About sellmath
sellmath is a small, free set of calculators for people who sell online: what a marketplace takes from each sale, what is left after your own costs, and what price you need to reach a target margin. It has no sign-up, no browser extension and no product database — you type in a price and a few costs, and the fee tables do the rest in your browser.
Who makes it
It is built and run by one person: a software developer with sixteen years of experience writing business applications, who now spends part of that time building automation for online sellers. The Korean edition, 셀러툴, launched first in 2026 for Coupang and Naver Smart Store sellers; this English edition applies the same approach to Amazon, Etsy, eBay and Shopify. The site is not a marketplace consultancy and does not sell products on any of these platforms — it is a developer's tool, made carefully, published openly.
Where the numbers come from
Every rate is taken from the platform's own published fee page — Amazon Seller Central's public help pages, Etsy's Fees & Payments Policy, eBay's selling-fees page and Shopify's pricing page — and the data sources page lists each one with the date it was last verified. A monitoring script re-reads those pages and related news every day; when something changes it is flagged, checked by hand and the "rates as of" date on the affected calculator is updated. The calculation formulas are described in plain language at the bottom of each tool.
What it is not
The results are estimates for planning. Marketplaces change fees, apply promotions and treat accounts differently, and only your seller account shows the exact fee on a given order. Nothing here is tax, legal or financial advice. sellmath is independent and is not affiliated with, endorsed by or sponsored by Amazon, Etsy, eBay, Shopify or any other company named on the site; their names are used only to describe their published fees.
How it is funded
The site may show advertising and may include affiliate links to seller software; when it does, the page says so, and it never changes the numbers. Currently: 11 calculators, 17 guides, updated as the platforms change their rates.
Found an error or a rate that changed? Send a note — every report is checked.