BY 
I spent years building projections for a living, and what finally pushed me into writing about betting was how much of the content out there runs on pure vibes. A price is just probability wearing a costume. Learn to undress the number and every line, and every capper record, gets much easier to judge.
We'll do this properly, with worked examples. The arithmetic is friendlier than it looks, I promise.
American odds: plus and minus
American odds are built around $100 as a reference point. The sign tells you which direction the $100 works.
- Negative odds like -150: the number is what you'd risk to win $100. Bet $150 at -150 and a win earns you $100 in profit.
- Positive odds like +200: the number is what you'd win on a $100 risk. Bet $100 at +200 and a win pays $200 in profit.
Minus means favorite, plus means underdog, and the size of the number tells you how lopsided the pricing is. A -300 favorite is expected to win far more often than a -120 one.
Decimal and fractional: same information, different costume
Decimal odds, which you'll see at most books outside the US, show total return per unit staked. A price of 2.50 means a $10 bet returns $25 total, which is $15 of profit plus your stake back.
Converting is quick. For positive American odds, decimal equals the odds divided by 100, plus 1, so +150 becomes 2.50. For negative American odds, decimal equals 100 divided by the odds, plus 1, so -200 becomes 1.50.
Fractional odds, the UK style, show profit over stake. A 3/1 price pays three units of profit per unit staked, the same bet as +300 American or 4.00 decimal. One piece of information, three outfits.
Books pick the format their market expects, and nothing else changes. A Las Vegas board and a London board can post the exact same opinion in different clothes. Don't let the presentation spook you.
Implied probability: what the line is really saying
This is the conversion I wish every beginner learned first, because every price is a probability statement in disguise. Decoding it takes one step.
- Negative odds: implied probability = odds / (odds + 100). So -150 implies 150/250, which is 60%.
- Positive odds: implied probability = 100 / (odds + 100). So +200 implies 100/300, which is 33.3%.
When you place a bet, you're asserting the true probability is higher than the implied one. Take -150 (60% implied) on a team you believe wins 55% of the time and you're lighting money on fire, no matter how good the team looks on TV.
A worked example, since that's how this sticks. Say a book posts your team at +200 and your own handicapping says the team wins 4 times out of 10. Implied is 33.3% and your estimate is 40%, so the bet carries positive expected value even though the team probably loses. That inversion trips up every beginner, and getting comfortable with it is the entire job.
Back in my projections days I kept a sticky note on my monitor asking what the price already knew. I still think that's the best one-line betting advice available anywhere.
The vig: why both sides add up to more than 100%
Take a standard spread bet priced at -110 on both sides. Each side implies 110/210, or 52.38%. Add them together and you get 104.76%. That extra 4.76% is the vig, the book's margin baked directly into the price.
Two consequences fall out of that math, and they're worth memorizing. First, break-even at -110 is 52.38% rather than 50%, so a bettor who picks winners at an exact coin flip rate loses about 4.5% of everything wagered, forever. Second, win percentage without odds context tells you nothing.
That second point is why the CAPTRACKER leaderboard ranks its 900+ tracked handicappers by units won and ROI instead of raw win rate. A 60% record built on heavy favorites can be worth less than a 45% record on underdogs. If terms like units and ROI are new to you, our betting glossary covers the vocabulary.
Reading a full board
A typical game listing shows three markets. The spread is a margin handicap, usually priced near -110 both ways. The moneyline is the straight winner, with odds that scale with team strength. The total is the combined score, bet over or under.
Try one: a board shows the underdog at +6.5 (-110), +220 on the moneyline, with a total of 219.5. The spread ticket cashes even if your team loses by six. The moneyline implies 100/320, about 31%, and only pays if they win outright. Same game, three different questions.
The formats above apply identically to all of them. Learn the conversions once and you can read any board at any book.
Putting it to work
Next time you see a pick anywhere, translate it before you consider it. What's the implied probability? What win rate would make this profitable after the vig?
Then ask the question almost nobody asks: does the person posting it have a verified record, over a real sample, at these kinds of prices? You can answer that in about ten seconds on CAPTRACKER's free daily feed, where every pick is timestamped and locked, then settled automatically against ESPN data.
The costume comes off fast once you practice. Give it a week of translating every line you see, and the board starts reading like plain English.