What are variable costs?
Variable costs are expenses that change in direct proportion to output or sales volume — raw materials, direct labour, payment-processing fees, cloud infrastructure that scales with users, sales commissions tied to revenue. Variable costs are the building blocks of COGS and contribution-margin analysis.
Variable costs in different business models
- Manufacturing: materials, direct labour, energy used in production.
- SaaS: hosting (compute, storage, bandwidth), customer-support staff that scales with users, third-party API calls, payment-processing fees.
- Marketplaces: payment processing, transaction-based partner economics, fraud prevention.
- E-commerce: COGS of goods sold, fulfilment, shipping, payment processing, returns.
Variable costs vs. fixed costs
- Variable: rise and fall with volume; per-unit cost relatively constant.
- Fixed: insensitive to volume in the short run; per-unit cost falls as volume rises.
- The variable-to-fixed mix determines operating leverage and break-even sensitivity.
Contribution margin
Variable costs feed the contribution-margin metric: Contribution Margin = Revenue − Variable Costs. The contribution margin is what is left to cover fixed costs and then deliver profit. Pricing decisions, especially discounts, hinge on what they do to contribution per unit — not gross margin alone.
SaaS-specific watch-outs
- Cloud costs (AWS, GCP) are increasingly variable with usage — what looked fixed in 2018 is now usage-priced.
- AI inference costs scale with usage and can dominate variable cost for AI-native products.
- Payment processing (Stripe, Adyen) is a steady ~2-3% variable cost on revenue.
Do: instrument variable cost by customer cohort and product line; understand how it shifts with scale and product mix.
Don’t: assume variable cost per unit is constant indefinitely — AI inference, cloud egress and support load all shift with scale.