How it's worked out

Left over from each sale = price − cost of the sale
Sales to break even = fixed costs ÷ left over from each sale (rounded up)

With the example figures: $10,170 of fixed costs ÷ ($45 − $18) = 376.7, so you need 377 sales a month, or about $16,965 in takings.

Common questions

What's the difference between fixed and variable costs?: Fixed costs stay the same however much you sell: rent, insurance, software, loan repayments and wages for staff who work set hours. Variable costs come with each sale: stock, packaging, card fees and commission.
Should I include my own pay?: Yes, if you want to know when the business also pays you. Add what you want to take each month as a fixed cost.
What's the safety margin?: How far your sales could drop before you start making a loss. The higher it is, the more room you have in a quiet month.