Polymarket and Kalshi Arbitrage: How to Profit from the Prediction Market Boom
James Whitfield
The prediction market explosion is creating one of the biggest arbitrage opportunities in betting history.
Polymarket processed over $3.7 billion in volume during the 2024 election cycle. Kalshi, now fully CFTC-regulated, is expanding aggressively into sports betting. And here's what sharp bettors have already figured out: these prediction markets are pricing the same sporting events as traditional sportsbooks, but they're doing it differently enough to create consistent, exploitable arbitrage opportunities.
The problem? Most bettors can't move fast enough to capture them. Prediction market arbitrage windows last seconds to minutes, and you need real-time data from both prediction markets and hundreds of sportsbooks to spot opportunities before they disappear.
That's where Odds-API.io changes everything. We're one of the only APIs that provides both traditional sportsbook odds AND prediction market data (Polymarket and Kalshi) in a single platform with sub-150ms REST response times and real-time WebSocket streaming for instant updates.
What Is Prediction Market Arbitrage?
Prediction market arbitrage means placing bets on all possible outcomes of an event across different platforms, exploiting price differences to lock in a guaranteed profit regardless of the result.
The math is simple: if the sum of implied probabilities across all outcomes is less than 100%, you've found an arbitrage opportunity. The hard part is finding these opportunities fast enough to act on them.
Here's why this works: traditional sportsbooks use professional oddsmakers and sharp money to set lines. Prediction markets like Polymarket and Kalshi use order books driven by crowd sentiment. When you compare prices between these two fundamentally different pricing mechanisms, you find gaps. And those gaps represent guaranteed profit.
Why Prediction Market Arbitrage Is Exploding Right Now
The opportunity isn't theoretical. It's happening at scale, right now, and growing fast.
Volume Is Surging
Polymarket didn't just have a good election season and disappear. The platform processed billions in volume, and that liquidity has shifted into sports betting, cultural events, and entertainment markets. More volume means deeper markets, which means more opportunities to find pricing inefficiencies between prediction markets and traditional sportsbooks.
Kalshi won its legal battle to offer event contracts and is now the only CFTC-regulated prediction market in the US. They're expanding aggressively into sports, and with regulatory approval comes institutional participation, which means even more liquidity and more arbitrage opportunities in sports betting.
Market Overlap Is Increasing
Every week, more sporting events are being priced on both prediction markets and traditional sportsbooks. NBA futures, NFL game outcomes, Champions League matches, tennis tournaments - if a sportsbook is offering odds on it, there's a good chance Polymarket or Kalshi is pricing it too.
More overlap means more opportunities. And because these platforms price events through completely different mechanisms, the gaps persist.
The Infrastructure Finally Exists
Until recently, building a prediction market arbitrage scanner meant scraping sportsbook websites (slow, fragile, often against ToS) and manually pulling data from prediction market APIs. The infrastructure problem alone kept most bettors out of this space.
That barrier is gone. Odds-API.io now provides both sportsbook odds (250+ bookmakers) and prediction market data (Polymarket and Kalshi) through a single, fast API with official Python and Node.js SDKs and real-time WebSocket streaming.
Settlement Speed Gives You an Edge
Polymarket runs on Polygon, which means near-instant settlement. You can recycle capital in minutes, not days. Compare that to waiting 24-48 hours for a traditional sportsbook to settle a bet, and you can see why sharp bettors are moving into this space.
Real Example: Premier League Match Arbitrage
Let's look at a real API response for a Premier League match between Leeds United and Arsenal FC.
Fetch odds from both prediction markets using Odds-API.io:
curl "https://api.odds-api.io/v3/odds?eventId=61300983&bookmakers=Polymarket,Kalshi&apiKey=YOUR_API_KEY"API Response:
{
"id": 61300983,
"home": "Leeds United",
"away": "Arsenal FC",
"date": "2026-01-31T15:00:00Z",
"status": "pending",
"sport": {
"name": "Football",
"slug": "football"
},
"league": {
"name": "England - Premier League",
"slug": "england-premier-league"
},
"urls": {
"Kalshi": "https://kalshi.com/events/KXEPLGAME-26JAN31LEEARS",
"Polymarket": "https://polymarket.com/event/epl-lee-ars-2026-01-31"
},
"bookmakerIds": {
"Kalshi": "KXEPLGAME-26JAN31LEEARS",
"Polymarket": "170912"
},
"bookmakers": {
"Kalshi": [
{
"name": "ML",
"updatedAt": "2026-01-28T10:06:39.424Z",
"odds": [
{
"home": "6.25",
"draw": "4.35",
"away": "1.56",
"layHome": "1.61",
"layAway": "7.14",
"layDraw": "4.76",
"depthHome": "9215",
"depthAway": "2164",
"depthDraw": "7670",
"depthLayHome": "5371",
"depthLayAway": "250",
"depthLayDraw": "1197"
}
]
}
],
"Polymarket": [
{
"name": "ML",
"updatedAt": "2026-01-28T09:55:32.185Z",
"odds": [
{
"home": "6.67",
"draw": "4.35",
"away": "1.56",
"layHome": "1.61",
"layAway": "7.14",
"layDraw": "4.55",
"depthHome": "12128",
"depthAway": "331",
"depthDraw": "26086",
"depthLayHome": "9401",
"depthLayAway": "2112",
"depthLayDraw": "1630"
}
]
}
]
}
}This shows the clean, structured JSON format you get from Odds-API.io. The response includes real-time odds from both prediction markets with timestamps, letting you instantly compare prices and spot arbitrage opportunities.
For a complete walkthrough of how to turn this data into profitable arbitrage opportunities, see the step-by-step guide below.
How Odds-API.io Makes Prediction Market Arbitrage Practical
Here's what you actually need to build a working prediction market arbitrage scanner:
- Real-time odds from hundreds of sportsbooks
- Real-time prices from prediction markets
- Fast API response times (arb windows close in seconds)
- Reliable uptime during peak betting hours
- Clean data format you can actually work with
Odds-API.io provides all of this in a single platform.
Complete Coverage: Sportsbooks AND Prediction Markets
Odds-API.io is one of the only sports betting APIs that provides both traditional sportsbook odds and prediction market data in a single platform:
250+ Sportsbooks including:
- All major US books (DraftKings, FanDuel, BetMGM, Caesars)
- European books (Bet365, William Hill, Unibet, Betfair)
- Sharp Asian books (SingBet, SBOBET)
- Regional and niche sportsbooks where the widest pricing gaps appear
Prediction Markets:
- Polymarket - Real-time contract prices for sports, politics, entertainment
- Kalshi - CFTC-regulated event contracts including sports betting markets
This means you can build a complete prediction market arbitrage scanner with a single API provider instead of stitching together multiple data sources, managing different authentication systems, and dealing with inconsistent data formats.
Speed That Actually Matters
Arbitrage windows between prediction markets and sportsbooks last seconds to minutes. Sub-150ms average response times mean your scanner can pull fresh odds from 250+ bookmakers and compare them against prediction market prices before the opportunity disappears.
When you're competing against other arb bettors for the same pricing gaps, 150ms versus 2 seconds is the difference between profit and watching someone else take the arb.
Real-Time WebSocket Streaming
For serious arbitrage scanners, REST API polling isn't fast enough. By the time your script makes another request, the arb opportunity might be gone.
Odds-API.io provides WebSocket streaming that pushes odds updates to your scanner the instant they change - no polling, no request limits, no delays. When a line moves on Polymarket or any of the 250+ sportsbooks, you know immediately.
WebSocket streaming means:
- Zero latency - Updates pushed instantly, not pulled every few seconds
- No API requests consumed - Stream stays open, your request quota stays untouched
- Lower infrastructure costs - One connection instead of thousands of API calls
- Competitive advantage - You see opportunities before bettors polling REST endpoints
For prediction market arbitrage where windows last seconds, WebSocket streaming is the difference between consistent profits and constant missed opportunities.
Built for Automation
Official SDKs for Python and Node.js with async support and WebSocket streaming. Clean, structured JSON responses. Consistent data format across all bookmakers and prediction markets.
No parsing HTML. No dealing with CAPTCHAs. No maintaining fragile scrapers. No managing multiple API authentication systems. No polling delays eating into your arb windows.
Reliability You Can Count On
99.9% uptime SLA means your scanner stays live during peak hours when arb opportunities are most abundant. Saturday afternoon during NFL season is not the time to discover your data source is down.
How to Find Polymarket Arbitrage Opportunities (6 Steps)
Whether you're building custom sports arbitrage software or a simple scanner script, the workflow is the same. While you could use a manual arbitrage betting calculator, most successful bettors automate this process for speed and accuracy.
Step 1: Connect to Odds-API.io via REST API or WebSocket Stream
For one-time queries or occasional checks, use the REST API:
curl "https://api.odds-api.io/v3/odds?eventId=61300983&bookmakers=Polymarket,Kalshi,Bet365,DraftKings&apiKey=YOUR_API_KEY"For continuous arbitrage scanning, use WebSocket streaming to receive real-time updates:
const WebSocket = require('ws');
const ws = new WebSocket('wss://api.odds-api.io/ws?apiKey=YOUR_API_KEY');
ws.on('open', () => {
console.log('Connected to Odds-API.io WebSocket');
});
ws.on('message', (data) => {
const update = JSON.parse(data);
console.log('Received odds update:', update);
// Process real-time odds update for arbitrage opportunities
});WebSocket streaming is recommended for arbitrage scanning because you receive updates the instant odds change across any platform, without consuming API requests or introducing polling delays.
Step 2: Parse the Response and Extract Best Odds
The API response includes odds in decimal format for home win, away win, and draw (for soccer) or other outcomes depending on the sport. Extract the best available odds for each outcome across all platforms.
Looking at our Leeds vs Arsenal example, you can pull traditional sportsbook odds alongside the prediction markets:
curl "https://api.odds-api.io/v3/odds?eventId=61300983&bookmakers=Polymarket,Kalshi,Bet365,DraftKings&apiKey=YOUR_API_KEY"Example response showing odds from multiple sources:
{
"bookmakers": {
"Polymarket": [
{
"odds": [
{
"home": "6.67",
"draw": "4.35",
"away": "1.56"
}
]
}
],
"Bet365": [
{
"odds": [
{
"home": "7.00",
"draw": "4.20",
"away": "1.52"
}
]
}
]
}
}Step 3: Convert All Odds to Implied Probabilities
Normalize sportsbook odds and prediction market prices to the same format:
Implied probability = 1 / decimal odds
For the example above, taking the best odds for each outcome across all platforms:
- Leeds win (Bet365): 1 / 7.00 = 14.3%
- Draw (Polymarket): 1 / 4.35 = 23.0%
- Arsenal win (Polymarket): 1 / 1.56 = 64.1%
Note that you're cherry-picking the best price for each outcome. Bet365 offers better odds for Leeds (7.00 vs 6.67), Polymarket offers better odds for the draw (4.35 vs 4.20), and Polymarket offers better odds for Arsenal (1.56 vs 1.52).
Step 4: Check If Combined Probability Is Below 100%
Calculate the sum of best available probabilities across all outcomes:
14.3% + 23.0% + 64.1% = 101.4%
This is over 100%, so not an arb yet. But if the draw was available at 4.76 on Kalshi (21.0%):
- 14.3% + 21.0% + 64.1% = 99.4%
That's a potential arbitrage opportunity before fees - a 0.6% edge that could be profitable after accounting for platform fees and vig.
When combined probability falls below 100%, you've found an arbitrage opportunity.
Step 5: Account for Fees and Calculate Stake Sizes
Before declaring an arb viable, subtract all platform fees:
- Polymarket: ~2% fee on winning positions
- Kalshi: Per-contract fees (varies by market)
- Sportsbooks: 4-5% vig built into odds
A theoretical 2% arb might become 0.5% after fees. Only arbs above your fee threshold are worth executing.
Use standard arbitrage staking formulas to size bets proportionally across platforms.
Step 6: Execute Trades on All Platforms Simultaneously
Place bets across all platforms as quickly as possible. Speed is critical - delays of even a few seconds can mean lines move and your arb disappears.
The best arb bettors automate this entire workflow using the Odds-API.io SDK, checking hundreds of events per minute and executing immediately when viable opportunities appear.
Critical Success Factors for Prediction Market Arbitrage
Account for All Fees
Polymarket: Roughly 2% fee on winning positions
Kalshi: Per-contract fees vary by market
Sportsbooks: Vig is baked into the odds (typically 4-5%)
An arb that looks like 2% profit might be 0.5% profit after fees. Make sure your scanner accounts for this before flagging opportunities.
Verify Settlement Criteria
"Will the Lakers win the NBA Championship?" on Kalshi might settle based on different criteria than a futures bet at a sportsbook. Always verify you're betting on truly identical outcomes.
Read the settlement rules for both platforms before placing bets. Mismatched settlement has killed many "arbs" that weren't actually arbs.
Manage Liquidity Across Platforms
Your capital will be split between:
- Multiple sportsbook accounts
- A crypto wallet for Polymarket
- A Kalshi account
- Cash reserves for quick deployment
Plan your capital distribution so you have liquidity available when opportunities appear. Running out of funds on one side means you can't execute.
Automate Everything You Can
Manual arbitrage is nearly impossible at scale. By the time you manually check five sportsbooks and compare to Polymarket, the line has moved.
The Odds-API.io Python SDK with async support and WebSocket streaming makes it straightforward to build a scanner that receives real-time updates and alerts you instantly when an arb appears.
Why Odds-API.io for Prediction Market Arbitrage?
Let's be direct about why bettors building prediction market arbitrage scanners choose Odds-API.io:
Single API for both data sources. You don't need to manage separate APIs for sportsbooks and prediction markets. Everything comes through one clean REST API with consistent formatting.
Coverage that actually matters. 250+ sportsbooks means you see pricing from sharp books, soft books, regional books, and niche operators. The widest arbs often appear on smaller books that other APIs don't cover.
Speed that wins arbs. Sub-150ms REST response times plus real-time WebSocket streaming mean you see opportunities before other bettors do. When everyone is running scanners, the fastest scanner wins - and WebSocket streaming is faster than REST polling.
Built for developers. Official Python and Node.js SDKs. Clean JSON. Comprehensive documentation. No reverse-engineering required.
Reliable when it matters. 99.9% uptime SLA. When there's a major sporting event with heavy betting volume, your scanner stays online.
Get Started with Prediction Market Arbitrage
The prediction market boom is creating arbitrage opportunities at scale. The infrastructure exists. The data is available. The question is whether you'll build a scanner that captures these opportunities or watch other bettors take them.
Odds-API.io gives you real-time access to 250+ sportsbooks and both major prediction markets (Polymarket and Kalshi) in a single API with sub-150ms response times and WebSocket streaming.
Start with the free tier to test your scanner logic, then scale to paid plans when you're ready to deploy capital.
Frequently Asked Questions
Can you actually arbitrage between Polymarket and sportsbooks?
Yes. Polymarket uses an order book driven by crowd sentiment. Traditional sportsbooks use professional oddsmakers and sharp money. These two pricing mechanisms frequently disagree on the same events, creating arbitrage opportunities. The gaps are real, the opportunities appear daily, and bettors are profiting from them right now.
Is prediction market arbitrage actually profitable?
Margins are typically 1-5% per arb after fees, and windows are short (seconds to minutes). Profitability depends on execution speed, fee management, and capital deployment. Bettors using automated scanners with fast data sources see the best returns. Manual arbing is nearly impossible at scale.
How do you find arbitrage opportunities on Kalshi?
Pull real-time sportsbook odds from Odds-API.io, fetch corresponding event contract prices from Kalshi via Odds-API.io's prediction market endpoint, convert both to implied probabilities, and check if combined probabilities sum to less than 100%. If they do, you've found a Kalshi arbitrage opportunity. The tricky part is speed and automation.
What tools do you need for cross-platform arbitrage betting?
At minimum: a real-time odds API that covers both sportsbooks and prediction markets (like Odds-API.io), a script or bot that compares prices and calculates arbitrage opportunities, and accounts on the relevant platforms (sportsbooks, Polymarket, Kalshi). Most successful arb bettors use Python or Node.js with the Odds-API.io SDK to build automated scanners.
Do I need multiple APIs to do prediction market arbitrage?
Not if you use Odds-API.io. We provide both traditional sportsbook odds (250+ bookmakers) and prediction market data (Polymarket and Kalshi) through a single API. Most other solutions require you to stitch together separate APIs for sportsbooks and prediction markets, which adds complexity, latency, and points of failure.
How fast do arbitrage opportunities disappear?
Typical windows last anywhere from a few seconds to a few minutes. The most obvious arbs (5%+ profit margins) disappear almost instantly. Smaller arbs (1-3%) can persist longer but still require fast execution. This is why API response time matters - a 2-second delay can mean missing the opportunity entirely. Using WebSocket streaming instead of REST polling gives you the fastest possible alerts.
Should I use REST API or WebSocket for arbitrage scanning?
For arbitrage scanners that need to monitor hundreds of events continuously, WebSocket streaming is the better choice. WebSockets push updates to your scanner instantly when odds change, without consuming API requests or introducing polling delays. REST API works well for one-time queries or lower-frequency checks, but serious arb bettors use WebSocket streaming to maximize speed and minimize costs. The Odds-API.io SDKs support both methods.
Do I need an arbitrage betting calculator?
While manual arbitrage betting calculators exist, most successful bettors automate the calculation process entirely. Building custom sports arbitrage software using Odds-API.io's SDK allows you to calculate implied probabilities, check for arbs, and size bets automatically - all in milliseconds rather than the minutes it would take to use a manual calculator.
Which prediction markets does Odds-API.io support?
We currently support Polymarket and Kalshi. Coverage includes sports betting markets, political events, entertainment outcomes, and cultural predictions. Check our prediction markets documentation for the complete list of supported markets and events.