The break-even formula
Break-even units = fixed costs / (price - variable cost per unit). The gap between price and variable cost is the contribution margin: what each sale contributes toward rent, salaries and other fixed costs. Every sale after break-even adds its full contribution margin to profit.
Fixed costs stay the same whatever you sell (rent, insurance, software, salaried staff). Variable costs rise with each sale (materials, shipping, card fees, commissions).