What Is Contribution Margin?
Contribution margin (CM) is the amount of revenue remaining after all variable costs have been deducted. It represents the "contribution" each unit of sale makes towards covering your fixed costs (rent, salaries, software subscriptions) and generating net profit.
Contribution Margin Formula
Contribution Margin = Revenue − Variable Costs
Or expressed as a ratio:
CM Ratio = Contribution Margin ÷ Revenue
Example: A product sells for £90. Variable costs total £51 (COGS £38, fulfillment £8, payment fee £2.61, return allowance £2.39). Contribution margin = £90 − £51 = £39. CM ratio = £39 ÷ £90 = 43.3%.
That £39 per unit sold is what builds toward covering your rent, team costs, and everything else — before a penny of net profit is made.
Contribution Margin vs Gross Margin
These terms are often used interchangeably — incorrectly. There's an important distinction:
| Metric | What It Deducts | Best Used For |
|---|---|---|
| Gross Margin | COGS (which may include fixed manufacturing overhead allocations) | Financial reporting, industry benchmarking |
| Contribution Margin | All variable costs only (COGS + fulfillment + fees + returns) | Per-unit decisions, break-even analysis, ad bid setting |
For e-commerce brands buying finished goods from suppliers, COGS is essentially fully variable — each unit purchased costs the same regardless of how many you buy at typical scale. In this case, gross margin and the COGS component of contribution margin are similar.
The difference appears in the additional variable costs that gross margin often excludes: payment processing fees (2.5–3.5% of revenue), per-order fulfillment costs, and return-rate adjustments.
Which to Use for Ad Bidding?
Always use contribution margin for advertising bid decisions. Gross margin overstates what you actually have available for ad spend because it ignores fulfillment and fee costs that are real and unavoidable per order.
What Counts as Variable Costs?
Variable costs are costs that change in direct proportion to the number of orders. Fixed costs are the same regardless of volume.
| Cost Type | Classification | Typical Range |
|---|---|---|
| COGS (purchase price + landed costs) | Variable | 40–70% of revenue |
| Pick, pack, and ship labor (per order) | Variable | £2–£8 per order |
| Packaging materials | Variable | £0.50–£3 per order |
| Outbound shipping | Variable | £2.50–£9 per order |
| Payment processing fees | Variable | 2.5–3.5% of revenue |
| Returns processing (blended) | Variable | 1–8% of revenue |
| Marketplace fees (if applicable) | Variable | 8–15% of revenue |
| Rent / warehouse fixed costs | Fixed | Not in CM calculation |
| Salaries (fixed headcount) | Fixed | Not in CM calculation |
| Software subscriptions | Fixed | Not in CM calculation |
Note: some costs are "semi-variable" — they're largely fixed but step up at volume thresholds. Warehouse rent, for example, is fixed until you outgrow your current space. For contribution margin purposes, treat semi-variable costs as fixed unless you're modelling a significant volume change.
Contribution Margin Ratio
The CM ratio (also called the contribution margin percentage) expresses contribution margin as a percentage of revenue. It tells you: for every £1 of revenue, how many pence contribute to fixed costs and profit?
CM Ratio Examples by Sector
Budget fashion brand (high COGS, high returns):
Revenue £80, variable costs £58 → CM £22 → CM ratio 27.5%
Premium beauty brand (strong margin, low returns):
Revenue £65, variable costs £28 → CM £37 → CM ratio 56.9%
Electronics retailer (tight margin, low returns):
Revenue £250, variable costs £215 → CM £35 → CM ratio 14%
The CM ratio is your most useful planning number. A 35% CM ratio means for every £100,000 in revenue, you have £35,000 to cover fixed costs and profit. If fixed costs are £25,000/month, you break even at £71,400 revenue (£25,000 ÷ 0.35).
Using Contribution Margin for Break-Even Analysis
Break-even analysis tells you how much revenue you need to cover all costs and make zero profit. Contribution margin is the core input.
Break-Even Formulas
Break-Even Revenue = Fixed Costs ÷ CM Ratio
Break-Even Units = Fixed Costs ÷ CM per Unit
Example: £30,000 monthly fixed costs, CM ratio 38%
Break-even revenue = £30,000 ÷ 0.38 = £78,947/month
Or per unit: CM per unit £38, fixed costs £30,000
Break-even units = £30,000 ÷ £38 = 789 units/month
This analysis is invaluable when making decisions like: should we invest in a larger warehouse? hire another team member? launch a new product range? Each decision adds to fixed costs — break-even analysis tells you how much extra revenue the decision requires to pay for itself.
Ad Spend Is a Variable Cost — Include It
When calculating break-even for advertising campaigns, include ad spend in your variable costs. The break-even analysis should account for the ad spend needed to generate the revenue, not just the product costs.
Per-Product Contribution Margin for Ad Bidding Decisions
Contribution margin is most powerful when calculated at the individual product (SKU) level. This is what enables rational, profit-based ad bidding decisions.
For Google Shopping specifically, per-product CM determines your maximum affordable ad spend per sale:
WORKED EXAMPLE: Setting Ad Spend from CM
Product: Running trainers, £120 sale price
- COGS: £52
- Fulfillment (pick, pack, ship): £7.50
- Payment processing (2.9%): £3.48
- Return allowance (12% × £15): £1.80
Total variable costs: £64.78
Contribution margin per unit: £120 − £64.78 = £55.22
CM ratio: 46.0%
Target: 12% net profit contribution after ads
Max ad spend per sale: £55.22 − (£120 × 0.12) = £55.22 − £14.40 = £40.82
Target ROAS: £120 ÷ £40.82 = 2.94×
This per-product calculation means you know exactly what ROAS to target for this specific product. Run this for every SKU in your catalogue and you have a complete, rational bid strategy — not a guessed, averaged one.
Contribution Margin Income Statement
A traditional P&L groups costs by function (cost of sales, operating expenses). A contribution margin income statement groups them by behaviour (variable vs fixed), making it far more useful for decision-making.
| Line Item | Amount | % of Revenue |
|---|---|---|
| Revenue | £500,000 | 100% |
| COGS (variable) | (£210,000) | 42% |
| Fulfillment costs (variable) | (£40,000) | 8% |
| Payment processing fees (variable) | (£15,000) | 3% |
| Returns (variable) | (£18,000) | 3.6% |
| Total Variable Costs | (£283,000) | 56.6% |
| Contribution Margin | £217,000 | 43.4% |
| Ad spend (variable) | (£75,000) | 15% |
| Contribution After Ads | £142,000 | 28.4% |
| Rent & warehouse (fixed) | (£18,000) | 3.6% |
| Salaries (fixed) | (£45,000) | 9% |
| Software & tools (fixed) | (£8,000) | 1.6% |
| Net Profit | £71,000 | 14.2% |
This format makes clear exactly where margin is being consumed and what levers are available. Note that ad spend is separated as a variable cost — this allows you to model the impact of changing ad budget directly on contribution margin.
Frequently Asked Questions
What is contribution margin?
Contribution margin is revenue minus all variable costs. It represents how much each sale "contributes" to covering your fixed costs and then generating profit. Unlike gross margin, it excludes fixed cost allocations.
How is contribution margin different from gross margin?
Gross margin typically deducts COGS (which may include fixed manufacturing overhead). Contribution margin deducts only variable costs — costs that change directly with each sale, like COGS, fulfillment, payment fees, and returns.
What is the contribution margin ratio?
Contribution margin ratio = Contribution Margin ÷ Revenue. If a £100 product has £42 contribution margin, the ratio is 42%. This tells you the percentage of each pound of revenue that contributes to fixed costs and profit.
How do I use contribution margin for Google Shopping bids?
Use per-product contribution margin to set your maximum allowable ad spend per sale. Your tROAS target = 1 ÷ (Contribution Margin Ratio − Target Net Profit %). This ensures bids are calibrated to what each product actually generates after variable costs.
What is a contribution margin income statement?
A contribution margin income statement separates variable costs from fixed costs, showing: Revenue − Variable Costs = Contribution Margin − Fixed Costs = Net Profit. This format is more useful for decision-making than a traditional P&L.
Next Steps
Contribution margin analysis gives you the foundation for profit-based bidding. Once you have per-product CM figures, you can set accurate ROAS targets for every SKU — automatically, at scale.
Automate Per-Product Margin Calculations
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