About MarginMath

MarginMath is a set of free calculators for people who sell online — on Amazon, eBay, Etsy or their own store. Every tool runs in your browser: nothing you type is sent to a server, there is no sign-up, and results update as you type.

Who builds it

How the numbers are checked

• Formulas are shown on every page so you can check any result by hand.
• Marketplace fees come from the platforms' own fee pages — Amazon, eBay and Etsy — and each fee calculator lists its sources and the date they were last checked.
• Calculations are tested against the platforms' published examples before each update (for example, eBay's own $424 sale example returns $58.06 in fees here too).
• Rates can change. Every fee calculator lets you edit or override rates, and the "Updated" date on each page shows when it was last reviewed.

Update log

• November 10, 2026 — Added the Amazon FBA calculator (all referral categories, 2026 FBA fees, peak season, fuel surcharge). eBay calculator now covers eBay Store rates and the Top Rated Plus discount. Etsy calculator now includes sales tax in the processing fee and country rates.
• November 10, 2026 — Added the eBay fee calculator, reviews, and the common-markups table.
• October 28, 2026 — Launched with markup, profit margin, margin-to-markup and Etsy fee calculators.

The calculators

• Markup Calculator — Enter any two values — cost, markup % or selling price — and the third is calculated instantly, along with your profit and profit margin.
• Profit Margin Calculator — Work out your gross profit margin from cost and revenue — or enter a target margin to find the selling price you need.
• Margin ↔ Markup Converter — Type a margin to get the matching markup, or type a markup to get the matching margin. Both directions update live.
• Etsy Fee Calculator — See exactly what Etsy takes from a sale, what you keep, and what to charge for the margin you want.
• eBay Fee Calculator — Every eBay category, with or without an eBay Store. See the final value fee, what you keep, and the price you need.
• Amazon FBA Calculator — Referral fees for every Amazon category and FBA fees worked out from your product's size and weight — so you see real profit before you buy inventory.
• ACoS & TACoS Calculator — Check whether your Amazon ads make money. Enter ad spend and sales for ACoS, TACoS and ROAS, and your product numbers for the break-even ACoS.
• Amazon vs eBay vs Etsy — Enter one product once and see what Amazon FBA, eBay and Etsy each take — and where you keep the most.

Spotted an outdated fee or want a new calculator? Get in touch.