Converting a negative price to a probability

Implied probability for a negative American price is the price divided by the price plus 100, using the absolute value. −150 becomes 150 ÷ 250, which is 60 per cent. −200 becomes 200 ÷ 300, or 66.7 per cent.

The negative sign says how much must be risked to win 100, so the formula is asking what share of the eventual payout your own stake represents. A price that requires more risk for the same return is describing a more likely outcome, and the arithmetic just makes that comparable across every price on the board.

This is the probability WITH the margin still in it, which means it always overstates the real chance. It is the book's asking price, not the book's opinion, and the two differ by roughly half the overround on each side.

Which is where it goes wrong. Prices near −110 look similar and are not. −105 and −115 differ by about 2.4 points of probability, which over a season is larger than most people's entire claimed edge.

What to do with it. Convert before you compare anything. Two prices in different formats, or the same format at different numbers, are not rankable by eye.

The same division is in the bet calculator, for a price you want to read rather than divide.

Work it out. The odds converter turns any American, decimal, fractional or percentage price into the other three.