business before a single pound goes toward advertising. Planning a budget against turnover rather than gross profit is the single most common way small businesses talk themselves into overspending on paid traffic, because the sale price flatters a number that was never available to spend.

No, and it is worth knowing where it came from. The figure was popularised by David Skok of Matrix Partners in his 2010 “SaaS Metrics 2.0” article on forEntrepreneurs.com, built from studying mature, publicly listed software companies with stable churn and multi-year customer relationships. Skok's own words were that “our guideline for a successful SaaS business is that this number should be higher than 3”, offered as a rule of thumb, not a derivation. Harvard Business School Online has since repeated three as a general benchmark for healthy lifetime economics. A one-person course or download business has none of the recurring-contract stability that number was built on, so treat 3:1 as a compass heading rather than a pass mark. Below 1:1 is a genuine warning sign in any business. Above that, the right ratio depends on how quickly cash needs to come back and how much room there is to reinvest.

Lifetime value is a promise paid out over months or years, but the invoice for acquiring the customer lands today. Stripe and most CAC-payback research treat 12 months or less as a healthy benchmark for recovering acquisition cost in a subscription business, and faster than that is considered strong. A one-person business rarely has 12 months of runway sitting spare, so a customer who takes that long to repay their acquisition cost can strangle cash flow even while the lifetime numbers look fine on paper. The 30-day figure exists to catch that gap before it becomes a real problem, not to replace the lifetime view, but to sit alongside it.

Work backwards from the ratio. If lifetime gross profit is £234 and the target ratio is 3:1, the acquisition cost should not exceed £78 per customer. If one in eight leads converts to a paying customer, that £78 has to cover eight leads, giving a maximum cost per lead of £9.75. That figure is the number worth taking into Meta Ads Manager or Google Ads and comparing against what the platform is actually charging, before a single pound is committed to a campaign.

Paid platforms run on auctions. The first pounds spent usually buy the cheapest, most responsive audience, and every pound after that competes for people who take more convincing. Meta and Google both widen delivery into cooler audiences as daily budgets rise, which is why cost per result tends to drift upward with scale rather than staying flat. Building in a 20% allowance keeps the monthly figure honest about that drift instead of assuming month one's efficiency holds indefinitely.

Take a rolling average across the last 50 to 100 orders rather than a single recent one. A figure built from one unusually cheap or expensive month will throw every number downstream out, from gross profit through to the suggested budget. The calculator is a planning tool built to sit alongside proper management accounts, not replace them.

Run a small test campaign first, ideally enough spend to generate at least 20 to 30 conversions before trusting the resulting cost per customer. Anything smaller than that is too thin a sample to plan a monthly budget around, since a handful of unusually cheap or expensive results can swing the average wildly. Until that data exists, leave the field blank and use the target CAC the calculator works out instead, as the ceiling to test against.

No. Net profit already has overheads, salaries and acquisition spend taken out of it, so using it here would count acquisition cost against itself. Gross profit keeps the acquisition decision separate from the rest of the business, which matters because acquisition and retention pull on profit in very different ways. Frederick Reichheld's research for Bain & Company found that increasing customer retention by just 5% can increase profits by 25% to 95%, depending on the business. Blending net profit into an acquisition calculation buries that distinction instead of letting it inform where effort goes next.

Monthly is sensible for most solo businesses, or sooner after a change to pricing, delivery costs or the channels used to find customers. Acquisition costs on paid platforms rarely stay flat for long, and a target CAC set six months ago against last year's ad costs is not a target worth spending against today.