A free prediction market tax calculator that never sees your data

Every venue hands you a different mess and none of them hands you a report. This turns any of them into per-position detail with matched lots, without your trades leaving your machine.

What it does

• Imports from any of the three venues. A Kalshi transaction CSV, a Polymarket wallet address, or a Robinhood event contracts export.
• Matches lots first in first out, so every disposal carries a real acquired date and a real cost basis instead of an average.
• Handles fees in the right place. Opening fees capitalized into basis, closing fees deducted from proceeds, per position, once.
• Books losing positions. On Polymarket it diffs your rebuilt history against live holdings and settles anything that resolved against you at zero, which is the step that stops your gain being overstated.
• Buckets by Eastern calendar day, so a settlement on the evening of December 31 lands in the year it belongs to instead of the next one.
• Splits short term from long term, which is the Part I and Part II division on Form 8949.
• Exports per-position detail with description, acquired, disposed, proceeds, basis and gain on every line.

Why it runs in your browser

Your trade history is a complete record of your finances and your judgment, and it is worth more to other people than it is to you. So there is no server to send it to. The parsing, the lot matching and the export all happen in the page, on your machine. Nothing is uploaded, nothing is stored, and there is no account to create.

That is not only a privacy stance, it is also what lets the tool be free. There is no infrastructure cost per user because there is no infrastructure per user.

You can check the claim rather than trusting it. Open your browser's network tab, import a file, and watch what leaves the page.

What it deliberately does not do

• It does not decide your tax treatment. Whether your gains are capital, gambling or other income depends on facts specific to you and is genuinely unsettled for event contracts. The tool produces the record. What you file with it is between you and a professional.
• It does not file anything. There is no e-file, no submission, no integration with a filing service.
• It does not touch your funds. Read only, everywhere. Polymarket rebuilds need nothing but a public wallet address, and there is no wallet connection anywhere in the product.

The whale board

The same engine that rebuilds your history rebuilds anyone else's, because Polymarket wallets are public. That is where the whale leaderboard comes from: the top traders by profit, with the option to reconstruct any of their full histories, losses included. Most leaderboards in this market count settled wins and quietly miss unredeemed losses, which is why their numbers read high.

Common questions

Is it free?

Yes, and there is no account and no payment. It runs entirely in your browser, so there is no per-user cost to cover.

Do my trades get uploaded anywhere?

No. Parsing, lot matching and export all happen locally in the page. Nothing is sent to a server and nothing is stored. You can verify it in your browser network tab while you import.

Which platforms does it support?

Kalshi via the transaction CSV, Polymarket via a public wallet address, and Robinhood event contracts via export. Multiple sources can be combined into one record.

Does it tell me what I owe?

No, and that is deliberate. It produces the per-position record every treatment requires. Which treatment applies to you is a question for a qualified professional, because it is unsettled for event contracts and turns on your own facts.