If you sell more than one product or service, you have probably asked yourself: which of these should my sales team be pushing this quarter? Revenue alone will not answer that question. A product that brings in the most turnover can still be the one making you the least money once you strip out the costs of actually delivering it. This article shows SME owners and finance managers how to use two simple but powerful tools — contribution margin and margin of safety — to work out which products genuinely deserve your marketing budget, your best staff, and your shelf space.
What contribution margin actually means
Contribution margin is simply what is left from a sale once you have paid the costs that change directly with that sale — things like raw materials, packaging, direct labour on a production line, or transaction fees. It does not include your fixed overheads such as rent, salaried office staff, or software subscriptions, because those costs do not go up or down depending on how many units you sell. The contribution margin is the amount each sale ‘contributes’ towards covering those fixed costs and, once they are covered, towards profit. A high contribution margin ratio tells you how efficiently each pound of revenue converts into money that helps pay the bills.
What margin of safety tells you
Margin of safety answers a different, equally practical question: how far could sales fall before the business stops covering its costs and starts losing money? It is the cushion between your actual (or forecast) sales and your breakeven point — the sales level at which contribution exactly equals fixed costs. A wide margin of safety means you can absorb a bad month, a lost customer, or a price war without tipping into losses. A narrow one means you are one disappointing quarter away from trouble. Together, contribution margin and margin of safety form the backbone of what accountants call cost-volume-profit, or CVP, analysis.
Worked example: three products, one decision
Imagine a small distribution business selling three products: A, B and C. Last month it sold 1,000 units of A at £50 each, 500 units of B at £80 each, and 2,000 units of C at £25 each. Fixed overheads for the month — rent, admin salaries, insurance — came to £15,000. The variable cost per unit (materials, packaging, delivery) was £30 for A, £64 for B, and £10 for C.
| Product | Units sold | Selling price | Variable cost per unit | Contribution per unit | Contribution margin ratio |
|---|---|---|---|---|---|
| A | 1,000 | £50 | £30 | £20 | 40% |
| B | 500 | £80 | £64 | £16 | 20% |
| C | 2,000 | £25 | £10 | £15 | 60% |
On revenue alone, Product B looks impressive at £80 a unit — nearly double the price of A and more than three times that of C. But once variable costs are stripped out, B is actually the weakest performer on margin efficiency, returning only 20p of contribution for every £1 of sales (a 20% contribution margin ratio). Although its unit contribution of £16 is slightly higher than Product C’s £15, Product C delivers 60p of contribution for every £1 of revenue — making it by far the most efficient converter of sales revenue into profit.
| Total revenue (A + B + C) | £140,000 |
| Total variable costs (A + B + C) | £82,000 |
| Total contribution margin | £58,000 |
| Overall contribution margin ratio (58,000 / 140,000) | 41.4% |
| Fixed costs for the month | £15,000 |
| Breakeven revenue (15,000 / 41.4%) | £36,200 (approx.) |
| Margin of safety (£140,000 − £36,200) | £103,800 |
| Margin of safety as a percentage of sales | 74.1% |
This tells the business two things. First, it only needs about £36,200 of sales a month to cover its fixed costs — comfortably below the £140,000 it is actually achieving, giving a healthy margin of safety of 74%. Second, and more useful for planning, if the sales team could shift the revenue mix towards more Product C and less Product B, the overall contribution margin ratio would rise, breakeven revenue would fall further, and the margin of safety would widen even without generating an extra pound of revenue overall.
| Account | Dr | Cr |
|---|---|---|
| Cash at bank | £2,500 | |
| Sales revenue — Product C | £2,500 |
Records the sale of 100 units of Product C at £25 each, recognising revenue in the management accounts.
| Account | Dr | Cr |
|---|---|---|
| Cost of sales (variable) — Product C | £1,000 | |
| Inventory — Product C | £1,000 |
Records the variable cost of the 100 units sold (£10 each), isolating it from fixed overheads so the contribution margin of £1,500 is visible in internal reporting.
Contribution margin is a management reporting concept, not a statutory one — it will not appear as a line on your published profit and loss account. But building it into your internal reports, even as a simple extra column next to each product, is one of the cheapest and most powerful upgrades you can make to your management information.
Do not let fixed cost allocations distort this decision. Many SMEs spread rent, admin salaries, or software costs across products using arbitrary rules — say, by revenue share — and then conclude that a product is ‘unprofitable’ once its share of overheads is deducted. For CVP and product-mix decisions, ignore allocated fixed costs entirely and focus only on contribution. A product with a strong contribution margin is helping the business regardless of how you happen to divide up the rent.
Turning the numbers into a decision
Once you know each product’s contribution margin, the question of what to push becomes far clearer. When allocating marketing budgets or targeting revenue growth where supply is unconstrained, you should generally push the product with the highest contribution margin ratio — in this example, Product C. Every additional pound of sales in Product C brings 60p towards fixed costs and profit, compared with just 20p for Product B. If individual sales effort is the bottleneck per unit sold, Product A leads with the highest absolute contribution per unit (£20 versus £16 for B and £15 for C). If you face a specific production bottleneck — machine hours, warehouse space, or skilled labour — the calculation shifts: you want the highest contribution per unit of that scarce resource. But the underlying discipline is the same: strip out variable costs, look at what is genuinely left over, and let that number — not the headline selling price — guide where you focus.
This same thinking also helps with pricing and discounting decisions. If a customer asks for a 10% discount on Product B, you now know that reduces the contribution per unit from £16 to £8 — the deal has to be genuinely worth it in volume or relationship terms, because you have effectively halved what that sale contributes to covering the rent. Running this calculation before agreeing to discounts, rather than after, avoids a lot of quietly eroded profitability.
- Calculate contribution margin (selling price minus variable cost) for every product or service line, not just overall gross margin — the differences between products are often larger than expected.
- Use the contribution margin ratio, not the selling price or unit volume, to decide which products to prioritise in sales targets, promotions, and marketing spend.
- Recalculate your breakeven point and margin of safety whenever your sales mix, pricing, or fixed costs change significantly, so you always know how much of a buffer you have.
- Never let arbitrary fixed cost allocations creep into product-level profitability decisions — keep contribution analysis clean and based on true variable costs only.
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