The Ecomm Analyst

Blended CAC is total marketing spend divided by the number of new customers acquired in the same period. Every dollar of marketing goes in the numerator, whether or not any platform claims credit for it, and only first time buyers go in the denominator. That is the entire formula. The difficulty is not the arithmetic. It is agreeing on what counts as marketing spend and what counts as a new customer.

What belongs in the numerator?

Media spend across every paid channel, obviously. Beyond that, I include agency retainers, freelance creative and production costs, influencer fees and gifted product at cost, affiliate commissions, and any marketing software that exists to acquire customers rather than serve them.

What I leave out is retention spend where I can cleanly separate it, plus salaries for in house staff. The salary question is genuinely contested. Some operators include a fully loaded marketing payroll and get a CAC that reflects the true cost of growth. Others exclude it so the number moves with media decisions rather than hiring decisions. Either is defensible. What matters is picking one and never quietly switching, because a CAC that improved 15 percent because someone reclassified a salary line is worse than no CAC at all.

What belongs in the denominator?

New customers, not orders. This is where most calculations quietly go wrong. If you divide total spend by total orders, your CAC improves automatically every month your repeat base grows, even if acquisition is getting more expensive. That is a metric that flatters you precisely when you need honesty.

Pull first time buyers from your commerce platform rather than from an ad platform. Shopify counts a customer as new on their first order, which is the definition you want. Watch for guest checkout creating duplicate customer records across separate emails, because that inflates your new customer count and understates CAC. On stores with heavy guest checkout, I have seen the gap run 5 to 8 percent.

What time period should you use?

Monthly is the default, and it is fine for most stores. The weakness is that spend and the resulting customers do not always land in the same month, particularly for considered purchases with long research cycles. If your average time from first touch to first order runs past a couple of weeks, monthly blended CAC will look volatile at period boundaries.

The practical fix is a trailing calculation. I usually look at a rolling 90 day blended CAC alongside the monthly figure. The monthly number catches sharp changes. The rolling number tells you whether the trend is real. When they disagree, the rolling number is almost always the one to believe.

Why does blended CAC differ from platform reported CAC?

Because platform reported CAC is calculated on attributed conversions, and attribution is generous. Add up the customers Meta, Google, TikTok, and your email tool each claim, and the total routinely exceeds the number of actual new customers your store recorded. Every one of those channel CAC figures is therefore too low.

Blended CAC has no such problem. It reconciles against your bank account and your customer table, which is exactly why it is the number I trust for board conversations and cash planning. Channel CAC has its uses for allocation, but it answers a different question, and I wrote about why I stopped leaning on it in why I stopped looking at channel level CAC.

A worked example

Say a store spent $84,000 in March. That breaks down as $58,000 in paid media, $12,000 in agency fees, $6,500 in creative production, $5,200 in affiliate and influencer payouts, and $2,300 in marketing software. Shopify recorded 1,940 first time buyers that month.

Blended CAC is $84,000 divided by 1,940, or $43.30. If the same operator had used media spend only and total orders instead of new customers, say 2,760 orders, they would have reported $21.01. That is less than half the real cost, and every downstream decision built on it would be wrong in the same direction.

What to do with the number once you have it

Compare it to contribution profit per new customer, not to revenue and not to a benchmark. Track the ratio of blended CAC to first order contribution over time. If that ratio is drifting upward while spend grows, you are buying worse customers at higher prices, which is the most common way a profitable store becomes an unprofitable one without anyone noticing for two quarters.

Write down your definitions once, in a document someone else can read, and recalculate the same way every month. Consistency is worth more here than precision.