Methodology

How EcomGauge calculators are built

A calculator is only worth using if you can see what it assumes. This page describes the rules every tool on this site follows, and how each one is checked before it is published.

Contribution first

Every calculator starts from the same figure: contribution per order. That is the order value minus the costs that scale with it — product, seller-paid shipping, percentage fees and other per-order variable costs. Fixed costs are handled separately, because they do not change when one more order arrives, and mixing the two is the most common way a per-order calculation goes wrong.

Using one shared definition is also why the tools agree with each other: the contribution margin shown by one calculator is the same number the next one starts from.

Published formulas

Each tool shows the exact formula it evaluates, in the same notation used in the code. Nothing is hidden behind a proprietary model, because there is no model — these are arithmetic relationships that you should be able to reproduce in a spreadsheet and get the same answer.

Stated assumptions and boundaries

Every calculator lists the costs it includes, the costs it excludes and the assumptions it makes. When a value is deliberately left out — refunds, tax, lifetime value — it is named rather than quietly folded into a default. A number you cannot audit is worse than no number.

How results are verified

• Calculation logic lives in its own module, separate from the interface, so it can be tested directly.
• Each formula has an automated test suite covering a profitable case, an exact break-even case, zero contribution, negative contribution, percentage fees and invalid or empty input.
• Worked examples in the written content are generated by the same engine as the calculator, so the article and the tool cannot disagree.
• Edge cases resolve to a plain-language explanation rather than a number. You will never see Infinity, NaN or a divide-by-zero result on this site.

Fee rates change — check yours

Payment providers, marketplaces and store platforms revise their fee schedules, and rates differ by country, card type, plan and sales channel. The calculators use the rate you enter rather than a built-in table, precisely so that a stale number in our code can never quietly distort your result.

Confirm your current rate against a recent statement or your provider's published pricing before relying on an output. The same applies to shipping tariffs and supplier pricing.

Review dates

Every calculator carries the date its formula and copy were last reviewed. Formulas are re-checked when the underlying commercial reality changes — a payment provider changing its fee structure, for instance — and the date is updated when that happens.

Site-wide review date: 2026-08-20.

Your numbers stay with you

All calculations run locally in your browser. Calculator inputs are not intentionally stored, transmitted or logged by this site, no account is required, and nothing is saved between visits. You can disconnect from the network after the page loads and the calculators still work. See the privacy policy for the full detail, including the standard technical logs any hosting provider keeps.

Who writes and reviews this

Calculators and guides are written and maintained by the people who run this site, and each page carries the date it was last reviewed. No claim is made to accounting, tax or financial credentials, and no external reviewer, certification or partnership is implied. If a named reviewer with relevant expertise is added later, they will be named explicitly on the pages they review.

Corrections

If a formula here is wrong, it should be fixed and the review date updated. Reports of calculation errors are treated as the highest-priority kind of feedback — see the contact page.

What these tools are not

Outputs are informational and educational. They are not accounting, tax, investment, legal or financial advice, they do not replace your books, and they are not a substitute for a qualified professional who can see your full circumstances. Every result is arithmetic performed on figures you supply — if an input is wrong, the output will be wrong in a way no calculator can detect. Read the full disclaimer.