Independent seller tools
About Seller Margin Lab
Seller Margin Lab is an independent web project built to make marketplace fees, shipping, show expenses, and estimated profit easier to understand. The tools are free, require no account, and are designed around questions a seller can answer with numbers they already have.
Why the site exists
A sale price is not the same as profit. Marketplace commission, payment processing, inventory, packaging, seller-paid shipping, giveaways, refunds, and other operating costs can all reduce what remains. Seller Margin Lab turns those separate deductions into practical planning estimates and post-sale summaries.
Before a sale
Plan the economics
The fee, shipping, giveaway, and show calculators help sellers test assumptions before choosing prices or promotions.
After a sale
Review actual reports
The CSV analyzer summarizes completed Whatnot report rows, including reported fees, refunds, shipping adjustments, and COGS.
How calculations are produced
Calculator assumptions are translated into explicit formulas, stored in shared configuration, and covered by regression tests at important rate and weight boundaries. Pages explain the major formulas and link to the primary sources used. Detailed formulas and limitations are published on the calculation methodology page.
Sources and update policy
Whatnot-focused estimates use public Whatnot Help Center documentation rather than undocumented fee guesses. US fee assumptions were last checked August 23, 2026, and published US shipping estimates were last checked August 23, 2026. Verification dates are updated only when the underlying rules are reviewed.
Privacy by design
CSV data stays in the browser
Seller report files are parsed locally by JavaScript in the browser. Seller Margin Lab does not upload or store CSV rows, buyer names, order IDs, financial amounts, or calculated report results on an application server. Read the full privacy policy.
Independence and limitations
Seller Margin Lab is not affiliated with, endorsed by, sponsored by, or operated by Whatnot. Marketplace programs, promotions, regional rules, report formats, taxes on fees, and seller-specific rates can change the final result. Estimates are informational and are not accounting, tax, legal, investment, or financial advice. Important decisions should be checked against official statements and qualified professional advice.
Additional terms and estimate limitations are available in the terms and disclaimer.