A £100,000 revenue month can still be a bad month. If discounting rises, shipping costs drift, returns climb and paid acquisition gets more expensive, top-line growth can quietly drain cash from the business.
That is why an ecommerce profitability metrics guide should start beyond ROAS. Return on ad spend matters, but it cannot tell you whether an order made money, whether a customer is worth acquiring, or whether more ad spend is genuinely scalable. Established retailers need a scorecard that connects media performance to contribution, cash flow and repeat purchase behaviour.
Ecommerce profitability metrics guide: start with contribution
Profitability is not revenue minus ad spend. It is the amount left after every variable cost required to win, fulfil and support an order. That is contribution margin, and it is the number that should shape your paid media targets.
Start with net sales, not gross sales. Remove VAT where appropriate, discounts, refunds and cancellations. Then deduct product cost, payment processing fees, pick-and-pack costs, packaging, shipping subsidies and any marketplace or platform fees directly tied to the sale. The remaining figure is contribution before advertising.
For example, a £100 order may look healthy at first glance. But if the product cost is £35, fulfilment and packaging are £8, payment fees are £3, subsidised delivery costs £5 and returns average £7 per order, contribution before ads is £42. A £30 acquisition cost leaves £12 to cover fixed overheads and profit. A £45 acquisition cost does not.
This is why a single account-wide ROAS target is often misleading. A retailer with a 60% gross margin, low return rate and strong repeat purchase can afford a different cost of acquisition from a retailer selling bulky, low-margin products with free returns. Profit targets have to be built from the commercial reality of the catalogue.
Contribution margin by product and category
Do not stop at a blended business average. Calculate contribution by SKU, category and product bundle where data allows. Your paid campaigns may be sending revenue to products that look popular in Google Ads or Meta reporting but contribute very little after fulfilment and returns.
This does not mean every low-margin product must be excluded from advertising. Some are valuable entry products, some lead to profitable bundles, and some support repeat purchase. The point is to make that decision deliberately. A loss-leading product is a strategy only when you can prove the wider customer economics, not when it is hidden inside a flattering revenue report.
The core metrics that expose profitable growth
A focused dashboard beats a long report full of disconnected platform numbers. These are the metrics that deserve attention each week and each month.
1. Blended ROAS and MER
Platform ROAS measures tracked platform revenue divided by spend in that platform. It is useful for campaign optimisation, but it is not a complete view of performance. Attribution windows, consent choices, cross-device journeys and branded demand can all make platform-reported ROAS look stronger than the underlying business result.
Marketing efficiency ratio, usually called MER, is total revenue divided by total marketing spend. If a business generates £500,000 in net revenue from £100,000 in total marketing spend, MER is 5.0. It gives leadership a clearer view of what the business is spending to generate revenue across all channels.
MER also has limits. It can improve because organic demand rises, an email promotion performs well or stock availability changes. Use it as a board-level efficiency metric, then use channel and campaign data to understand what actually moved it.
2. Customer acquisition cost
Customer acquisition cost, or CAC, is total acquisition spend divided by the number of genuinely new customers acquired. The word new matters. Reporting all orders as if they were new customer wins inflates performance, especially for brands with established repeat demand.
Track new-customer CAC separately from blended CAC. A strong retention programme can make blended figures look excellent while new-customer acquisition is deteriorating. That is not necessarily a problem, but it changes the decision. You may need to protect profitable prospecting rather than simply optimise towards the cheapest reported conversion.
Where possible, distinguish between first-time purchasers and returning purchasers in your reporting. Google Shopping, Performance Max and paid social can all capture demand from existing customers. That revenue has value, but it should not be confused with customer acquisition.
3. Break-even cost of sale
Break-even cost of sale is the highest percentage of net sales you can spend on advertising before contribution is exhausted. If contribution before ads is £42 on a £100 net order, your break-even cost of sale is 42%, equivalent to a break-even ROAS of 2.38.
That figure is a guardrail, not a target. Spending at break-even leaves nothing for salaries, software, warehousing, rent, tax or profit. A sustainable target must sit below it, with enough room for fixed costs and the return the business expects from growth.
Set targets by category when margins vary materially. A single 4x ROAS rule applied across a mixed catalogue can restrict profitable high-margin lines while allowing inefficient spend on low-margin ones. Feed structure, campaign segmentation and product labels should reflect these economics.
4. Contribution after advertising
This is one of the most useful measures for a paid media account: net sales minus all variable costs and attributable advertising spend. It turns a campaign conversation from “How much revenue did it produce?” into “How much did it contribute?”
Monitor both pounds and percentage. A campaign producing £20,000 of contribution is more meaningful than one producing £100,000 of sales at a thin margin. The percentage shows efficiency; the pound figure shows whether the activity is large enough to matter.
Be disciplined about attribution here. You do not need false precision. It is better to use a consistent methodology, review it against backend sales data and make decisions with sensible ranges than to treat a platform’s reported revenue as accounting truth.
5. Repeat purchase rate and customer lifetime value
A first order can be unprofitable if the customer reliably returns and later orders carry strong contribution. That is where repeat purchase rate and customer lifetime value, or LTV, matter.
Calculate LTV from contribution, not revenue. A customer who spends £250 over a year is not automatically worth £250 to the business. Deduct product, fulfilment, support, refund and promotional costs to understand the contribution they generate over the relevant period.
The trade-off is timing. A 12-month LTV may justify higher CAC, but it will not solve an immediate cash-flow problem. Brands should set an acquisition payback period that matches their working capital position. A well-funded subscription business may accept a longer payback period than a retailer buying stock upfront for seasonal demand.
6. Refund rate, return rate and discount rate
These are often treated as ecommerce operations metrics. They are also paid media metrics because they change the true value of the sales your campaigns generate.
Check returns and refunds by product, source and customer cohort. If a campaign drives a disproportionate volume of poor-fit purchases, the initial ROAS can look excellent while the eventual contribution is weak. The cause could be misleading creative, broad targeting, a weak product page or an offer that attracts deal-led customers unlikely to return.
Discount rate deserves the same scrutiny. If paid revenue is growing only because discount depth is increasing, your reported sales can rise while profit falls. Promotions can be commercially sound, particularly for clearance, seasonal stock or customer acquisition, but they must be measured against margin after the discount rather than full-price assumptions.
Build a reporting cadence that leads to decisions
Weekly reporting should focus on direction and action: spend, net revenue, blended efficiency, new-customer CAC, contribution after advertising and major product-level changes. Do not overreact to one weak day or a short attribution delay, particularly with higher-consideration products.
Monthly reporting should go deeper. Compare actual contribution against plan, review cohort quality, returns, discounting, stock availability and the difference between platform reporting and backend results. This is where you decide whether to scale budgets, restructure campaigns, improve feeds, pull back on unprofitable products or fix conversion issues before buying more traffic.
Your reporting stack needs clean inputs. Make sure product costs are current, shipping assumptions reflect reality, refunds are captured, and new versus returning customers are identifiable. If the numbers are unreliable, no amount of campaign optimisation will create a dependable profit target.
Scale only when the economics hold
Scaling paid media is not raising budgets because ROAS looks good for three days. It is increasing spend while contribution remains healthy, stock can support demand, conversion rate is stable and cash can tolerate the acquisition payback period.
Expect efficiency to soften as spend rises. The first £5,000 may capture high-intent demand; the next £20,000 may require broader audiences, more creative testing and more patience. That does not automatically make the additional spend bad. The question is whether the marginal spend still produces enough contribution for your business.
For serious ecommerce brands, the best reporting does not make every channel look successful. It makes the next decision obvious. Measure what remains after the sale, protect the products and campaigns that create real contribution, and let profitable capacity – not vanity metrics – dictate the pace of growth.