Math you can follow
ShopMathLab uses deterministic formulas with explicit inputs. Every calculator displays its formula, an example and the assumptions that affect its answer. Change an input and the result updates immediately in your browser.
Precision and rounding
Calculations use full JavaScript numeric precision. Currency and percentages are rounded only for display. Reorder points round up to the next whole unit. Displayed totals may differ by a cent from the sum of rounded components. Marketplace invoices may use different line-level rounding.
Revenue, tax and returns
Pass-through sales tax is excluded from seller revenue. Where a platform charges fees on tax, the relevant calculator includes a separate tax input in the fee basis. Expected return loss is return probability multiplied by the unrecovered cost of a return; avoid duplicating losses already reflected in net revenue.
Profit has a specific meaning
Results generally describe contribution profit before fixed overhead and income taxes. You can allocate additional expenses using the other-cost fields. Break-even advertising calculations assume that variable cost behavior remains constant as sales scale.
Marketplace fee updates
Marketplace baselines are stored in versioned configuration separately from the interface. The fee-source log states what was verified and what remains an unverified baseline. Rates are editable because plans, categories, payment methods and individual seller terms differ. Official sources take precedence over an estimate.
Worked example
With $100 net order revenue and $60 in non-ad variable costs, contribution margin is $40, contribution margin percentage is 40%, break-even ROAS is 2.50× and maximum CPA is $40. At a 4× ROAS, advertising costs $25 and contribution profit is $15.
Limitations
These tools model the inputs you provide. They do not connect to your store, infer tax obligations, classify customs codes or retrieve live marketplace quotes. Review estimates against actual invoices and seek qualified advice for accounting, tax or legal decisions.