About MarginBase

MarginBase is a small set of calculators for US shop owners and online sellers — markup and margin, break-even, reverse sales tax, food cost, and rough marketplace fees. The point is to figure a number before you print a tag or hit “publish,” not to replace your books.

Everything runs in your browser. No account, no saved inputs, no email gate. Costs and prices stay on your device. Close the tab and they’re gone.

Who it’s for

People who sell physical goods — retail, Etsy, Shopify, FBA, kitchens — and need plain arithmetic between customers. If you already live in a full accounting package, you may not need this. If you keep mixing markup with margin on a sticky note, you might.

Where the numbers come from

Formulas are grade-school arithmetic: markup, margin, break-even, tax split, and fee stacks you can check on paper. Marketplace fee defaults are starting estimates — your seller dashboard wins when numbers differ. Sales tax rates are typed by you; we don’t look up ZIP codes or scrape rate tables.

Every assumption and known limit is listed on our data sources & methodology page. Formulas and fee defaults were last reviewed in July 2026.

We also publish short guides when they’re ready — useful if you want to stop mixing markup with margin. Not a replacement for a bookkeeper or tax advisor.

What this is not

Not tax advice, not accounting advice, and not a substitute for your marketplace fee schedule or state sales-tax rules. These are planning estimates — useful before you print tags or list a product. We don’t file returns, reconcile payouts, or store inventory.

Contact & corrections

Found a wrong formula or calculator bug? Email contact@shopmarginbase.com. Include the page URL and the inputs you used. Incorrect pricing math is treated as urgent once we can reproduce it.