How to Read Betting Odds
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What Odds Actually Tell You
Odds do two things at once: they tell you how much you'll be paid if you win, and they encode the sportsbook's view of how likely each outcome is. Every betting strategy — including matched betting and arbitrage — depends on reading odds quickly and accurately.
There are three formats. US sportsbooks default to American odds. European books use decimal. UK bookmakers often show fractional. You will encounter all three, so it is worth knowing each one.
American Odds (+/−)
American odds use a plus or minus sign followed by a number. The sign tells you whether you're looking at an underdog (+) or a favorite (−).
Positive odds (+) — the underdog
The number shows how much profit you make on a $100 bet.
Example: +130 → bet $100, win $130 profit → collect $230 total
Negative odds (−) — the favorite
The number shows how much you need to bet to win $100 profit.
Example: −150 → bet $150, win $100 profit → collect $250 total
You don't have to bet exactly $100. These are just ratios. At +130 you win $1.30 for every $1 you stake. At −150 you win $0.67 for every $1 you stake.
The Quick Formula
Positive odds: profit = stake × (odds ÷ 100)
Negative odds: profit = stake × (100 ÷ |odds|)
Total return = profit + original stake
Decimal Odds
Decimal odds are the simplest format to calculate with. Multiply your stake by the decimal to get your total return— that's profit plus your original stake back.
2.30 → $100 × 2.30 = $230 total return ($130 profit)
1.67 → $100 × 1.67 = $167 total return ($67 profit)
2.00 → even money ($100 profit on a $100 bet)
Decimal odds below 2.00 are favorites. Decimal odds above 2.00 are underdogs. Even money (a coin flip) is exactly 2.00.
Converting American → Decimal
Positive American: decimal = (odds + 100) ÷ 100
+130 → (130 + 100) ÷ 100 = 2.30
Negative American: decimal = (|odds| + 100) ÷ |odds|
−150 → (150 + 100) ÷ 150 = 1.67
Fractional Odds
Fractional odds are written as X/Y (spoken "X to Y"). They show your profit relative to your stake — how much you win for every Y dollars you risk.
5/2 → win $5 for every $2 staked → $100 bet wins $250 profit ($350 total)
1/1 (evens) → win $1 for every $1 staked → even money, equivalent to +100 or 2.00 decimal
2/5 → win $2 for every $5 staked → $100 bet wins $40 profit ($140 total)
Most US sportsbooks don't use fractional odds, but you may see them on international sites or when reading about futures markets. Once you can convert to decimal, the math is the same.
Quick Reference Table
The same odds across all three formats, with implied probability:
| American | Decimal | Fractional | Implied prob. |
|---|---|---|---|
| −200 | 1.50 | 1/2 | 66.7% |
| −150 | 1.67 | 2/3 | 60.0% |
| −110 | 1.91 | 10/11 | 52.4% |
| +100 | 2.00 | 1/1 | 50.0% |
| +130 | 2.30 | 13/10 | 43.5% |
| +200 | 3.00 | 2/1 | 33.3% |
| +400 | 5.00 | 4/1 | 20.0% |
Implied Probability
Every set of odds implies a probability — the chance the sportsbook thinks that outcome will happen. This is called implied probability, and it is one of the most important concepts in all of betting.
How to Calculate It
From positive American odds: implied prob = 100 ÷ (odds + 100)
+130 → 100 ÷ 230 = 43.5%
From negative American odds: implied prob = |odds| ÷ (|odds| + 100)
−150 → 150 ÷ 250 = 60.0%
From decimal: implied prob = 1 ÷ decimal
2.30 → 1 ÷ 2.30 = 43.5%
Notice: 43.5% + 60.0% = 103.5%, not 100%. That extra 3.5% is the sportsbook's margin. The implied probability & vig guide explains this in detail — it's the same margin that matched betting and arbitrage work around.
Why This Matters for Matched Betting
When you place a matched bet, you're comparing odds at two different sportsbooks. Being able to read and convert odds quickly means you can spot which combination of bets produces the best outcome without needing to open a spreadsheet every time.
In practice: most US books default to American odds, so that's your working format. The calculator handles the exact math, but understanding the numbers behind it lets you sanity-check outputs and catch errors before money is on the line.