The best prediction market tools are mostly free, and most traders use none of them. They check one venue, buy at the ask, and find out afterwards what the fee was. A handful of free tools fixes all of that: a cross-venue price comparison, a few calculators, on-chain data for the markets that settle on a blockchain, and the official sources that decide how each contract resolves.
This guide covers the tools worth bookmarking — what each does, when to use it, and what it tells you that the venue's own screen does not. Some are built by PredictReport, some by the venues themselves, and some are public data sources that every serious trader relies on. None requires a subscription.
| Tool | What it does | Cost | Best for |
|---|---|---|---|
| Arbitrage scanner | Compares Polymarket and Kalshi prices and flags fee-inclusive arbitrage | Free | Buying on the cheaper venue |
| Market fee calculator | Runs Kalshi's and Polymarket's fee formulas | Free | Knowing the cost before you trade |
| Vig calculator | Measures overround and strips it out | Free | Finding fair value |
| Expected value calculator | Compares your probability with the price | Free | Deciding whether to trade |
| Odds converter | Converts cents to American, decimal and fractional odds | Free | Comparing with sportsbooks |
| Parlay calculator | Combines probabilities and payouts | Free | Checking multi-leg bets |
| Take-home calculator | Applies fees and tax to a profit | Free | Planning a year's trading |
| Dune Analytics dashboards | On-chain Polymarket data | Free tier | Tracking large wallets |
| Venue APIs | Programmatic market data and trading | Free | Builders and automation |
Price Comparison: See Both Venues at Once
The single most useful habit in prediction markets is checking the other venue before you buy. Polymarket and Kalshi run separate order books, and the same outcome regularly trades at different prices on each. On September 21, 2026, J.D. Vance was 46¢ on Kalshi and 51.3¢ on Polymarket to be the Republican nominee — the same $1 payout for about 10% less money on one venue.
PredictReport's arbitrage scanner does that comparison for every market it tracks, updating every ten minutes. For each outcome it shows both venues' prices, the gap between them, and — when the gap is large enough to survive fees on both sides — a fee-inclusive arbitrage with its return and annualised yield. It only flags opportunities where both venues show a genuine two-sided book, because a lone quote with nothing behind it is not a price anyone can trade.
The same data powers the market pages for individual events, such as the live Super Bowl odds and the 2028 Democratic nominee market. Our articles on Super Bowl LXI odds and 2028 Republican nominee odds show how to read the comparison, and our arbitrage guide covers turning a gap into a trade.
Calculators for Every Trade
Each calculator answers one question that the venues' own screens do not.
Market fee calculator. Kalshi charges 0.07 × contracts × price × (1 − price), rounded up to the next cent, and Polymarket charges takers 0.75–1.80% depending on the category. The fee calculator runs both for any price and size, which matters most near 50¢, where Kalshi's fee peaks at about 1.75¢ a contract.
Vig calculator. Enter the prices of every outcome in a market and the vig calculator shows the overround and the no-vig fair probabilities. It is the fastest way to see how much margin a set of prices carries, whether on an exchange or a sportsbook. Our vig guide explains the concept.
Expected value calculator. Enter your probability and the market's price, and the EV calculator shows the edge per contract. It is the calculation behind every good trade, explained in our expected value guide.
Odds converter. The odds converter translates a prediction market price into American, decimal and fractional odds, and back again, which is what you need to compare an exchange price with a sportsbook line. Our explainer on prediction market odds walks through the formulas.
Parlay and take-home calculators. The parlay calculator shows why a combined bet's payout looks better than its probability deserves, and the take-home calculator applies fees and tax to a year's gross profit so you know what you actually keep.
On-Chain Data and Wallet Tracking
Because Polymarket settles on the Polygon blockchain, every trade is public. That makes it possible to see what the largest and most successful wallets are doing, which is information no traditional exchange publishes.
Dune Analytics is the most widely used way to explore it. Community-built dashboards on Dune track Polymarket volume, open interest, the biggest wallets and their positions, and Dune's free tier lets anyone query the underlying data. Our Dune Analytics review covers what the dashboards show and how traders use them. A block explorer such as Polygonscan lets you verify individual transactions directly.
Wallet tracking is the basis of copytrading — following the positions of traders with strong records. It can be informative, but it has obvious limits: a wallet's history does not guarantee its future, large traders hedge in ways you cannot see, and by the time you copy a trade the price has often moved. Our guide to copytrading on Polymarket covers the approach and its pitfalls.
On-chain data also has blind spots. It covers Polymarket's international platform, where trades settle on Polygon, but not Kalshi or other regulated exchanges, whose order books are private. A wallet address tells you nothing about who controls it or whether one trader runs several. And a large position on one venue may be hedged on another, so what looks like a bold bet can be one leg of an arbitrage. Treat wallet data as a clue about where informed money might be, not as a signal to follow blindly. It is most useful as a check on your own view: if the wallets with the best records are all on the other side of a trade you are about to make, that is a reason to look again at your reasoning, not necessarily to change it.
APIs for Builders
Both major venues publish free APIs. Polymarket offers a market data API and a separate order book API for trading, and Kalshi offers a trading API with market data, order placement and portfolio access. PredictReport's own odds comparison is built on the public market data from both.
APIs are how automated traders operate, and they are the only practical way to monitor dozens of markets at once or to act within seconds of news. They also let you build personal tools — a price alert, a spreadsheet that updates itself, a dashboard for the markets you care about. Our guide to the best prediction market APIs compares them in detail, and our analysis of AI trading bots in prediction markets covers what automation is doing to the category.
Resolution Sources Worth Bookmarking
Every contract settles on a named source, and the source — not the headline — decides whether you are paid. Bookmarking the sources for the markets you trade is one of the most practical things you can do.
| Market type | Typical settlement source |
|---|---|
| Fed decisions | The FOMC statement, released at 2:00 p.m. ET on decision day |
| Inflation and jobs | Bureau of Labor Statistics releases, first published figure |
| GDP | Bureau of Economic Analysis advance estimate |
| Daily temperatures on Kalshi | National Weather Service Daily Climate Report for the named station |
| Bitcoin on Kalshi | 60-second average of the CF Benchmarks Real-Time Index |
| Short-dated crypto on Polymarket | Chainlink price data; hourly and daily markets use Binance candles |
| Awards and prizes | The awarding body's official announcement |
Always confirm the source in the specific market's rules, because details vary. Our explainer on how prediction markets resolve covers what to check, and our guides to Kalshi weather markets and Kalshi economic data markets go through two of these sources in depth.
Forecasting Inputs
For economic markets, a few public forecasts are worth watching. The Federal Reserve Bank of Atlanta's GDPNow model updates its estimate of the current quarter's growth as each data point arrives. The Cleveland Fed publishes a similar nowcast for inflation. CME's FedWatch tool derives rate-decision probabilities from fed funds futures, a useful cross-check on the Fed markets tracked in our Fed rate odds.
For elections, polling averages and statistical models are the standard inputs, and markets react to them within minutes. For weather, the National Weather Service's forecasts and the major weather models are free and updated several times a day. None of these guarantees an edge — markets watch them too — but trading without them means trading with less information than the people on the other side.
Tools for Specific Market Types
Different markets reward different inputs. For sports, the most useful information is injury news and the sportsbook closing line, which is among the most efficient forecasts in any market; prediction markets on the same games tend to converge toward it. Futures markets move more slowly and reward patience — our NBA championship odds show how quiet a board can be before the season starts.
For politics, polling averages, statistical models and early-vote data drive prices, and the largest markets are deep enough that single polls rarely move them far. Our 2026 midterm election odds show how chamber and race markets fit together.
For crypto, the settlement feed is the tool. A bitcoin contract settles on a specific index or exchange at a specific moment, and knowing which is the difference between trading the market and guessing at it. Our guide to bitcoin prediction markets covers the sources each venue uses.
Alerts and Monitoring
Prices move while you are not watching, so a way to be told when they do is worth setting up. Both venues let you follow markets inside their apps, and anyone comfortable with a little code can build a price alert from the public APIs in an afternoon. PredictReport's market pages refresh every ten minutes while open, which is enough for most futures and political markets.
Be sceptical of alerts on thin markets. A sudden move on a quiet book is often a single order rather than news, and it frequently reverses. Before acting on any alert, check whether the other venue moved and whether there is size behind the new price; our guide to prediction market liquidity explains what to look for.
Putting the Tools Together
A simple routine uses most of these tools in a few minutes. Start with the price comparison to see where each venue stands and whether a gap exists. Confirm the price and the depth on the venue itself. Run your own probability through the expected value calculator, and the trade size through the fee calculator. Place a limit order rather than a market order — our guide to limit orders explains why that single change saves the most money. Then log the trade in your journal.
None of these steps takes long, and together they remove most of the avoidable costs that turn a reasonable view into a losing trade. The edge in prediction markets rarely comes from a secret tool; it comes from using the ordinary ones every time.
A Trading Journal: The Most Underrated Tool
The most valuable tool is one you build yourself: a simple record of every trade. For each one, write down the market, the date, the price you paid, the probability you believed, your reason, and the outcome.
After thirty or forty trades, that record answers the only question that matters — whether your probabilities are better than the market's. Group your trades by the probability you assigned and check how often each group came true. If your 70% calls happen about 70% of the time and you bought them at 60¢, you have an edge. If not, the journal has saved you from finding out the expensive way. Our guide to prediction market accuracy explains the calibration idea behind this.
The journal also makes tax time easier. Kalshi issues a 1099-MISC and Polymarket issues nothing, so a record of your trades is often the only complete account of your profits. Our prediction market tax guide covers how gains are reported.
Choosing a Platform
Tools help most once you know where you will trade. Our ranking of the best prediction market apps compares every major venue on fees, markets, access and depth, and Polymarket vs Kalshi goes head to head on the two largest. Our offers page compares what each platform gives new users, from interest on idle cash to fee rebates. Whichever venue you choose, complete identity verification before you deposit; our guide to prediction market KYC explains what each platform asks for and how to avoid the mismatches that delay withdrawals. Regulated venues ask for more than offshore ones, and our explainer on how the CFTC regulates prediction markets covers why. If you are starting from zero, how prediction markets work covers the basics.
Frequently Asked Questions
What is the best free prediction market tool?
A cross-venue price comparison, because the same outcome often trades at different prices on Polymarket and Kalshi. PredictReport's arbitrage scanner compares both every ten minutes and flags gaps that survive fees.
How do I compare Polymarket and Kalshi prices?
Use a tool that shows both venues side by side, or check each venue's order book before buying. Buy on whichever venue is cheaper for the side you want.
Is there a calculator for Kalshi fees?
Yes. Kalshi's fee is 0.07 × contracts × price × (1 − price), rounded up to the next cent, and our market fee calculator runs it along with Polymarket's taker fee.
How can I track Polymarket whales?
Through on-chain data, because every Polymarket trade is recorded on Polygon. Dune Analytics dashboards track the largest wallets and their positions; see our Dune Analytics review.
Do Polymarket and Kalshi have free APIs?
Yes. Both publish APIs for market data and trading at no cost. Our guide to the best prediction market APIs compares them.
How do I convert prediction market prices to sportsbook odds?
Use an odds converter. A 25¢ contract equals +300 in American odds and 4.00 in decimal odds; a 75¢ contract equals −300 and 1.33.
What should a prediction market trading journal include?
The market, date, price paid, the probability you believed, your reason and the outcome. After thirty or more trades, compare your probabilities with results to see whether you have an edge.
Do I need paid tools to trade prediction markets?
No. The most useful tools — price comparison, fee and EV calculators, on-chain data, venue APIs and official data sources — are free.
