MER vs CAC: Which should guide your budget?

MER shows how much store revenue your marketing spend supports. Blended CAC shows what you spend per new customer. Use both before raising your budget, then check whether the customers you acquire leave enough profit.

The MER vs CAC question matters most when the numbers disagree. Returning buyers can keep revenue healthy while acquiring new customers gets more expensive. A steady MER gives you a reason to investigate, rather than automatic permission to spend more.

What do MER and blended CAC tell you?

MER, or marketing efficiency ratio, describes total store revenue relative to marketing spend over the same period. It includes revenue from new and returning customers, including people who arrived through email, search, or direct visits.

Blended customer acquisition cost describes spending per new customer across acquisition sources. A customer placing another order belongs in your revenue total, but does not become another new customer. Leads, signups, and platform purchase conversions are different counts.

Cost scope matters before you compare either number. Some teams use ad spend alone; others include agency fees, creative work, software, and marketing salaries. A figure based on ad spend will look different from one that includes those additional costs.

Label that choice and keep it consistent. Fully loaded CAC includes the broader acquisition costs you have assigned to winning new customers. Paid CAC focuses on customers attributed to paid acquisition; blended CAC includes new customers from other sources too.

Keep the dates consistent as well. Compare spending, revenue, and new customers over the same period, then compare similar trading periods. A promotion can lift revenue while discounts reduce what you keep. A longer buying cycle can also separate the spending from the orders it helps earn.

How much does blended CAC vary?

The observed spread is wide enough to make a single spending target unhelpful. In BTN's 90-day blended CAC benchmark ending November 8, 2026, the median was $31 across 117 stores. The middle half ran from $10 to $92.

This benchmark describes ad spend per new customer, rather than the full cost of marketing. Compare it with a figure using that same cost scope. Use your broader costs when judging whether acquisition is affordable.

An expensive first order can support acquisition when it leaves enough contribution to cover the cost. A cheaper customer can still be unprofitable after discounts, returns, and fulfillment. The benchmark cannot tell you which situation describes your store.

Why can MER hold while acquisition gets worse?

MER can hold steady because repeat purchases support revenue while the number of new buyers falls. That is useful revenue, but it answers a different question from whether your acquisition efforts are improving.

Consider a hypothetical store with unchanged ad spend and total revenue across comparable periods. In the later period, fewer new buyers order, while established customers purchase more. MER stays the same, yet blended CAC rises because the store acquired fewer new customers for the same spending.

The owner should check why new buyers fell before adding budget. Perhaps the offer appeals more to existing customers, or prospecting traffic converts less often. Perhaps a repeat purchase promotion changed the mix. Those are explanations to investigate, rather than conclusions the two metrics prove.

The opposite pattern needs attention too. Falling CAC can look encouraging when a discount brings in more first orders. If those orders leave less contribution or lead to fewer repeat purchases, cheaper acquisition may not improve the business.

What should you check before raising spend?

Start with the definitions, then work toward the budget decision. This order helps you separate a reporting change from a change in customer economics.

Confirm the comparison. Check that the cost categories, dates, revenue basis, and definition of a new customer stayed consistent. Investigate changes in refunds or discounts before treating a better MER as stronger performance.

Look at new and returning business separately. Review new customer counts alongside revenue from returning buyers. When returning revenue rises and new customer counts fall, investigate acquisition before using total revenue to justify expansion.

Check what first orders leave. Account for product costs, shipping, packaging, payment fees, and expected returns before deciding what you can spend. Leave room for overhead and profit. Neither MER nor CAC alone shows that room.

Check the wait for repeat profit. If acquisition relies on later orders, examine what recent customer groups have bought since their first purchase. Use observed behavior rather than assuming new customers will resemble your most loyal buyers. Your cash balance must support the wait until their contribution covers acquisition costs.

Make a limited budget change. Choose a spending amount you can afford to test, with a stopping point tied to margin and cash. Review MER, CAC, and customer contribution together after allowing for your usual buying cycle. Additional spend may acquire customers at a different cost from your historical CAC.

For the wider budget decision, use the Shopify advertising budget guide. In By the Numbers, advertising reporting brings spend, ROAS, CPC, and campaign results together across connected ad platforms. Those campaign details help you investigate where a change may have started.

Keep campaign attribution in its place. ROAS and MER answer different questions, while how to calculate ROAS explains the campaign measure. Neither blended metric identifies which campaign caused an extra sale. A controlled test can help answer that separate question.

How to use these benchmarks

Treat the CAC distribution as a reference for reviewing your own spending assumptions. It does not establish a profitable CAC for your products, your customer mix, or your cash position.

The benchmark does not show MER moving alongside CAC over time. It cannot tell you how often returning buyers hide weaker acquisition, or whether increasing spend will improve results. Use the comparison to ask better questions, then make the budget decision from your own margins and observed payback.

Methodology

These benchmarks use anonymized, aggregated data from Shopify stores in the By the Numbers network. No individual store is identifiable from any figure. Stores that use an analytics app are not a random sample of all Shopify stores, so read the figures as a reference point, not as an average for the whole market.

Keep reading

If you want the full picture on ROAS and attribution, start with the ROAS and attribution guide.

Related reading: UTM naming conventions for a Shopify team, ROAS vs MER: Which Metric to Trust on Shopify and Marketing Attribution Models Explained for Shopify.

By the Numbers builds this into the dashboard. See advertising ROI.