How to calculate e-commerce contribution margin and CAC payback

Revenue growth on its own does not tell you whether growth is affordable. An online store can double its sales in a quarter and end that quarter with less cash than it started, because every extra order carried discounting, carriage, payment fees and advertising spend that together cost more than the order contributed.

Two calculations answer the affordability question: contribution margin per order, and how long a cohort of customers takes to repay what it cost to acquire them. This article works through both, line by line, with a fictional brand and figures you can follow. The numbers are illustrative only — they are not benchmarks, and they are not drawn from any survey or industry average.

Gross margin and contribution margin are not the same thing

Gross margin is net revenue less the cost of the goods sold. It is a useful buying and pricing measure, and it is what most accounting systems report by default. What it excludes is everything else that varies with the order: picking, packing and packaging materials, the net cost of outbound delivery after any delivery charge the customer pays, card and wallet processing fees, and marketplace commission.

Contribution margin includes those costs. It answers a narrower and more useful question: after fulfilling one more order, how much money is left to pay for marketing, overheads and profit? A brand with a healthy gross margin and a thin contribution margin is usually one that has never separated the two.

Definitions of contribution margin vary between businesses, advisers and software packages — some stop before marketing, some deduct acquisition cost, some treat fulfilment labour as overhead. There is no single universal definition. This article uses one clearly defined operating convention, set out line by line below, and applies it consistently. What matters more than which convention you adopt is that you state it, and use the same one every month.

Building contribution margin, line by line

Start at gross revenue — the full ticket price before anything is deducted — and work down. The order of the lines matters, because percentages calculated on gross revenue and on net revenue are different numbers and are easily confused.

1. Gross revenue, excluding VAT. Mixing VAT-inclusive and VAT-exclusive figures is a common arithmetic error in this calculation.
2. Less discounts and promotions — codes, sitewide sales, bundle pricing, welcome offers.
3. Less returns and refunds, measured as an expected rate across the period rather than order by order.
4. Equals net revenue. Every percentage below is expressed against this line.
5. Less landed product cost: unit cost plus inbound freight, duty and any import handling, not the supplier invoice alone.
6. Less variable fulfilment: pick and pack labour and packaging materials only. Outbound delivery is deliberately excluded from this line so that it is counted once, in the line below.
7. Less shipping subsidy: the cost of outbound delivery charged by your carrier, less any delivery income collected from the customer. Where delivery is free or capped, the whole (or most) of the carrier cost falls here; where the customer pays full carriage, this line can be nil.
8. Less payment processing: card, wallet and buy-now-pay-later fees, including fixed per-transaction elements.
9. Less marketplace or channel fees: referral commission, fulfilment fees, storage and any advertising billed by the platform as a fee.
10. Equals contribution before marketing.

Customer acquisition cost: what belongs in it

Customer acquisition cost is the money spent to win a new customer, divided by the number of new customers won. The judgement sits in the numerator. Media spend clearly belongs. Agency retainers, creative production and affiliate commission usually do too, because they scale with the acquisition effort. Salaries of a permanent in-house team are a matter of policy — include them if you want a fully loaded figure, exclude them if you want a marginal one, but be consistent and say which you have done.

Blended CAC divides total acquisition spend by all new customers, including those who arrived organically. Channel-level CAC divides one channel's spend by the new customers that channel is credited with. The two answer different questions: blended CAC tells you what growth costs overall, channel CAC tells you where the next pound should go. Using blended CAC to judge a single channel flatters paid channels, because organic customers subsidise the average.

Contribution after marketing

Subtracting CAC from first-order contribution gives contribution after marketing on the first order. In the example above, contribution before marketing is £31.80. If the brand's own-site CAC is £42.00, the first order loses £10.20. That is not automatically a problem — it is only a problem if customers do not come back.

Where repeat purchasing is meaningful, it changes the answer

This section applies only to businesses where customers realistically buy again — consumables, replenishable products, subscriptions and similar. Where a product is genuinely one-off or bought once every several years, first-order contribution has to stand on its own and cohort payback beyond month one is largely academic.

Where repeat purchasing is meaningful, a second order carries no acquisition cost, so its contribution falls straight against the original CAC. That arithmetic means an increase in the proportion of customers who order again shortens payback, and in some businesses it moves payback more than a comparable change in first-order margin would. Whether that is true of your business, and whether it is cheaper to achieve than a margin improvement, depends on your own retention data and costs — it should be measured, not assumed.

Payback by cohort

A cohort is the group of customers acquired in one period. Track their cumulative contribution month by month and compare it with what they cost to acquire. Payback is the month in which cumulative contribution first exceeds CAC. Cohorts matter because a blended average mixes customers acquired under different offers, at different prices and through different channels, and can hide a deteriorating trend for months.

This cohort repays its acquisition cost during month four. Whether that is comfortable depends entirely on the brand's own circumstances — how long inventory sits before it sells, what payment terms suppliers give, and how the gap is funded. There is no universal payback threshold that makes scaling safe, and any figure quoted as one should be treated with caution.

Payback is not the same as cash breakeven

Contribution payback ignores the working-capital cycle. Stock is usually bought, shipped and paid for well before the customer orders it, and advertising is typically settled monthly while repeat revenue arrives over quarters. A cohort that repays in four months on a contribution basis can still consume cash for considerably longer once inventory purchases and supplier terms are included.

That gap is a cash-timing question rather than a margin one, and it is best answered with a short-horizon forecast — see our guide to the 13-week cash flow forecast.

Comparing routes to market on the same basis

Run the same build-up for each route to market. These are acquisition segments rather than mutually exclusive businesses: the same brand can sell on its own site to customers who arrive organically or directly, sell the same product through a marketplace, and sell on its own site to customers acquired through paid social. Segmenting this way keeps like with like, because the difference between the columns is how the order was won and where it was transacted, not what was sold.

The cost structures differ in shape, not only in size. A marketplace charges referral commission and fulfilment fees, and separately recorded CAC for that route may look low because platform discovery is partly paid for through those fees rather than through media spend. Own-site sales avoid commission but carry any acquisition cost directly, and that cost is visible as media spend when the route is paid social.

The comparison is genuinely useful, but it misleads in three ways worth naming. First, the marketplace column's low recorded CAC is not free acquisition — part of the cost of being found sits in the commission line above it, so the two lines should be read together. Second, direct retention of marketplace customers can be constrained, because the customer relationship and the contact data sit with the platform rather than with the brand. Third, shared overheads are excluded throughout, so no column shows profit, and channel CAC rests on attribution. Reporting this consistently each month is the work behind channel and KPI reporting for e-commerce brands, and the same discipline applied across a whole business is profitability and margin analysis.

Common mistakes

• Mixing gross and net revenue between lines, or mixing VAT-inclusive and VAT-exclusive figures, so the percentages do not reconcile.
• Leaving returns out of the calculation, or netting them off at the year end only, which overstates contribution all year.
• Treating marketplace commission and fulfilment fees as overhead rather than variable cost of the order.
• Using blended CAC to assess a single paid channel.
• Ignoring the shipping subsidy created by a free-delivery threshold, which can quietly consume several percentage points of net revenue.
• Reading payback as cash breakeven, and being surprised by the inventory funding requirement.
• Reconciling from platform dashboards rather than from settled bank and payout data — see ecommerce accountants in the UK on marketplace reconciliation and inventory accounting.

A practical test you can run this month

Take last month's orders. Build the contribution table above once, for the business as a whole, using actual settled figures rather than platform estimates. Then build it again for your largest route to market alone. If the two contribution percentages differ by an amount that is material in the context of your business, your blended view has been hiding something, and that is the number worth investigating before any budget decision.

If you would like this built and maintained monthly rather than assembled in a spreadsheet each quarter, that is what our e-commerce CFO services cover.