Kelly Calculator

Note

The app interface is in Spanish. This guide references the actual labels you will see.

The Kelly Calculator tells you how much of your bankroll to wager on a bet based on your edge. The Kelly Criterion is a mathematical formula that maximizes long-term bankroll growth by sizing bets proportionally to your advantage. The tool shows three fractions — Full, Half, and Quarter Kelly — so you can choose the aggression level that fits your risk tolerance.

This tool is available to all users at Tools > Kelly Calculator (/tools/kelly-calculator). All calculations run entirely in your browser.

How to Use

Enter your inputs in the calculator form:

Input	Label	Description
Odds Format	Odds Format	Choose between American, Decimal, or Fractional. Your selection is saved to localStorage and shared across all calculator tools.
Odds	Odds	Enter the sportsbook odds for your bet. When Fractional is selected, two input fields appear separated by a / divider.
Win Probability	Win Probability (%)	Your estimated probability that the bet wins, from 0 to 100. The implied probability from the odds is shown as a hint below the field.
Bankroll	Bankroll ($)	Your total bankroll in dollars. Used to convert percentages into dollar amounts.

Results update instantly as you change any input.

Understanding the Results

When your estimated probability gives you an edge over the implied odds (Kelly > 0), you see three rows:

Output	Label	Description
Quarter Kelly	Conservative	The smallest recommended stake. Shows the percentage of bankroll and the dollar amount. Best for bettors who want minimal variance.
Half Kelly	Recommended	The middle ground. Captures most of the growth benefit with significantly less volatility than Full Kelly.
Full Kelly	Aggressive	The mathematically optimal fraction for maximum long-term growth. High variance — only for bettors comfortable with large swings.

When there is no edge (Kelly is zero or negative), the calculator displays a red No Bet card with the message: “No edge detected at these odds and probability.” This means the sportsbook odds already price in more probability than your estimate — there is no value in placing this bet.

A Show Math collapsible section displays the full formula with your actual numbers substituted in.

How It Works

The Kelly Criterion formula is:

f* = (b x p - q) / b

Where:

• f* = fraction of bankroll to wager (Full Kelly)
• b = decimal odds - 1 (the net profit per dollar wagered)
• p = your estimated win probability
• q = 1 - p (loss probability)

The fractional variants are:

• Half Kelly = f* / 2
• Quarter Kelly = f* / 4

If f* is zero or negative, the formula says “do not bet” because you have no mathematical edge.

Tips

Half Kelly Is the Sweet Spot

Most professional bettors use Half Kelly. Full Kelly maximizes long-term growth mathematically, but it assumes your probability estimates are perfectly accurate. In practice, Half Kelly gives you roughly 75% of the growth rate with far less volatility and drawdown risk.

• The Kelly formula is only as good as your probability estimate. If you overestimate your win probability, you will overbet.
• Never bet more than Full Kelly. Overbetting (even slightly) turns long-term growth into long-term ruin.
• Use the EV Calculator first to confirm the bet has positive expected value, then come here to size it.
• Start with Quarter Kelly if you are new to bankroll management. You can move to Half Kelly as you gain confidence in your probability estimates.
• The bankroll field should reflect your actual betting bankroll, not your total savings. Only risk money you have set aside specifically for betting.