Calculators
Prediction market calculators: fees, expected value and arbitrage math
Free calculators for Kalshi and Polymarket fees, expected value, implied probability and post-fee arbitrage, checked against the venue fee schedules.
Checked September 8, 2026 · every price read off the live site
Every calculators tool
6 reviewed · alphabetical-
Bet Hero prediction market calculator One-screen Kalshi payout math that prints the actual fee formula it uses, so you can check its work.
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DefiRate calculators 13 free calculators: odds conversion, EV, fees, Kelly, hedging, parlays, Monte Carlo bankroll simulation.
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Market Math Comparing every venue's fee schedule in one table and running fee-adjusted EV before a trade.
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OddsAssist Kalshi calcs Kalshi fee calculator with maker and taker sides (maker orders cut fees roughly 4x), plus payout calculators.
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Prediction Markets Picks 25+ quant tools: Kelly, EV, Bayes updater, correlation-adjusted parlay edges, even an MCP server for AI assistants.
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Start Polymarket tools Polymarket-only fee, profit and loss, EV and Kelly math, plus a country availability check, on one independent site.
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| Tool | Venues | Pricing | Type |
|---|---|---|---|
| Bet Hero prediction market calculator | Kalshi, Polymarket, PredictIt | Free. | web app |
| DefiRate calculators | Polymarket, Kalshi, Robinhood, PredictIt, Crypto.com, ForecastEx, OG.com, FanDuel, DraftKings, Coinbase, Gemini | Free. | web app |
| Market Math | Kalshi, Polymarket, Robinhood, DraftKings, ForecastEx, FanDuel, PredictIt, Coinbase | Free. | web app |
| OddsAssist Kalshi calcs | Kalshi, Polymarket, Robinhood, Crypto.com, DraftKings Predict, Fanatics Markets, FanDuel Predicts, OG.com, Underdog Predicts, Novig | Free to use without an account. | web app |
| Prediction Markets Picks | Kalshi, Polymarket, DraftKings, FanDuel Predicts | Free tier. | web app |
| Start Polymarket tools | Polymarket | Free. | web app |
How these tools work
Calculators do the arithmetic between a price and a decision: what a contract costs after the venue's fee, what your edge is worth in dollars, how much of the bankroll to put on it, and what it takes to hedge or arbitrage it. None of the tools in this category pull live prices; you bring the numbers.
The fee model is what separates them. Kalshi charges a parabolic taker fee (0.07 x P x (1 minus P) per contract, highest at 50 cents), Polymarket's U.S. exchange charges a taker fee with a maker rebate while the offshore book prices cost into the spread, Robinhood charges a flat per-contract amount, and the newer venues (DraftKings, FanDuel, Fanatics, Underdog, Crypto.com) each have their own. A calculator that gets the fee wrong gets EV wrong.
Choose by venue coverage and by whether the tool shows its formula. A range on a label is not auditable; a formula is.
How to choose
- Check the fee schedule the tool uses against the venue's current published fees, and prefer tools that print the formula (Bet Hero) or a dated fee table (Market Math, DefiRate).
- Match venue coverage to where you trade: OddsAssist models ten venues one at a time, Market Math and DefiRate compare eight to eleven in a table, Start Polymarket is Polymarket-only.
- Decide whether you need a callable tool. Only Prediction Markets Picks exposes its calculators through an MCP server; the rest are browser-only.
- Separate calculators from model outputs. Edge screens and mispricing scanners are opinions with a price attached; EV, Kelly and fee math are arithmetic.
- Ignore anything that claims to find arbitrage without live data. A calculator confirms a spread you found; it does not discover one.
Questions about calculators tools
What is the best free Kalshi fee calculator?
OddsAssist, Bet Hero, DefiRate, Market Math and Prediction Markets Picks all offer one free. Bet Hero prints the fee formula on the page; OddsAssist adds a fee-inclusive break-even price and maker/taker toggle.
Is there a Polymarket fee calculator?
Yes. OddsAssist models Polymarket's U.S. exchange (taker 0.05% to 2.5%, maker rebate 1.25%), Start Polymarket computes fees by market category, and DefiRate and Market Math include Polymarket in their fee tables.
Do prediction market calculators pull live prices?
The calculators in this category do not. You enter the contract price; the tool returns fee, EV, payout or position size.
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.