How to Calculate Odds in Betting: Formulas & Tools
Learn how to calculate betting odds in every format, convert to implied probability, and find value in the lines.
By
Eric Pauly
9 min read
What Does "Calculating Odds" Actually Mean?
When a sportsbook posts -110 on a spread or +250 on a moneyline, those numbers aren't random — they encode two things: the implied probability of that outcome and the sportsbook's margin (vig). Knowing how to calculate odds means you can translate any line into a probability, compare it to your own estimate, and decide whether there's value in the bet.
This guide covers how to calculate odds in all three major formats — American, decimal, and fractional — plus how to find implied probability and what tools can do the math automatically.
article Summary
Quick version: American odds of -110 mean implied probability of 52.4%. American odds of +200 mean implied probability of 33.3%. To find value, your estimated probability needs to be higher than the implied probability. That gap is your edge.
How to Calculate American Odds
American odds (also called moneyline odds) are displayed as a positive or negative number. The math is different depending on which sign you're looking at.
Negative odds (favorites): These tell you how much you need to bet to win $100.
-110 means bet $110 to profit $100
-200 means bet $200 to profit $100
-350 means bet $350 to profit $100
Positive odds (underdogs): These tell you how much you profit on a $100 bet.
+150 means bet $100 to profit $150
+300 means bet $100 to profit $300
+550 means bet $100 to profit $550
To calculate your payout on any bet size, use this formula:
Negative odds: Profit = (Stake / (Odds / 100))
Positive odds: Profit = (Stake x (Odds / 100))
How to Calculate Implied Probability
Implied probability is the key number. It's what the sportsbook thinks the outcome's chance of happening is, baked into the line. Knowing how to convert odds to implied probability is how you identify whether a bet has value.
For negative odds:
Implied probability = |Odds| / (|Odds| + 100) x 100
For positive odds:
Implied probability = 100 / (Odds + 100) x 100
If the implied probability is lower than your own estimated probability of the outcome, you have a positive expected value (+EV) bet.
How to Calculate Decimal and Fractional Odds
If you're betting on international sportsbooks or looking at exchange prices, you'll run into decimal and fractional formats. Both are actually simpler to work with than American odds.
Decimal odds: Multiply your stake by the decimal to get your total payout (stake included).
2.50 odds on a $100 bet = $250 payout ($150 profit)
1.91 odds on a $100 bet = $191 payout ($91 profit) — this is roughly equivalent to -110
Implied probability from decimal odds = (1 / Decimal) x 100
Fractional odds: Common in UK markets. The left number is profit, the right number is the stake.
5/2 means profit $5 on every $2 wagered
1/2 means profit $1 on every $2 wagered (a heavy favorite)
Implied probability from fractional odds = Denominator / (Numerator + Denominator) x 100
How to Convert Between Odds Formats
Sometimes you'll need to compare lines across books or formats. Here's how to convert cleanly:
American to Decimal:
Negative: (100 / |Odds|) + 1 — so -110 becomes (100/110) + 1 = 1.909
Positive: (Odds / 100) + 1 — so +200 becomes (200/100) + 1 = 3.00
Decimal to American:
If decimal >= 2.0: (Decimal - 1) x 100 — so 3.00 becomes +200
If decimal < 2.0: -100 / (Decimal - 1) — so 1.909 becomes -110
Most odds comparison tools handle this automatically. But knowing the math means you're never surprised when shopping lines.
Using Tools to Calculate Odds Faster
Manual math is useful to understand the fundamentals, but sharp bettors use tools to speed up line analysis and catch value in real time.
Odds calculators: Input any odds format and get implied probability, no-vig fair probability, and payout instantly.
Odds comparison tools: Show lines across 20+ sportsbooks side by side. Useful for line shopping and finding the best available price.
+EV scanners: These tools remove the vig from sharp books (Pinnacle, Circa) to find the true probability, then flag any soft-book line that pays more than fair value.
The combination of understanding the math and using the right tools is what separates informed bettors from recreational ones.
Calculating odds is foundational. Every serious bettor should be able to look at -115 and know it implies 53.5% probability. Once you're comfortable with the math, the next step is building a system to find lines where the market has mispriced the real probability. That's where the edge lives.
The formulas above are worth knowing by heart. But the bigger skill is recognizing when the implied probability doesn't match your edge estimate — and having the process to act on it consistently.
Frequently Asked Questions
Additional Resources
Explore our curated selection of guides and tools to help promote responsible gambling.
Golf Betting Guide: Types of Golf Bets and How to Find Value
Golf offers some of the softest betting markets in sports. Learn the bet types, how odds work, and strategies for profitable golf wagering.
Soccer Betting Tips: How to Bet Smarter on the Beautiful Game
Soccer is the most bet-on sport in the world, yet most bettors ignore the markets where value actually lives. These tips help you find it.
Spread Betting Guide: How Point Spreads Work and How to Beat Them
How point spreads work, why lines move, and where sharp bettors find value.
Online Sports Betting: How to Place Your First Bet
New to online sports betting? Learn how to place your first bet, choose a sportsbook, and avoid common beginner mistakes.
