Understanding cost behaviour helps a business estimate how much it needs to sell and how changes in volume affect profit. The same cost can behave differently depending on the time period and business model.
What are fixed costs?
Fixed costs generally do not change directly with each additional unit sold within a relevant range and period. Examples may include rent, a fixed software subscription or a salaried role. Fixed does not mean permanent: costs can change when contracts renew or capacity expands.
What are variable costs?
Variable costs tend to rise or fall with activity. Examples may include raw materials, packaging, sales commissions or per-transaction fulfilment fees. Some costs are mixed, with a fixed base plus a usage-based component.
Why the distinction matters
Break-even analysis uses fixed costs and contribution per unit to estimate the sales volume needed to cover costs. If variable cost per unit rises, contribution falls and the break-even volume generally increases.
Use consistent periods
Do not compare monthly fixed costs with annual sales volume. Convert costs and expected sales to the same time period and use a realistic sales mix.
Review assumptions regularly
Supplier prices, staffing, rent, shipping and payment fees can change. Revisit the model when prices, products or operating conditions change.
A quick checklist before you act
- Write down the assumptions and the period covered by each number.
- Check whether important costs, fees, taxes or risks have been left out.
- Test a more cautious scenario, not only the optimistic one.
- Revisit the calculation when your circumstances or the underlying numbers change.
Common mistakes to avoid
- Assuming every monthly bill is fixed.
- Mixing weekly, monthly and annual figures in one calculation.
- Forgetting that some costs are mixed or increase in steps.
Key takeaways
- Fixed costs are not directly driven by each unit within a relevant range.
- Variable costs usually move with activity or units sold.
- Some costs are mixed or step-fixed.
- Use consistent periods in break-even calculations.
Useful calculators
Editorial note: This is general educational content. Verify local rules, product terms, tax treatment and current figures with relevant official sources. Last updated: November 2026. Please check linked primary sources before relying on this guide.
Further reading and primary sources
- U.S. Small Business Administration — Break-even point calculator
- U.S. Small Business Administration — Plan your business
Sources are provided as starting points for further research. Some links describe U.S. rules or institutions and should not be treated as legal, tax or regulatory guidance for other countries. Check the relevant authority in your own jurisdiction.