About SharpTally

SharpTally is a small set of calculators for the arithmetic that sits behind an online store: what a marketplace actually takes off a sale, what a discount does to a margin, and whether an ad campaign is paying for itself.

Who it is for

SharpTally is for people selling physical products online in the US: resellers on eBay, Poshmark, Mercari, Depop and Whatnot, makers on Etsy, Amazon FBA sellers, TikTok Shop sellers and anyone running their own Shopify store. It covers marketplace and payment fees, pricing and margin, ad maths, customer economics, unit economics and inventory.

It is not accounting software, and it does not connect to your seller account. It will not know your category's special rate, your store subscription discount or your country's VAT unless you type them in. For those, the calculators take any rate you give them.

Why it exists

Most seller maths is simple but easy to get subtly wrong. eBay charges its fee on the shipping you collect as well as the item. A 50% markup is a 33% margin. A 20% discount can wipe out more than half your profit. None of that is difficult — it is just fiddly enough that people guess, and guessing on price compounds.

How the numbers are kept honest

• Every fee percentage on this site lives in one file, with the rate card it came from and the date someone last checked it. Nothing is hardcoded in a page.
• The build fails if any fee figure has gone more than 400 days without being re-checked, so stale rates break the site rather than quietly misinform you.
• Every calculator is a pure function with its own unit tests covering normal, boundary and invalid inputs.
• Each page shows the formula it uses. If you disagree with our maths, you can see it.

Rates still change without notice, and some categories carry their own rates. Treat these as planning figures and confirm against your own seller account before you price anything.

How the site pays for itself

Two ways, both disclosed, and neither is switched on yet. Once there is enough traffic the site will carry advertising, placed below the calculator and never above it. Some pages may also link to seller software we think is worth the money; those links will be marked on the page and may earn us a commission. Neither one changes a single number in a calculator, and no tool is ranked or ordered by what it pays.

What we do not do

No sign-up walls, no email capture before a result, no "unlock the full calculation" upsell. The tool works the moment the page loads, including with JavaScript turned off. We use Cloudflare Web Analytics, which sets no cookies and builds no profile of you. See the privacy policy for specifics.

Corrections

If a rate is wrong or a formula is off, tell us and we will fix it. The contact page has the address.