Comparing Odds Across Sportsbooks: The Right Way to Find Value

If you want to find the best prices on sports events—or even spot arbitrage opportunities—you need to compare odds across sportsbooks accurately. That sounds simple, but real-world complications make it trickier than it seems. Odds are quoted in different formats, fees are often hidden, and what you can actually bet (the fillable size) varies. This guide walks through the math of comparing sportsbook odds the right way, with practical examples and pitfalls.

The Basics: What Are You Comparing?

Sportsbooks offer odds on the outcomes of events (e.g., Team A wins). You want to know: Which book is offering the best price? Is there a pricing gap to exploit? But to answer, you have to:

1. Convert odds to a common format (implied probability or decimal odds)
2. Account for all fees and juice (the built-in sportsbook edge)
3. Check the fillable size (can you actually get that price?)
4. Factor in withdrawal, deposit, or exchange-rate costs (sometimes significant for international books)

Odds Formats: Apples to Apples

Let’s say you see these on a simple NBA moneyline:

• Sportsbook A: +150 (American odds)
• Sportsbook B: 2.5 (Decimal odds)
• Sportsbook C: 3/2 (Fractional odds)

They’re all the same price! But you need to be sure. Here’s how to convert:

• American to Decimal:
  • Positive odds: Decimal = (American/100) + 1
  • +150: (150/100) + 1 = 2.5
• Fractional to Decimal:
  • 3/2: (3/2) + 1 = 2.5

Key: Always convert to decimal odds or implied probability.

Implied Probability

This is the inverse of decimal odds:

• Implied Probability = 1 / Decimal Odds
• For 2.5: 1 / 2.5 = 0.4 (or 40%)

If you see odds of +150, 2.5, or 3/2, they all “imply” a 40% chance of winning (before fees).

The Vig (Juice): Finding the Real Price

Sportsbooks build in a profit margin called the “vig” or “juice.” This means the sum of implied probabilities for all possible outcomes is usually more than 100%—the extra is their edge.

Let’s work through an example:

• Sportsbook D:
  • Team A: -110 (Decimal: 1.91) → Implied Prob: 1/1.91 = 52.36%
  • Team B: -110 (Decimal: 1.91) → Implied Prob: 1/1.91 = 52.36%
  • Total: 104.72%

The “overround” is 4.72%—the vig. To compare true odds (fair prices), you have to remove this.

Removing the Vig

For a two-way market:

• For each side: True Probability = (Implied Prob) / (Sum of Implied Probabilities)
• For Team A and Team B:
  • Team A: 0.5236 / (0.5236 + 0.5236) = 0.5
  • Team B: 0.5236 / (0.5236 + 0.5236) = 0.5

So the “fair” odds with no vig are 2.0 (decimal), or +100 (American).

Note: Some sites quote “no-vig” odds, but always check their math.

Fees, Exchanges, and Marketplaces

Traditional sportsbooks price the vig into their odds. But on prediction markets like Kalshi or Polymarket, trading fees are charged on top of the price.

Example:

• You buy YES at 60c on Kalshi (pays $1 if correct)
• Kalshi charges a 2% trading fee on profits
• If YES wins, you get $1 - 60c = 40c profit, minus 0.8c fee (2% of 40c), so net profit is 39.2c
• Effective payout = 1 / (0.6 + (0.008)) ≈ 1.635
• Implied probability after fee ≈ 61.15%

If you compare this to a sportsbook quoting 1.62 decimal odds (61.73% implied), Kalshi would actually be the better price—but only after adjusting for the fee.

Fillable Size: Can You Actually Get That Bet?

All this math is useless if you can’t actually place a bet at the quoted odds. Sportsbooks may limit the amount you can bet, especially if they suspect you’re hunting for value.

Prediction markets (like Polymarket) are order books: the size at each price is visible, but may be small.

Example:

• Sportsbook E offers +200 with a $50 max bet
• Sportsbook F offers +150, unlimited
• Polymarket has $80 available at 2.95 (implied +195)

Depending on how much you want to bet, the “best” odds may not be available for your full amount. When comparing, always check the fillable size at each price.

Worked Example: Step-by-Step Odds Comparison

Suppose you want to compare a market across three venues:

• Venue 1 (Sportsbook): -105 on Over, -115 on Under, $500 max
• Venue 2 (Kalshi): Over at 51c, Under at 51c, $200 available per side, 2% fee on profits
• Venue 3 (Polymarket): Over at 0.52, Under at 0.50, $100 at each, 2% fee on profits

Step 1: Convert All Odds to Decimal Odds

Sportsbook

• Over: -105 → Decimal = 1 + (100/105) ≈ 1.952
• Under: -115 → Decimal = 1 + (100/115) ≈ 1.870

Kalshi

• Over: Buy at 0.51, pays 1 if correct
  • Raw payout: 1 / 0.51 ≈ 1.961
  • Net payout after 2% fee: Profit = 0.49, Fee = 0.0098, Net profit = 0.4802, Net payout = 1.4802 / 0.51 ≈ 1.902

Polymarket

• Over: Buy at 0.52, same payout logic
  • Raw payout: 1 / 0.52 ≈ 1.923
  • Profit = 0.48, Fee = 0.0096, Net profit = 0.4704, Net payout = 1.4704 / 0.52 ≈ 1.849

Step 2: Find the Best Price

• Sportsbook Over: 1.952
• Kalshi Over: 1.902
• Polymarket Over: 1.849

Best price: Sportsbook Over at 1.952

But for Under:

• Sportsbook Under: 1.870
• Kalshi Under: (Same math as Over) ≈ 1.902
• Polymarket Under: Buy at 0.50, payout after fee = 1.480 / 0.50 = 1.84

Best price: Kalshi Under at 1.902

Note: This assumes you can fill your desired size. If you want to bet more than $200 on Under, Kalshi isn’t enough. If you want to bet more than $500 on Over, you’ll need to combine venues or accept a worse price.

Step 3: Adjust for Any Additional Fees

If you’re using international sportsbooks, currency conversion or withdrawal fees can eat into profits. Always check the terms for each venue.

Automating the Process

Manually comparing odds, adjusting for fees and fillable size, is tedious and error-prone—especially with 20+ sportsbooks and multiple prediction markets updating constantly. ArbMonster automates this across 20+ sportsbooks, Kalshi, and Polymarket, accounting for all fees and liquidity, and updates live. See more on the math and methodology at ArbMonster Learn.

Conclusion

Comparing odds the right way means:

• Converting all odds to a common metric (decimal or implied probability)
• Adjusting for vig and trading fees
• Checking fillable size at each venue
• Factoring in all transaction costs

Done right, this reveals the real pricing gaps—and sometimes, arbitrage opportunities—that simpler methods miss.

FAQ

Q1: Why do sportsbooks limit the size of bets at good odds?
A: Sportsbooks manage risk and want to avoid losing too much to sharp bettors. If they spot someone targeting mispriced lines, they may lower limits.

Q2: How often do odds really differ enough to create arbitrage?
A: True arbitrage is rare and usually lasts seconds to minutes, especially after accounting for all fees and size. It’s more common to find small value edges.

Q3: Can you trust automated odds comparison tools?
A: Automation reduces errors, but always double-check the numbers—especially for fees, fillable size, and any site-specific quirks.

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.