Prediction Market Arbitrage: A Plain-English, Numbers-Driven Guide

Prediction markets let people bet on real-world events—elections, sports, weather, and more—by buying and selling shares tied to outcomes. These markets, like Kalshi and Polymarket, set prices that reflect the probability of those outcomes, at least as judged by traders. But here's where things get interesting: not all markets agree. And sometimes, prediction markets and traditional sportsbooks (like FanDuel or BetMGM) post prices that are out of sync—with enough of a gap that, in theory, you could lock in a profit by betting both sides. This is called prediction market arbitrage.

Let's break down what that means, how the math works, and what you need to know to spot (and understand) these opportunities.

What is Arbitrage?

Arbitrage is the practice of exploiting price differences between two or more markets for the same outcome. In the context of prediction markets and sportsbooks, it means finding a "yes" price that's higher in one place than the "no" price somewhere else, so that betting both sides guarantees you more than you risked—before fees and limits are factored in.

Simple Example: Election Outcome

Suppose there's a prediction market for "Will Candidate X win the 2024 election?" On Polymarket, the YES shares are trading at $0.58 (implying a 58% chance), while on a sportsbook, the odds for NO (implying Candidate X loses) are +110 (decimal odds 2.10).

Translating Odds

First, let's get everything in the same units:

• Polymarket YES at $0.58: For every $1 you spend, you get $1 if YES wins, $0 if NO. So the market thinks the chance is 58%.
• Sportsbook NO at +110: Bet $1, win $1.10 profit ($2.10 back) if NO; lose $1 if YES. Implied probability is 1 / 2.10 = 47.6%.

If the YES on Polymarket is 58% (implies NO is 42%), and the sportsbook is offering 47.6% for NO, there's a mismatch.

Can You Lock in Arbitrage?

Let's try an example with $100 max stakes:

• Buy $100 YES on Polymarket at $0.58:

  • Cost: $58
  • If YES: Receive $100 ($42 profit)
  • If NO: Receive $0 (lose $58)

• Bet $55.24 NO on sportsbook at +110:

  • Cost: $55.24
  • If NO: Receive $55.24 × 2.10 = $115.99 ($60.75 profit)
  • If YES: Lose $55.24

Total staked: $58 + $55.24 = $113.24

Now, check the outcomes:

• If YES wins:

  • Polymarket: +$42
  • Sportsbook: -$55.24
  • Net: $42 - $55.24 = -$13.24

• If NO wins:

  • Polymarket: -$58
  • Sportsbook: +$60.75
  • Net: -$58 + $60.75 = +$2.75

Not a locked-in profit! In this case, the prices aren't quite far enough apart to guarantee a win. But if the sportsbook NO odds were longer—say, +140 (decimal 2.40)—the math could work. Let's recalculate:

• Sportsbook NO at +140: Bet $1, get $2.40 back if NO.

• Bet amount to cover $100 payout: $100 / 2.40 = $41.67

• Total staked: $58 (Polymarket) + $41.67 (Sportsbook) = $99.67

  • If YES: $100 from Polymarket, lose $41.67 on Sportsbook. Net: $100 - $58 - $41.67 = $0.33
  • If NO: Lose $58 on Polymarket, get $100 on Sportsbook. Net: $100 - $41.67 - $58 = $0.33

Now, it's arbitrage: no matter who wins, you get a small profit (before fees and possible withdrawal costs).

Why Do These Pricing Gaps Exist?

• Different participants: Sportsbooks set lines based on their risk and customer flow, sometimes shading odds away from true probabilities. Prediction markets reflect the consensus of traders, who may be better informed (or not!).
• Regulatory silos: Not everyone can access every venue; US residents can't use many offshore sportsbooks, and prediction markets may be limited by state.
• Market inefficiency: Especially in less-trafficked or "exotic" markets, prices don't always update instantly.
• Fees and spreads: Both types of markets charge fees—sportsbooks bake it into the odds (the "vig"), prediction markets often take a % of profits or trades.

Arbitrage Math: The Core Principle

The basic idea: If you can buy YES for less than the implied probability of NO elsewhere (after fees), arbitrage is possible.

Assume:

• Market A: YES at price P
• Market B: NO at implied probability Q
• Fees F_A and F_B

Arbitrage exists if:

• P + (1 - Q) < 1 - (F_A + F_B)

In other words, the total amount you must bet to cover both outcomes is less than the guaranteed payout, after costs.

A Concrete, Numerate Example

Suppose:

• Polymarket YES at $0.42 (42% chance)
• DraftKings NO offered at -120 (decimal 1.83, implied probability 54.6%)
• Both venues charge 2% fee on profits

Step 1: Stake calculation

• Buy $100 YES on Polymarket: Cost $42
• To cover NO on DraftKings (payout $100 if NO): Stake = $100 / 1.83 ≈ $54.64
• Total staked: $42 + $54.64 = $96.64

Step 2: Fees

• If YES: Polymarket pays $100, profit $58, after 2% fee: $58 - $1.16 = $56.84; lose $54.64 on DK. Net: $56.84 - $54.64 = $2.20
• If NO: Lose $42 on Polymarket; DK pays $100, profit $45.36, after 2% fee: $45.36 - $0.91 = $44.45. Net: $44.45 - $42 = $2.45

Result: In this case, after fees, you're ahead by a small margin either way.

Real-World Complications

• Limits: Prediction markets and sportsbooks cap bet sizes. Sometimes the arbitrage only exists for tiny stakes.
• Fees: Always include every fee (including withdrawal or currency conversion fees, if any).
• Timing: Prices change fast, especially for popular events. By the time you execute both legs, the opportunity may be gone.
• Execution risk: If one leg is filled and the other leg's price moves, you may not have a true arb.

How to Spot Prediction Market Arbitrage

In theory, you could scan every prediction market and sportsbook for every event, crunch the numbers, and check for opportunities. But in practice, this is a huge, ongoing task—especially with dozens of venues, fees, and changing prices.

That's where tools like ArbMonster come in. ArbMonster automatically scans 10+ sportsbooks and both Kalshi and Polymarket, showing only true arbitrage after all fees, at fillable sizes. For a deeper dive on the mechanics, see ArbMonster Learn.

Summary Table: Arbitrage Example

Market	Bet	Price/Odds	Stake	Outcome YES	Outcome NO
Polymarket	YES	$0.42	$42	$58 profit	-$42
DraftKings	NO	-120 (1.83)	$54.64	-$54.64	$45.36
Total			$96.64	$3.20	$3.36
After 2% fees				$2.20	$2.45

FAQ

Q1: Are prediction market arbitrage opportunities common?

A: They do arise, especially in less-liquid markets or when news breaks. But after accounting for fees, limits, and execution risk, most are razor-thin or fleeting.

Q2: Is arbitrage risk-free?

A: In theory, yes—if you fill both bets at locked-in prices, cover all fees, and both venues pay out as expected. In practice, venue risk, changing prices, and execution timing all introduce potential pitfalls.

Q3: How does ArbMonster help with prediction market arbitrage?

A: ArbMonster automates the search, scanning 10+ sportsbooks and both Kalshi and Polymarket, showing only net-of-fees, fillable-size arbitrages. Learn more at ArbMonster.com.

ArbMonster is a data service. Nothing here is financial or betting advice; markets carry risk, venues have age and jurisdiction restrictions, and you are responsible for verifying everything before acting.