Understanding how costs behave — how they change (or don’t) as business activity changes — is the foundation of managerial accounting and budgeting. Costs aren’t monolithic; they respond to volume in predictable patterns.
Variable costs change in proportion to activity. If a bakery makes more loaves, it uses more flour. The flour cost per loaf stays roughly constant, but total flour cost increases with volume. Other examples: raw materials, sales commissions, shipping costs per unit.
Fixed costs don’t change with activity — at least not within a relevant range. The bakery’s rent is $3,000/month whether it bakes 100 loaves or 10,000. Other examples: salaries (of employees not paid by the hour), insurance, equipment leases. Fixed costs per unit decrease as volume increases — this is why businesses seek scale.
Mixed costs (semi-variable) have both a fixed and variable component. A delivery truck has a fixed lease payment plus fuel costs that increase with miles driven. A phone plan might have a base fee plus per-minute charges. The formula: Total Cost = Fixed Component + (Variable Rate × Activity Level).
Why it matters:
- Break-even analysis: If fixed costs are 10 after variable costs, the business needs to sell 3,000 units to break even. Below that, it’s losing money; above it, each additional unit adds $10 of profit.
- Budgeting: Variable costs can be budgeted as a rate per unit. Fixed costs are budgeted as a lump sum. This distinction is essential for flexible budgets — adjusting the budget for actual volume.
- Pricing: A price must at minimum cover variable costs per unit, and over enough units, cover fixed costs too. Selling below variable cost loses money on every unit.
- Decision-making: When evaluating whether to accept a special order, take on a new project, or discontinue a product line, the relevant question is often about variable (incremental) costs, not total costs including allocated fixed overhead.
The relevant range: Fixed costs are only “fixed” within a range of activity. If the bakery needs to lease a second oven to handle increased volume, fixed costs step up. Cost behavior analysis assumes the business is operating within a range where the cost relationships hold.