Calculation Methodology

Start with the scenario and your own terms

Selected Stripe and PayPal Level B scenarios use confirmed published rates and conditions, with site half-up rounding. Square remains custom-only. Custom scenario labels describe your payment context. PayPal Checkout, goods-and-services and invoice contexts stay separate; invoices require a payment method. Square contexts distinguish in-person, online and manual-entry channels and require plan/account terms.

Exact money and explicit rounding

USD inputs are parsed into integer cents, with at most two decimal places. Percentage inputs use exact integer units with up to six decimal places. Signs, exponent notation, grouping commas and extra precision are rejected rather than silently corrected.

The custom model adds percentage charges on the same gross amount, rounds the combined percentage fee once to cents, then adds exact fixed charges. Choose half-up (half-cent ties round up) or half-even (ties round to the nearest even cent). Neither convention is verified provider policy. Displayed component percentage amounts are shown before rounding, not separately rounded fees.

Net = gross − total fees. Effective rate = total fees ÷ gross × 100, displayed to two decimal places. Zero gross has no defined effective rate. A zero-amount transaction still carries any fixed charge you enter; if fees exceed gross, the tool reports an error instead of clamping net to zero.

Minimum-gross reverse calculations

The engine finds the smallest whole-cent gross amount whose rounded net meets your target. Because supported combined rates are below 100% and percentages round once, fees increase by at most one cent when gross increases by a cent. Net is therefore monotonic for this model.

A bounded binary search finds the first qualifying amount, then verifies the result and the preceding cent. One cent less must miss the target. Gross → net → reverse may produce a smaller gross than the original because rounding can create equal-net plateaus.

Synthetic examples, tested against the engine

These use a made-up custom rate of 3% plus $0.30; they are not Stripe, PayPal or Square prices. Half-up rounding is used.

• Transaction: $100.00 gross → $3.30 fee → $96.70 net.
• Reverse: a $100.00 net target requires $103.40 gross, with a $3.40 fee. One cent less misses the target.
• Monthly: 100 transactions averaging $100.00, plus $20.00 recurring cost → $10,000.00 gross, $350.00 total costs and $9,650.00 estimated net.

Monthly estimates are not statement reconstructions

Monthly mode assumes every transaction equals the entered average ticket. It rounds each assumed transaction fee, multiplies by the count and adds the monthly fixed cost once. A real mix of payment amounts can produce different fees despite the same average and gross. With zero transactions, no per-transaction charges apply; recurring costs still apply.

For example, at a synthetic 25% rate with no fixed fee and half-up rounding, 2 assumed $0.02 payments incur $0.02 total fees. Actual payments of $0.01 and $0.03 have the same gross but incur $0.01 total fees. Monthly mode does not offer reverse calculation.

Limits and unsupported fee rules

Each entered monetary amount and estimated monthly gross is limited to USD 1 billion. Monthly count is limited to 1,000,000 whole transactions. Combined percentage rates must be below 100%. An unreachable reverse target reports the gross limit rather than returning an approximation.

The model does not include currency conversion, refunds, disputes, tax, automatic interchange tables, cascading percentages, separately rounded provider components, minimum/capped/tiered charges or thresholds. Entering a custom rate does not verify that it matches your actual provider agreement.

Official evidence is still required

Evidence Level A requires documented fees, applicability and provider calculation behavior with independent policy review. No Level A preset is enabled. Level B confirms published rates and conditions but uses an explicit site convention: round the percentage of gross once to nearest cents, half-cent ties upward, then add fixed cents. Provider rounding and fee order remain undocumented; actual charges may differ. Independent reference calculations check arithmetic, not provider policy. Level C stays disabled when applicability, plan mapping or unsupported behavior is unresolved.

Five Level B estimates are available: Stripe domestic online cards; PayPal domestic Checkout; Send/Receive Goods & Services; invoice via PayPal Checkout; and invoice via standard credit/debit card. Each exposes its US/USD restrictions, exclusions and official sources. Goods & Services uses the published percentage-only row, with no inherited Checkout fixed fee. International transactions, conversion, special account terms and additional products are excluded. Effective dates remain unset.

Square plan-column mapping is unresolved in retrieved official HTML, so no Square preset is enabled. Stripe international and FX records remain disabled. Custom rates remain available for estimates under your own assumptions.

Editorial responsibility and source review

This independently operated personal project is responsible for its explanations and corrections. It has no claimed editorial team, professional certification or independent provider-policy auditor. Arithmetic reference checks are separate implementations, not proof of undocumented provider policy.

Review begins with official public pricing for the relevant country, currency, payment method, channel and plan. Percentage and fixed components, additions and exclusions are recorded separately from site rounding assumptions. Unresolved applicability keeps a preset disabled; confirmed published rates may support a clearly labeled estimate only within its reviewed scope.

The recorded pricing/applicability review for the current evidence is 2026-10-13; this is not a new verification or a fee effective date. Each enabled scenario links to its official source and shows its recorded review date. Provider rounding and fee order remain unverified; Level B uses site half-up percentage rounding once, followed by fixed cents. Actual charges can differ.

When reviewing a source change or correction, preserve its URL, applicable conditions and actual review evidence, record the change, and check the related explanation and worked examples against the tests before release. A source effective date is recorded only when documented. No automatic refresh or fixed review schedule is claimed. A passing arithmetic test does not establish current provider pricing.

Once a verified contact channel is configured, corrections can be reported with the page, scenario and supporting source or reproducible example. Avoid private payment or account information. See the contact status and correction guidance. Useful guides remain unpublished until source, example and editorial review and human publication approval are complete.