Marginal cost is the cost of producing one additional unit of output. It drives pricing decisions, production-volume choices, and the point at which adding more output stops being profitable.
Marginal cost = change in total cost / change in quantity. If producing 100 units costs $10,000 and producing 101 units costs $10,080, the marginal cost of the 101st unit is $80. This number is more decision-relevant than average cost because it tells you the true economics of the next unit — not the blended historical economics. In classical economics, marginal cost eventually rises as production scales (diminishing returns). For many digital products and SaaS businesses, marginal cost is near zero — serving the 10,001st customer costs almost nothing incremental. This creates different scaling economics than physical-product businesses. For SMB lending purposes, understanding marginal cost helps with: (1) break-even analysis on loan-funded capacity expansion — does the revenue from additional capacity exceed its marginal cost? (2) pricing decisions — pricing above marginal cost but below average cost can still contribute to fixed cost coverage. (3) evaluating whether a new product line, location, or channel is worth funding. Marginal cost analysis pairs with contribution margin analysis. Contribution margin = revenue - variable costs (which approximates marginal cost for pricing decisions). Any unit priced above marginal cost contributes to fixed cost coverage, even if the total business isn't profitable yet.
Average cost = total cost / quantity. Marginal cost = change in total cost / change in quantity. Average cost blends past fixed and variable costs; marginal cost isolates the economics of the next unit. A business should produce as long as marginal cost < marginal revenue (price), even if average cost is still above price, because each additional unit still covers its own costs.
When borrowing to fund expansion, the loan creates fixed debt service costs. For the expansion to pay for itself, the revenue generated by the additional capacity must exceed both the marginal cost of the new output and the loan payments. Lenders look for this logic in projections — 'we're borrowing $200K to expand capacity by 30%, which will generate $80K/year in additional contribution margin against $40K/year debt service.'
In the short run, marginal cost approximates variable cost because fixed costs don't change with volume. Over longer time horizons, some 'fixed' costs become variable (adding a production line, hiring a manager), so marginal cost analysis should adjust for these step-function costs when modeling significant expansion.