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EventArb: what it does, what it costs, where it stops
Last checked 2026-09-08
EventArb is a free web calculator that tells you whether buying opposite sides of the same event on two venues locks in a profit after fees. Its title page names Kalshi, Polymarket, Robinhood and Interactive Brokers as the venues it handles.
It suits a trader who has already found a candidate pair and wants the fee math done properly. It is not a scanner; you type the prices in yourself.
What it does
You pick a market type, binary or multiple choice, enter the contract prices from each venue, and optionally a principal and a contract length. The tool returns the position sizes on each side, the profit, and an annualized ROI based on the time to settlement.
The calculator includes per-contract and per-transaction fees in its math and adds a gas-fee line for Polymarket trades. Calculations can be shared by link, and the site links to a page showing the formulas it uses.
- Binary and multiple-choice market modes
- Fee-inclusive spread, opportunity cost and annualized ROI
- Gas-fee input for Polymarket legs
- Shareable calculation links
- Email signup for alerts when opportunities open
Pricing
There is no pricing page and no paid tier. The interface asks users who found it useful to buy the author a coffee. The alert signup is labeled free.
Venues and data
The venue list is Kalshi, Polymarket, Robinhood and Interactive Brokers, per the site's own meta description. The README notes the math works for any two markets with identical outcome conditions and matching timeframes.
The source code is public at akhan280/event-contract-arbitrage, whose GitHub homepage field points at eventarb.com. It is written in TypeScript, had 27 stars and 4 forks when checked, and was last pushed on 2026-09-07.
Where it falls short
Prices are entered by hand. The repository roadmap lists automated market matching and pulling market lists from Kalshi and Polymarket as unchecked items, so today the tool does not watch the books for you.
It also does not execute anything. Once the calculator confirms a spread, you place the two legs on the venues separately or through a cross-venue terminal.
The repository has no license file, so reuse terms are unstated.
Who should use it
Traders who find pairs manually or through another scanner and want a fee-correct sizing check before committing. If you need live discovery, pair it with a scanner such as ArbiTrack or the Prediction Hunt arbitrage page.
Questions about EventArb
Is EventArb free?
Yes. There is no paid tier. The site accepts donations through Buy Me a Coffee and the alert signup is labeled free.
Does EventArb work with Kalshi and Polymarket?
Yes. Its venue list is Kalshi, Polymarket, Robinhood and Interactive Brokers, and the math applies to any two markets with matching outcome conditions.
Does EventArb find arbitrage automatically?
No. You enter prices by hand. Automated market matching and live market lists are unchecked items on the repository roadmap.
Is EventArb open source?
Yes. The code is at github.com/akhan280/event-contract-arbitrage, 27 stars when checked, written in TypeScript, with no license file.
Trackers find the edge. Calculators size it. Alerts time it.
When it is time to execute, the free Kairos terminal trades Kalshi, Polymarket and Predict.fun from one book.