The model in one line
Break-even units = fixed costs ÷ (price − variable cost).
The denominator is the contribution — what each unit gives you towards covering the fixed costs. Everything interesting follows from it.
The case the tool refuses to answer
If price is at or below variable cost, contribution is zero or negative and there is no break-even point at any volume. This is worth stating explicitly because a spreadsheet will happily return a large number or a negative one, and someone will read it as a target.
Selling below variable cost can be deliberate — a loss leader, a launch price — but it must be a decision, not the output of a formula nobody checked.
Sensitivity is the real lesson
Try raising the price by 10% and watch the break-even volume fall by far more than 10%. Contribution is a difference of two numbers, so a small change in either moves it disproportionately. That is why pricing usually beats cost-cutting, and why a small discount is more expensive than it looks.
Questions
What counts as a fixed cost?+
Anything you pay regardless of how much you sell: rent, salaries, software subscriptions, insurance. Variable costs scale with each unit — materials, packaging, payment fees, shipping. Getting a cost in the wrong column moves the break-even point substantially.
What if the contribution is negative?+
Then there is no break-even point, and the tool says so instead of showing a number. If each unit sells for less than it costs to produce, selling more increases the loss. Volume cannot fix a negative contribution — only price or cost can.
Does this include tax?+
No. It is a unit-economics model: fixed costs, price and variable cost. Tax, financing and one-off costs sit outside it.
Why is contribution margin the number to watch?+
Because it tells you what share of each additional sale becomes profit once fixed costs are covered. A 40% contribution margin means every unit past break-even adds 40% of its price straight to the bottom line.