Fixed vs. Variable Costs

Fixed costs are expenses that remain roughly constant regardless of production or sales volume over a relevant range — rent, salaried headcount, insurance, equipment depreciation. Variable costs change in direct proportion to volume — raw materials, per-unit labor, shipping, payment processing fees. Many costs are semi-variable (mostly fixed with a small variable component) or step-fixed (constant until a threshold, then jump).

Almost every quantitative case problem depends on correctly classifying costs. Breakeven, pricing, make-vs-buy, capacity decisions, and scenario analysis all use a fixed-versus-variable split. Misclassifying a cost leads to the wrong breakeven volume, the wrong decision on a special order, and the wrong recommendation on scaling or cutting capacity.

Total Cost = Fixed Cost + (Variable Cost per Unit x Units) Breakeven Units = Fixed Cost / (Price - Variable Cost per Unit) Average Cost per Unit = Total Cost / Units (falls as volume rises because fixed cost is spread) Contribution Margin = Price - Variable Cost per Unit

Fixed costs are stated as currency per period (e.g., $2M/month). Variable costs are per unit (e.g., $4 per widget) or as a percent of revenue (e.g., 30% of sales). Fixed-cost intensity varies widely: airlines and telecoms have huge fixed bases, marketplaces and agencies are mostly variable.

• Convert fixed costs to variable (outsource, use contractors, pay-per-use cloud)
• Negotiate step-down points in fixed contracts (rent escalators, minimum commitments)
• Reduce variable cost per unit through procurement or process efficiency
• Scale volume to dilute fixed costs over more units
• Right-size fixed base during downturns to match lower throughput

• Profitability: identifying whether a margin drop came from fixed-cost inflation or variable-cost pressure
• Special order: should we accept a discounted order? Only if price exceeds variable cost (fixed costs are sunk)
• Capacity planning: do we need a new plant, or can we flex within existing fixed capacity?
• Pricing: how much can we discount before we stop covering variable costs?

• Cost-volume chart: horizontal line for fixed cost, sloped line for variable cost, steeper total line
• Cost structure stacked bar comparing fixed and variable share by product or business unit
• Breakeven diagram: revenue line crossing total cost line at the breakeven volume

A bottled-water plant has $1.2M/month in fixed costs (rent, plant managers, depreciation). Each bottle costs $0.15 to produce in variable costs (plastic, water, labels, labor) and sells wholesale for $0.45. How many bottles must it sell to break even, and what are profits at 5 million bottles/month?

1. Step 1: Contribution margin per bottle = $0.45 - $0.15 = $0.30
2. Step 2: Breakeven volume = Fixed Costs / CM = $1,200,000 / $0.30 = 4,000,000 bottles/month
3. Step 3: At 5M bottles: Total contribution = 5,000,000 x $0.30 = $1,500,000
4. Step 4: Profit = Total contribution - Fixed costs = $1,500,000 - $1,200,000 = $300,000
5. Step 5: Note that each incremental bottle above 4M adds $0.30 of pure profit — classic high-fixed-cost economics

Answer: Breakeven is 4 million bottles per month. At 5 million bottles the plant earns $300K profit, and every extra bottle beyond 4M drops $0.30 straight to the bottom line.

Treating every cost as variable just because it is on the income statement

Salaries, rent, insurance, and depreciation do not flex with one more unit. Classify them as fixed and watch how margins move with volume.

Ignoring step-fixed behavior

Adding a second shift or a new plant is a discrete jump, not smooth. Model step changes explicitly so you do not underestimate the cost of growth.

Allocating fixed costs per unit and calling it 'cost per unit'

Average fixed cost per unit falls with volume and rises with idle capacity. For decisions on incremental sales, use variable cost only.

Confusing 'direct' with 'variable'

A direct cost is traceable to a product. A variable cost changes with volume. A dedicated plant manager is direct but fixed.

Airlines

Roughly 80% of costs are fixed in the short run (aircraft, crew, airport fees), so filling a marginal seat at any price above fuel and food costs is accretive — the source of deep last-minute discounts.

Professional Services

Consultants' and lawyers' salaries are technically fixed, but utilization lets firms flex by adjusting hiring and layoffs, effectively turning labor into a slow-moving variable cost.

Cloud Software

A huge share of cost is semi-variable (cloud infrastructure scales with users but with minimum commitments), which is why unit economics improve as revenue scales past the fixed base.