What is contribution margin?

Revenue minus cost of goods sold, fulfilment, and returns. The number that tells you whether a sale actually made money. The foundation of POAS bidding.

How do I calculate contribution margin per SKU?

Pull cost data from your ecommerce platform (Shopify, Magento, BigCommerce) covering COGS, fulfilment cost, and average return rate per SKU. Subtract from revenue. The result is the input to BOI® job assignment and POAS bidding.

How do I feed contribution margin into Smart Bidding?

Via custom labels in the product feed and value rules at the campaign or asset group level. The bidder receives margin-adjusted conversion value as the optimisation signal rather than gross revenue.

What's the difference between contribution margin and gross profit?

Gross profit deducts only COGS. Contribution margin deducts COGS plus variable selling costs (fulfilment, returns, payment processing). Contribution margin reflects actual unit economics; gross profit overstates them.

Why does Google Ads default to gross revenue rather than contribution margin?

The platform can only optimise against the signal you feed it. Default conversion value is gross revenue because it's what Shopify, WooCommerce, and other platforms send by default. Contribution margin requires explicit configuration - which is exactly the BOI® implementation work.

Methodology

Contribution margin is the commercial truth

Revenue means nothing if the margin structure does not support it. This framework builds contribution margin into every bidding decision, every report, and every scaling conversation.

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The Margin Waterfall

Five layers between revenue and profit

Most agencies stop at layer one. Your finance team lives at layer five. The gap between these two views is where margin destruction happens invisibly.

Revenue

What Google reports

Platform-attributed revenue. The number that appears in your Google Ads dashboard.

Gross Margin

After COGS

Revenue minus cost of goods sold. The first layer most agencies ignore when setting ROAS targets.

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Contribution Margin 1

After fulfilment

Gross margin minus picking, packing, shipping, packaging, and payment processing fees. This is where many 'profitable' campaigns start to look different.

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Contribution Margin 2

After ad spend

CM1 minus the cost of the ad spend that generated the sale. This is the true number. If CM2 is negative, the campaign is losing money regardless of what ROAS says.

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Net Contribution

After returns

CM2 adjusted for the actual return rate of products sold through that campaign. A 30% return rate can turn a 4x ROAS campaign into a loss-maker.

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Why this changes everything

ROAS hides margin destruction

A 5x ROAS on a product with 30% gross margin, 15% return rate, and £4.50 fulfilment cost per unit can be unprofitable. ROAS does not tell you this. Contribution margin does.

£100 sale at 5x ROAS: £20 ad cost, £70 COGS, £4.50 fulfilment, £2.50 payment fees = -£7 loss per sale.

Bidding without margins is guessing

Smart Bidding optimises for conversion value. It does not know your COGS varies by SKU, your return rate differs by category, or your fulfilment cost changes by product weight. Without margin data, the algorithm optimises blind.

Scaling reveals the truth

At £10k/month, margin inefficiencies are tolerable. At £50k/month, they compound. The brands that scale profitably are the ones who built margin into their bidding logic before they needed to.

Before & After

What changes when margin drives bidding

Before: Revenue-Led

Target metric4x ROAS

Bidding signalRevenue (selling price)

Return rate adjustmentNone

Fulfilment cost awarenessNone

Breakeven definitionVague / account-level

Finance alignmentMonthly reconciliation shows gap

After: Margin-Led

Target metric1.8x CM2 POAS

Bidding signalContribution margin by SKU

Return rate adjustmentCategory-level, updated monthly

Fulfilment cost awarenessBuilt into conversion values

Breakeven definitionPer campaign, per product group

Finance alignmentWeekly reporting matches P&L logic

Implementation

How we build it into your account

Map your margin architecture

Week 1-2

We work with your finance team to map COGS, fulfilment costs, payment fees, and return rates at category or SKU level. This creates the foundation for everything that follows.

Build margin-aware conversion values

Week 2-3

Instead of sending Google your selling price as the conversion value, we send contribution margin. This means Smart Bidding optimises for profit, not revenue.

Set breakeven thresholds by segment

Week 3-4

Every campaign, product group, or asset group gets a defined breakeven point. Below this threshold, spend is paused or reallocated. No more 'give it another week' on losing campaigns.

Layer in return rate adjustments

Ongoing

Products with high return rates get adjusted conversion values that account for the probability of return. A £100 sale on a product with a 25% return rate is valued at £75 in the bidding model.

Report on commercial truth