Open any sportsbook and the screen looks like a foreign language. A -110 here, a +250 there, numbers blinking everywhere. Most people just tap the team they like and hope for the best. But the board is not random, it is math, and once you can read it you stop making bets that were losers before the game even started. This is the single most important skill in betting, and it takes about ten minutes to learn.

What do the + and - mean in betting odds?

American odds are built around 100 dollars, and the sign in front of the number tells you everything you need to know.

A - sign means the favorite (like -110). It tells you how much you have to bet to win 100 dollars. At -110, you bet 110 to win 100. At -200, you bet 200 to win 100.
A + sign means the underdog (like +250). It tells you how much you win on a 100 dollar bet. At +250, you bet 100 to win 250. The bigger the plus number, the bigger the underdog.

That is the surface-level translation, and most bettors never get past it. But the dollar amount is not the point. The probability hiding inside the number is where the real information lives.

What is implied probability, and how do you calculate it?

Every set of odds is just a win probability wearing a costume. The price tells you the exact chance the book is saying you need to win, simply to break even. The conversions are quick:

For a minus (favorite): divide the odds by the odds plus 100. So -110 becomes 110 divided by 210, which is 52.4%.
For a plus (underdog): divide 100 by the odds plus 100. So +250 becomes 100 divided by 350, which is 28.6%.

This is the most useful habit you can build. Before you place any wager, convert the line to its break-even percentage, then ask one honest question: do I really believe this happens more often than that? If a +250 underdog needs to win 28.6% of the time and you think it wins closer to 40%, that is a bet worth making. If the answer is no, you are not backing a team, you are donating.

What is the vig, and why does it matter?

Here is the part the casual bettor never notices. A standard game is priced -110 on both sides. Add the two break-even numbers together: 52.4% plus 52.4% equals 104.8%. That extra 4.8% does not exist in nature. It is the house cut, called the vig or the juice, baked silently into the price of every bet you place.

It is the reason "I win about half my bets" is quietly a losing record. At -110 you actually have to win closer to 53% just to stay even. The coin flip the book offers you is not a fair coin flip, it is a coin flip minus a tax, and that tax is exactly how sportsbooks make money whether you win or lose.

How to actually use this when you bet

Reading the board well comes down to one repeatable move. Turn the line into a probability, then compare it to what you genuinely believe the real chance is.

When your number is higher than the book's number, that gap is your edge. That is a bet with value.
When your number is lower, walk away, no matter how much you like the team.
Remember that a high win rate at a bad price still loses money. The smartest bettors hunt the gap between the real probability and the price, not the favorite.

This is also why two sportsbooks offering the same bet at different prices matters so much. The exact same wager at -105 instead of -120 can be the difference between a long-term winner and a long-term loser, purely because of the implied probability you are paying.

Frequently asked questions

What does -110 mean? It means you bet 110 dollars to win 100. It also implies a 52.4% break-even win rate, which is why standard bets are priced this way.

What does +250 mean? It means a 100 dollar bet wins 250. It implies the book thinks this outcome happens about 28.6% of the time.

How do you convert American odds to a percentage? For favorites, odds divided by (odds + 100). For underdogs, 100 divided by (odds + 100). That percentage is the break-even win rate.

Here's the bottom line

The book is counting on you not doing this math. Every juiced line, every "safe" favorite, every parlay that pays less than it should, all of it works because most people never convert the odds into a real probability and check it against reality. You do not need to be a genius to beat that, you just need to stop guessing and start comparing. That is the whole idea behind DataStreak: we show you the real over/under hit rate for every player prop, computed from actual game logs, sitting right next to the current line and the odds. So the moment the book's implied probability drifts from how often something actually happens, you see the gap instead of guessing at it. Read the board, then beat it.

See the real over/under hit rate next to the line for every prop in the DataStreak Streak Finder.