How our estimates are calculated

What this page explains. Every estimate on Margin Ruler is produced by a deterministic formula applied to your inputs and to source-dated published industry data. This page describes how the numbers are computed, what data they are based on, and what they do and do not represent. If you came here from an estimate and want to know "where did that number come from," you are in the right place.

The short version. For each calculator, a deterministic formula takes your inputs and source-dated industry rates (marketplace fees, carrier base rates, payment-processor fees, storage cost-per-cube-per-month) to produce a planning estimate. The calculator uses source-dated Amazon US marketplace/FBA fee logic unless a local calibration is explicitly shown. No machine learning, no black-box model — the formulas and the rate-card constants live in the open JavaScript served to your browser.

Each of the six calculators in the catalog applies industry rate cards and published midpoints to inputs you enter. Fees change; the formulas are reviewed against the most recent published rate cards and midpoints at the time of each release:

• FBA profit — Amazon Seller Central referral-fee logic and simplified fulfillment-fee midpoint tables, plus your COGS and inbound shipping.
• Return cost — labor-per-return, restocking/resalability, return shipping, and non-refunded processor fees.
• Margin leak, shipping overpay, break-even ad spend, inventory carrying — each anchored to published seller-cost benchmarks with documented assumptions in the source code. The formulas are deterministic and reviewable.

The calculators run entirely in your browser. The inputs you enter are not sent to our servers and are not used for advertising, profiling, or tracking. See our privacy policy for details on what is and is not collected.

Fees, ad CPCs, carrier rates, and payment-processor fees change — often quarterly, sometimes mid-quarter. The calculators use published rate cards and industry midpoints as defaults. Your actual figures will differ based on your tier, your negotiated rates, your category, and your specific contracts. Use the calculator as a planning baseline; re-run with your actual last-30-day figures before making a binding decision.

Is: a deterministic point estimate based on your inputs and source-dated published rate cards, meant to help you compare levers (pricing, inventory, shipping, ads) before committing to tooling or policy changes.

Is not: a quote, a contract, a binding figure, an accounting statement, or a guarantee. Margin Ruler does not provide services directly and is not a party to any transaction between you and a vendor or platform.

Why the output can shift quickly. E-commerce seller economics are sensitive to small changes — a 1% fee change, a 2% return-rate change, or a 5% CPC shift can move the outcome meaningfully. Re-run the calculator any time your inputs change.

The formulas and data sources are in the open: the calculation functions live in the publicly-served JavaScript that runs in your browser. If you want to see how a specific number is derived, open a calculator, adjust the inputs, and watch the result change. Each input maps to a specific factor in the formula.

If you find a calculation that looks wrong or want to submit a correction, contact us at contact@marginruler.com.

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