How the numbers are calculated

Every calculator publishes its formula next to the inputs. These tools use ordinary JavaScript number arithmetic and display money to two decimal places. They are planning tools, not an accounting ledger.

Rounding and boundaries

Intermediate results retain their precision. Break-even unit counts round upward to a whole unit, with a small floating-point tolerance at exact integer boundaries. Selling prices designed to meet a margin round upward to cents. Percentage outputs display up to two decimal places. Rounding fees on individual transactions can make actual statements differ from estimates.

Inputs must be finite, non-negative numbers no larger than one trillion. Expected unit counts must be whole numbers. Discounts and fee rates cannot exceed 100%. Selling-price target margin plus percentage fees must be below 100%. The tools reject results too large for reliable display. Blank fields are not silently treated as zero.

Undefined is different from zero

Margin at zero revenue, ROI at zero total spending and hourly profit at zero hours are not defined. They appear as “Not available”. Positive fixed costs cannot be recovered when each sale contributes zero or less. The tool explains this instead of displaying a misleading break-even target.

Cost definitions

Margin divides profit by revenue. Markup divides profit by cost. Contribution subtracts variable costs from revenue and still has to cover fixed costs. Simple ROI divides net gain by total included spending. Currency is a display choice only; there are no exchange-rate conversions.

Method references

Our break-even and contribution methods follow the standard cost-volume-profit relationships described in OpenStax, Principles of Accounting: break-even in units and dollars. The Shopify explanation of profit margin is a useful reference for why different included costs lead to different margin measures. PlainProfit's examples and calculator explanations are written for this site.

Verification and limitations

Automated tests check worked examples, zero and negative outcomes, rounding boundaries and invalid input handling. This does not establish that your forecast is realistic. Demand, risk, capacity, income tax, financing and many other business factors require separate assessment.