Odds Converter Calculator – From Price to Probability, EV and Kelly Stake
Odds show you a price, but what matters for long-term betting is probability and expected value. This Odds Converter Calculator from MyTimeCalculator helps you move between American, decimal and fractional odds, translate prices into implied win rates, compute expected value per bet and apply the Kelly Criterion for bankroll sizing.
Throughout this article, decimal odds are written as d, American odds as A, fractional odds as n/d and implied probability as p (between 0 and 1). Stake is denoted by s and bankroll by B.
Odds Formats and Conversion Formulas
Different markets and sports use different odds formats, but they all describe the same underlying idea: how much you win if the bet succeeds compared with how often it is expected to succeed.
Decimal odds
Decimal odds d represent total return per unit staked, including stake. If you stake s at decimal odds d and the bet wins, your total return is d·s, of which d·s − s = s(d − 1) is profit and s is the original stake.
American odds
American odds are written as a positive number +A for underdogs or a negative number −A for favorites.
- If A is positive, the odds show how much profit you win on a 100 stake: profit = A for every 100 staked.
- If A is negative, the odds show how much you must stake to win 100 profit: you stake A (in absolute value) to win 100.
The conversion from American odds to decimal odds is:
- For A > 0 (positive odds): d = 1 + A / 100.
- For A < 0 (negative odds): d = 1 + 100 / |A|.
Fractional odds
Fractional odds show profit compared with stake as a fraction n/d. For every d units staked, you win n units of profit if the bet succeeds. The conversion to decimal odds is
d = 1 + n/d.
Implied probability from odds
Once you have decimal odds, the implied probability of a win under those odds (ignoring bookmaker margin) is
p = 1 / d.
The implied probability percentage is p × 100. The calculator applies these formulas behind the scenes so you can enter odds in any supported format and see the implied win rate.
Expected Value (EV) of a Bet
Expected value combines the odds with your own win probability estimate. For a single bet with stake s, decimal odds d and personal win probability estimate p, there are two outcomes:
- Win with probability p, profit s(d − 1).
- Lose with probability 1 − p, profit −s.
The expected value EV per bet is
EV = p·s(d − 1) + (1 − p)(−s).
This simplifies to
EV = s(pd − 1).
The expected value per unit stake is
EV per unit stake = EV / s = pd − 1.
EV as a percentage of stake (a return on investment) is
EV% = (EV / s) × 100 = (pd − 1) × 100.
If EV% is positive, the bet is favorable under your probability estimate. If EV% is negative, the bet is unfavorable on average.
Break-even Probability and Break-even Odds
The break-even win probability is the value of p that makes EV equal to zero. Setting EV per unit stake equal to zero gives the break-even condition
pd − 1 = 0.
Solving for p yields the break-even probability
pBE = 1 / d.
In percentage terms, the break-even win rate is
pBE% = 100 / d.
This is the same formula used for implied probability, which shows that if your personal win probability p is higher than the implied probability, the bet has positive expected value. The calculator converts American and fractional odds to decimal odds first, then applies these formulas to show break-even percentages and equivalent odds in each format.
Kelly Criterion for Bankroll Management
The Kelly Criterion is a formula for deciding what fraction of your bankroll to risk on a favorable bet. For decimal odds d and win probability estimate p, define
- b = d − 1, the profit-per-unit-stake multiplier.
- q = 1 − p, the loss probability.
The Kelly fraction f* is
f* = (b·p − q) / b.
This is the fraction of your bankroll B to stake on the bet in order to maximize long-run logarithmic growth when f* is between 0 and 1. The optimal stake is
sKelly = f*·B.
If f* is negative, the edge is negative and the Kelly strategy says not to bet. If f* is greater than 1, the formula suggests risking more than your entire bankroll, which usually indicates extreme input values and is not practical.
Many bettors use fractional Kelly strategies, such as half-Kelly, to reduce drawdowns:
- Half Kelly stake: shalf = 0.5·f*·B.
The Kelly tab of the calculator computes f*, the dollar stake for full Kelly, the stake for half Kelly and the size of your edge.
Using the Odds Converter Calculator Effectively
- Start with the odds converter tab to normalize any price into decimal odds, American odds, fractional odds and implied probability.
- Use the implied probability tab when you only care about the win rate the market is pricing in for a single set of odds.
- Switch to the EV tab when you have your own probability estimate and want to see if the bet is positive or negative expected value.
- Use the break-even tab to see what hit rate you need and compare it to your historical win rate or model estimates.
- Use the Kelly tab only when you are comfortable with volatility and risk. Even then, consider half Kelly or smaller fractions for more conservative bankroll management.
This Odds Converter Calculator is intended for education, planning and analysis. It does not guarantee profits and does not replace professional financial or betting advice. Always gamble responsibly and never risk money you cannot afford to lose.
Odds Conversion & Betting Math FAQs
Frequently Asked Questions About Odds, Probability and EV
Quick explanations that connect American, decimal and fractional odds with implied probability, expected value and Kelly stake sizing.
Different regions developed different odds conventions. American odds are common in the United States, decimal odds dominate in Europe and Australia, and fractional odds have a long history in the UK and horse racing. All formats carry the same information once converted to probabilities and payouts, so a converter tool helps you compare them fairly.
No. Expected value is a long-run average. Even with a positive EV, any single bet can still lose. The benefit of positive EV appears only over many independent bets, where the average outcome tends to move toward the expected value predicted by the math.
Full Kelly is aggressive and leads to high volatility, especially when edges or probabilities are uncertain. Many experienced bettors use reduced Kelly fractions such as half or quarter Kelly to limit drawdowns. Treat the Kelly fraction as an upper bound rather than an automatic staking rule.
If your probability estimate is less accurate than the market, EV and Kelly calculations can be misleading and may suggest bets that are not actually profitable. Building reliable probability models and tracking your long-term results is essential before relying heavily on EV or Kelly formulas.
No. The calculator provides mathematical quantities based on the odds and probability estimates you enter. Deciding which bets to place depends on your strategy, risk tolerance, bankroll and personal circumstances. Always make decisions carefully and within your limits.