Betting odds look intimidating until you realize they're just a way of expressing one number: the probability the sportsbook is pricing into an outcome. Once you understand that one idea, everything else — the +150s and −200s, the 2.50s, the 3/2s, the point spreads, the totals, the parlays — becomes arithmetic.
Betting odds look intimidating until you realize they're just a way of expressing one number: the probability the sportsbook is pricing into an outcome. Once you understand that one idea, everything else — the +150s and −200s, the 2.50s, the 3/2s, the point spreads, the totals, the parlays — becomes arithmetic.
This guide takes you from "I have no idea what +150 means" to "I can spot a bad line in five seconds." It covers all three odds formats, the math of implied probability and the vig, the major bet types you'll see at any sportsbook, and the small habits that separate disciplined bettors from people who hand sportsbooks money.
1 ÷ decimal odds, expressed as a percent. If a price implies 40% but you think the true chance is 50%, that's a +EV bet.Sportsbooks display odds in one of three formats depending on geography and tradition. They are mathematical translations of one another. None is "better" — they just present the same probability through different lenses.
The U.S. moneyline default. Shown as a positive or negative integer relative to a $100 unit.
+150 means a $100 bet wins $150 (and you get your $100 stake back, for $250 total).−200 means you bet $200 to win $100 (and get your $200 stake back, for $300 total).+100, −100, or simply "EVEN".American odds carry an implicit reference to a 50% probability: the line at which positive flips to negative. +100 implies 50%; anything more positive implies under 50% (underdog), anything more negative implies over 50% (favorite).
The global online standard, dominant outside the U.S. Shown as a decimal number ≥ 1.0.
This is the format most professional bettors mentally use. The "what's my payout" calculation is one multiplication. No mental arithmetic to figure out implied probability either — see below.
The traditional UK and Irish format, surviving primarily in horse racing and at high-street bookmakers.
3/2 means "win 3 for every 2 staked." A £10 bet returns £15 profit + £10 stake = £25 total.5/2, 4/1, 10/1 are all "win N per 2/1/1 staked."1/2 means "win 1 for every 2 staked" — a heavy favorite. 1/4 means risk 4 to win 1.6/4 and 3/2 are mathematically identical, but UK racing tradition prefers fourths and eighths because punters at the track historically thought in those denominators.The three formats side by side at common probabilities. Bookmark this — once you see a few values, the pattern clicks.
| Decimal | American | Fractional | Implied prob. |
|---|---|---|---|
| 1.10 | −1000 | 1/10 | 90.9% |
| 1.25 | −400 | 1/4 | 80.0% |
| 1.50 | −200 | 1/2 | 66.7% |
| 1.91 | −110 | 10/11 | 52.4% |
| 2.00 | +100 / EVEN | 1/1 | 50.0% |
| 2.50 | +150 | 3/2 (or 6/4) | 40.0% |
| 3.00 | +200 | 2/1 | 33.3% |
| 4.00 | +300 | 3/1 | 25.0% |
| 6.00 | +500 | 5/1 | 16.7% |
| 11.00 | +1000 | 10/1 | 9.1% |
| 51.00 | +5000 | 50/1 | 2.0% |
For any value not on this table, type it into the Odds Converter — it does the conversion in three formats simultaneously plus implied probability and bet payout.
Implied probability is the single most useful piece of math in sports betting. It's the chance the price implies your bet will win.
The formula: implied probability = 1 ÷ decimal odds, expressed as a percentage.
Examples: - Decimal 2.00 → 1/2.00 = 50%. - Decimal 1.50 → 1/1.50 = 66.7%. - Decimal 5.00 → 1/5.00 = 20%.
For American odds, the formula bifurcates:
- Positive (+N): 100 ÷ (N + 100). So +200 → 100/300 = 33.3%.
- Negative (−N): N ÷ (N + 100). So −150 → 150/250 = 60%.
Use the Odds Converter and the implied probability shows up automatically when you type any odds value.
A sportsbook can't run a market that fairly prices both sides at exactly the right probabilities — they'd make zero money. So they shade every price slightly toward the house. Sum the implied probabilities of every outcome in a market and you get something like 104–110%, not 100%. The amount over 100% is the vig.
Worked example. A market lists Team A at −115 and Team B at −105. Convert to decimal: 1.870 and 1.952. Implied probabilities: 53.5% + 51.2% = 104.7%. The 4.7% over 100% is the vig — the sportsbook's expected profit margin on this market.
The lower the vig, the better the bettor's chances. Sharp markets (NFL spreads, MLB totals, tennis moneylines) carry tighter vig (2–5%) than soft markets (futures, exotic props, half-time markets) which can carry 15–30% vig. Always check overround before betting. A 30% vig market is a sucker bet regardless of which side you take.
Below are the bet types you'll see at any sportsbook. Each carries its own structure, math, and pitfalls.
A straight bet on who wins, no spread or handicap. The simplest sportsbook bet.
The moneyline price reflects the implied probability of winning straight up. Heavier favorites have larger negative numbers; bigger underdogs have larger positive numbers.
A handicap added to one team to even out the market. Common in NFL, NBA, college football, college basketball.
Spread bets are typically priced at −110 each side ("standard juice"), implying a 52.4% break-even rate. The team you bet must cover the spread by your bet's win condition for the bet to pay.
A bet on the total combined score of both teams.
Totals share the −110 standard juice and the same 52.4% break-even structure. For full-game totals, late-game scoring patterns matter — many bettors who bet Overs end up rooting for garbage-time scoring. For first-half totals, lower scoring is more predictable.
Bets on specific events within a game — typically player or game props.
Props are popular but high-vig. A typical NFL passing-yards prop carries 8–12% overround (vs. 4.5% on the spread). Sharp bettors hunt for prop value when sportsbooks haven't priced an injury or matchup carefully; recreational bettors get crushed by the vig.
Long-term bets on outcomes that resolve weeks or months later — Super Bowl champion, Premier League winner, MVP awards.
Futures are entertainment for many bettors and a research project for sharps. Approach with a clear EV model or you're paying the book a tax for the fun.
A single bet combining multiple legs; every leg must win for the parlay to pay out. Each leg's decimal odds multiply together.
Parlays are the most popular bet at U.S. sportsbooks because they pay big — but they're heavily juiced and a long-term −EV proposition for most bettors. The Parlay Calculator shows the math and breaks down system bets (Trixie, Yankee, Lucky 15, Heinz) which are smarter alternatives when you want partial-coverage upside.
A typical bet slip looks like:
Patriots −7 −110 $50 to win $45.45
Decoded: - Patriots — the team you backed. - −7 — the spread (Patriots must win by 8+). - −110 — the price (you risk $110 to win $100, or $50 to win $45.45 here). - $50 — your stake. - $45.45 — your profit if it wins. - Total return if it wins: $50 + $45.45 = $95.45.
For a moneyline bet at +180:
Bills +180 $50 to win $90
$50 stake → $90 profit → $140 total return.
For a 3-leg parlay at +600:
Patriots −7
Lakers −5
Yankees −150
$25 parlay +600 to win $150
All three must hit. $25 stake → $150 profit if all win.
Some final scores happen far more often than others. Lines that cross these "key numbers" are systematically more valuable than lines that don't.
Two sportsbooks rarely price the same game identically. The bettor who consistently takes the better price compounds an edge over thousands of bets.
Game: Yankees vs. Red Sox.
| Sportsbook | Yankees ML | Implied prob. |
|---|---|---|
| Book 1 | −150 | 60.0% |
| Book 2 | −145 | 59.2% |
| Book 3 | −155 | 60.8% |
Best price: Book 2 at −145. Difference vs. Book 1 (−150): about 0.8% implied probability. Tiny — but compound across 1,000 bets a year and the line-shopping bettor extracts hundreds of dollars in expected value vs. the bettor who reflexively uses the same sportsbook every time.
A −150 favorite isn't 60% to win because it's priced at 60%. The price is the sportsbook's opinion plus margin. Markets are usually wise but not always — your job as a bettor is to find spots where you think the true probability is higher than the implied. That's +EV betting.
A market priced at −110/−110 has 4.5% vig. A market priced at −115/−105 has 4.7% vig. Both look similar; both bleed money on every bet. The break-even win rate at −110 is 52.4%. To beat the book net of vig, you need to win significantly above 50% on these markets — and that's hard.
Stacking 5 legs at +100 each gives you 32× your stake. Sounds great. The hit rate is 1 in 32, exactly. Each individual leg also has its own vig, which compounds. Most parlay players lose at higher rates than straight-bet players because the long-shot psychology + compounded vig is a brutal combination.
Same Game Parlays look like 3 independent legs but aren't — books correlate the legs and re-price at higher juice. A 3-leg SGP at "implied" +600 might actually price at +500. Always read the slip total, not the implied multiplier from the legs.
Variance is real. A −EV bet can win; a +EV bet can lose. Long-term profit comes from edge, not from any single result. Sizing bets at 1–5% of bankroll, not "let me get my money back this weekend," is what separates disciplined bettors from broke ones.
When a price catches your eye:
This loop, applied consistently, is the difference between dabbling and disciplined betting. The math is simple. The discipline is hard.