ROAS — Return on Ad Spend — has become one of the most quoted metrics in ecommerce marketing. It's clean, it's simple, and it gives you an immediate sense of whether your ads are "working." But used in isolation, it's one of the most misleading numbers in your dashboard.

I've seen businesses scale campaigns aggressively on the back of a strong ROAS figure, only to find that profit margins were being eroded at the same time. Others have killed channels that were actually profitable because their ROAS looked weak against an arbitrary benchmark.

"A high ROAS doesn't mean you're making money. It means your ads are generating revenue. Those are not the same thing."

What ROAS Actually Tells You

ROAS tells you how much revenue you generated for every pound spent on advertising. A 4x ROAS means for every £1 you spent, you brought in £4 of revenue. Simple enough.

The problem is that revenue isn't profit. Before you've made a penny, you need to account for:

  • Cost of goods sold (COGS)
  • Fulfilment and shipping costs
  • Returns and refunds
  • Platform fees and payment processing
  • Overheads allocated to that revenue

A product with a 40% gross margin and a 4x ROAS might be barely breaking even once you account for all of the above. Meanwhile a product with a 70% margin at 2.5x ROAS could be highly profitable.

MER Marketing Efficiency Ratio Total revenue ÷ total marketing spend. A far more honest view of overall paid performance.

The Break-Even ROAS Calculation

The first number every ecommerce business should know is their break-even ROAS — the point at which ad spend neither makes nor loses money.

The formula is straightforward:

The Formula

Break-Even ROAS = 1 ÷ Gross Margin %

If your gross margin is 40%, your break-even ROAS is 1 ÷ 0.40 = 2.5x. Anything below that and your ads are losing money at the gross margin level — before overheads.

This number changes everything. A 3x ROAS at 40% margin is barely profitable. A 3x ROAS at 65% margin is very healthy. Without knowing your break-even point, ROAS targets are essentially guesswork.

What to Use Instead

ROAS still has its place — it's a useful signal at the campaign and ad set level for relative performance. But it should never be the primary metric by which you judge whether paid media is working. Here's what to use alongside it:

  • MER (Marketing Efficiency Ratio) — total revenue divided by total marketing spend. Gives you a blended view across all channels
  • nCAC (New Customer Acquisition Cost) — what it actually costs to bring in a net new customer, separated from returning customer revenue
  • Contribution Margin per Order — revenue minus variable costs per transaction. The truest measure of whether a sale was worth making
  • LTV:CAC ratio — are you acquiring customers at a price that makes sense given what they're worth over time?
Key Takeaways
  • Always calculate your break-even ROAS before setting targets
  • ROAS varies significantly by product margin — set targets per product category
  • Use MER for a blended view of total paid media efficiency
  • Separate new customer acquisition cost from returning customer revenue
  • Contribution margin per order is the most honest measure of ad profitability

The goal of paid media isn't a strong ROAS number — it's profitable, scalable customer acquisition. Keep your eye on that, and ROAS becomes one useful signal among many rather than the metric everything is judged against.