ACoS: How to Calculate Ad Cost of Sales and What It Should Be

When advertising is meant to drive sales, the owner’s key question is simple: what share of revenue goes on advertising, and is it too much? The answer comes from a metric called ACoS — advertising cost of sales, or ad spend as a share of revenue. Let’s look at how to calculate it, how it differs from ROMI and how to use it when planning a budget.

What ACoS is

ACoS shows what percentage of the revenue generated by advertising was spent on the advertising itself:

ACoS = advertising spend / revenue from advertising × 100%

If you spent 2,000 dollars on advertising and the orders it generated brought in 20,000 dollars of revenue, ACoS = 2,000 / 20,000 × 100% = 10%. In other words, every 10 cents of advertising brings in 1 dollar of revenue.

ACoS vs ROMI — what’s the difference

ROMI measures return based on profit rather than revenue:

ROMI = (gross profit from advertising − advertising spend) / advertising spend × 100%

• ACoS is convenient for day-to-day work: revenue is visible straight away, and you don’t need to calculate the cost of goods. That’s why it is popular in e-commerce and retail.
• ROMI gives a more honest picture of whether the business actually made money. Two campaigns with the same ACoS can deliver different profits if they sell products with different margins.

Good practice is to plan the budget by ACoS and review the period’s results by ROMI.

What ACoS counts as normal

There is no universal figure. An acceptable ACoS depends on your margins: advertising shouldn’t eat up the entire mark-up.

A simple rule of thumb: maximum ACoS = margin − target profit.

An example with hypothetical figures: the product’s gross margin is 40%, and the business wants to keep at least 15% of revenue. That means up to 25% of the revenue that advertising brings in can be spent on it. If ACoS rises above 25%, the campaign is losing money relative to plan.

How to calculate ACoS correctly

Include all advertising costs

Costs include not only the budget in ad accounts but also the agency fee, creative production and contractors’ work. Otherwise your ACoS will be understated.

Link revenue to its source

To know how much revenue advertising generated, you need end-to-end attribution: UTM tags in links, goals in your analytics, the source recorded in your CRM — or at the very least, a manager asking “how did you hear about us?”. Without this, ACoS becomes guesswork.

Remember repeat purchases

If a customer keeps coming back after their first purchase, the ACoS on the first order may be high but perfectly reasonable over the customer’s lifetime. For businesses with repeat sales, it is useful to look at ACoS together with LTV — the revenue a customer brings over their entire relationship with you.

How to reduce ACoS

1. Switch off campaigns and placements that generate clicks but not sales.
2. Improve your landing pages: a higher site conversion rate lowers ACoS without increasing the budget.
3. Promote products and services with higher margins.
4. Work on repeat sales: email campaigns, loyalty programmes, retargeting past customers.
5. Regularly reconcile advertising data with actual sales, not just with leads.

ACoS is a simple metric that both a marketer and a finance director can understand. Agree on a target value in advance, and the “is advertising working or not” debate turns into a routine check of the numbers.