Explain contribution margin and how you use it.
A core FP&A interview question — asked in analyst and associate interviews across IB, PE, and the Big 4.
THE SHORT ANSWER
Contribution margin is revenue minus variable costs — what each unit, product, or customer 'contributes' toward covering fixed costs and then profit (CM% = CM ÷ revenue). It's central to decision-making because, unlike full/absorption gross margin, it isolates the truly variable economics: breakeven = fixed costs ÷ CM per unit; it drives pricing floors (never price below variable cost with spare capacity), product-mix optimization (push high-CM lines), special-order and make-vs-buy decisions, and operating-leverage analysis (high fixed cost + high CM = profits swing hard with volume). The trap is treating allocated fixed costs as variable — they don't change with one more unit, so they don't belong in a marginal decision. So CM is the right lens for short-term, incremental decisions; absorption costing is for external reporting.
WHAT INTERVIEWERS LISTEN FOR
- ✓CM = revenue − variable costs; CM% = CM/revenue
- ✓Drives breakeven, pricing floors, product mix, special orders
- ✓Links to operating leverage (fixed cost + CM → profit swing)
- ✓Don't treat allocated fixed costs as variable in marginal decisions
COMMON MISTAKES
- ✗Confusing CM with gross/absorption margin
- ✗Including allocated fixed cost in a marginal decision
- ✗Pricing below variable cost with spare capacity
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