About FBAbase
Last updated 14 August 2026
FBAbase is a free Amazon FBA profit calculator and a growing set of plain-language answers about FBA fees and unit economics. It is built and maintained by one person.
Why it exists
I am a software developer, not a full-time Amazon seller. This started because I went looking for a calculator that would tell me what a product actually nets after Amazon takes its cut, and found three kinds of tool: ones behind a paywall, ones that wanted an email address before showing a number, and free ones that were quietly wrong.
The wrongness is the part that bothered me. Most free calculators use the shorthand formula — price, minus cost of goods, minus the referral fee — and stop there. That formula omits every cost that depends on time: monthly storage at the correct seasonal rate, the aged-inventory surcharge once a unit passes roughly 180 days, and the cost of the capital frozen in stock while it sits. Two products can look identical under the shorthand and diverge sharply once turnover is in the model. A calculator that hides that is not simpler, it is misleading.
How the numbers are produced
The methodology is deliberately boring, and I would rather explain it than ask you to trust it:
- You supply the rates, not us. The calculator ships no built-in fee table. Referral percentages, fulfillment fees, and storage rates change over time and vary by category, size tier, and season, so a hardcoded table goes stale silently and you never find out. You enter your own figures from Seller Central, and the result reflects what Amazon actually charges you.
- The aged-inventory surcharge is summed across bands. Stock that reaches 365 days is not billed the top rate for its whole life. Charging it that way is a common spreadsheet error and it overstates the cost.
- Peak storage is modelled separately. The October–December rate is roughly triple the rest of the year. Blending it into an annual average hides the spike that decides whether carrying stock through Q4 works.
- Cost of capital is an explicit input. Nobody invoices you for it, which is exactly why it goes missing from most models.
Where a calculation involves a judgment call, the page says so rather than presenting one number as the answer.
What this site will not do
There are no accounts, because there is nothing to sign up for. There is no analytics, no tracking, and no server to send anything to — the calculator runs entirely in your browser, and the figures you type never leave your device. Saved records live in your browser's local storage; clearing your browser data deletes them, and nobody, including me, can read them. The privacy policy is short because there is very little to disclose.
The site is free and stays free. If that ever changes, it will be said plainly on this page rather than discovered at a paywall.
Corrections
Amazon's fee structure moves, and I get things wrong. If a number here looks off or a calculation does not match your Seller Central statement, tell me and I will fix it or publish why it is right — [email protected], or @SukeJ56505 on X. Corrections to published answers are noted on the page itself rather than edited in silently.
Elsewhere
Try the calculator or read the FBA answers.