Get the definitions right before you judge the result
Customer acquisition cost, or CAC, answers what it costs your business to acquire a new paying customer. For a fully loaded view, include the sales and marketing costs associated with acquisition: media, people, creative, fees and relevant systems. Allocate shared costs consistently. Divide that amount by new customers—not leads, appointments or all orders.
Marketing efficiency ratio, or MER, gives you a broader view. Here, I use your business's net revenue divided by total marketing spend for the same reporting period. That spend includes the team, tools, production and outside support as well as advertising. A media-only ratio is useful too; label it so your team knows the difference.
Read the two numbers together
MER includes revenue from existing customers, referrals and past marketing. A strong month does not prove your latest campaign caused every sale. CAC focuses on acquisition, but an attractive CAC can still buy customers who return products, cancel or require expensive support.
Ask your team to show three things beside these ratios: contribution after variable delivery costs, the proportion of customers who are new, and when cash actually arrives. Revenue, signed contracts and payments collected belong on separate lines. Shopify, for example, separates sales reporting from payments reporting; your internal review should make that distinction clear too. [1]
The useful question is not whether your ratio looks impressive. It is whether the next dollar you spend is likely to leave your business stronger.
Work backward from what you keep
Suppose your business retains 40% of revenue after the variable costs of delivering the sale, before marketing. On $100,000 of revenue, that leaves $40,000 to cover marketing, fixed overhead and profit. If marketing costs $25,000, your MER is 4.0 and $15,000 remains before those fixed costs. This is an illustration, not a target.
The contribution break-even MER is 1 divided by that pre-marketing contribution margin: 1 ÷ 0.40 = 2.5. At 2.5, marketing consumes all of the contribution in this example. There is nothing left for fixed overhead, financing or tax. Your actual business break-even requires those costs too. Count each cost once, and use the same revenue and cost scope throughout.
Give your results time to develop
A customer may click today, buy next month and pay later. Google documents that conversion delay can make recent CPA look worse and ROAS look lower before additional conversions are recorded. [2] Do not compare an unfinished week with a fully developed month as if they are equally complete.
I want to understand your current cash position and see customers grouped by when they first bought, followed through cancellations and repeat business. That helps you see whether a lower CAC represents better acquisition or simply a different mix of customers.
Make your 2027 budget a set of decisions
Before increasing your budget, model what happens if acquisition costs rise, your margin falls or customers take longer to pay. Then identify the condition that would change your decision. You might expand a campaign when mature customer cohorts cover acquisition costs within a cash window your business can support.
Use the marketing efficiency calculator to test your own numbers. I would rather see a defensible spending limit and a clear review date than a borrowed claim that every business should achieve a particular MER or CAC.
Sources & context
Checked October 7, 2026. I use these sources for the facts and definitions noted above. The questions and suggested actions reflect my perspective; adapt them to your business.
- Shopify: Finance reports
Sales, payments and gross-profit reporting are distinct; reporting definitions should be reconciled before comparing performance.
- Google Ads: Conversion lag reporting
Explains how delayed conversions affect recent CPA and ROAS. Formulas and numerical scenarios in this article are educational calculations, not industry benchmarks.