Implied Probability

Odds and probability carry exactly the same information in different notation. Learning to move between them instantly is the first real skill in betting, because you cannot judge a price you have not translated.

The conversions

American odds. Negative prices show what you risk to win 100; positive prices show what you win from 100.

So −150 becomes 150 ÷ 250 = 60%, and +250 becomes 100 ÷ 350 = 28.6%.

Decimal odds. Far simpler: implied probability is 1 divided by the decimal. Odds of 2.50 imply 1 ÷ 2.50 = 40%.

Fractional odds. Convert to decimal first by dividing and adding one, then take the reciprocal. 5/2 becomes 3.50, which implies 28.6%.

The odds converter does all three at once, and the implied probability calculator runs the conversion in both directions.

A few worth memorising

The −110 row is the one to know by heart. It is the standard price on spreads and totals, and 52.4% is the win rate you need just to break even.

The number is not the truth

Implied probability includes the bookmaker's margin, which means it always overstates the real chance of the outcome. Add both sides of a market and you get more than 100%:

This matters enormously in practice. If you compare your own estimate to the posted implied probability, almost every bet will look bad, because the posted number is deliberately worse than fair. The correct comparison is against the no-vig fair probability.

Going the other way

You will often want the reverse: you think something happens 42% of the time, so what price would be fair? Take the reciprocal to get decimal odds, then convert.

1 ÷ 0.42 = 2.381 decimal, which is about +138 in American terms. If the book is offering +160 on that outcome, you have found a value bet. If it is offering +110, you have not.

That single comparison — your probability against the fair price — is the whole of betting analysis. Everything else is about making the probability more trustworthy.