Kelly Criterion Calculator

Q: What is the Kelly Criterion formula?

The Kelly Criterion formula is f* = (bp − q) / b . In this formula, b is the net odds you get from the sportsbook, p is your chance of winning, and q is your chance of losing (1 − p). The answer, f* , is the fraction of your bankroll you should bet. If the answer is zero or less, you should not bet at all.

Updated Jul 19, 2026 By Jehan Wadia

Introduction

The Kelly Criterion is a math formula that tells you how much of your bankroll to bet. It uses two things you already know: the odds a sportsbook gives you and how likely you think the bet is to win. From those inputs, it finds the bet size that grows your money the fastest over time without risking too much on a single wager.

This Kelly Criterion calculator does all the math for you. Enter your bankroll, your win probability, and the sportsbook odds in American, decimal, or fractional format. The calculator then shows your optimal bet size, expected value, and a full step-by-step breakdown of the formula. It also lets you use a fractional Kelly multiplier, which most sharp bettors prefer because it lowers risk while still capturing most of the long-term growth.

Bet too much and a losing streak can wipe you out. Bet too little and your edge barely matters. The Kelly Criterion finds the sweet spot between the two. Use this tool before you place a bet to make sure the size of your wager matches the size of your edge.

How to Use Our Kelly Criterion Calculator

Enter your bankroll, win probability, and odds below. The calculator will tell you the best amount to bet based on the Kelly Criterion formula. It also shows expected value, win/loss outcomes, growth rate, and a step-by-step breakdown of the math.

Bankroll ($): Type in the total amount of money you have set aside for betting. This is the full balance you are willing to wager from, not just what you want to bet on one game.

Win Probability (%): Enter how likely you think the bet is to win, as a number from 1 to 99. Be honest and conservative. If you guess too high, the calculator will tell you to bet more than you should.

Odds Format: Pick the format your sportsbook uses. Choose American (+110, -150), Decimal (2.10), or Fractional (7/4). The input field will change to match your choice.

Sportsbook Odds: Type in the odds from your sportsbook. For American odds, use a plus sign for underdogs and a minus sign for favorites. For Decimal odds, enter a number greater than 1.00. For Fractional odds, enter the numerator and denominator separately.

Fractional Kelly Multiplier: Use the slider or type a number between 0.10 and 1.00. A value of 1.00 means full Kelly, and 0.50 means half Kelly. Most bettors use half Kelly or less to lower risk.

Round Bet Down to Nearest ($): Set a dollar amount to round your bet down to. For example, if you enter 5.00, a $47 bet rounds down to $45. Set this to 0 if you do not want rounding.

Minimum Bet ($): This field is optional. If your sportsbook has a minimum bet, enter it here. The calculator will warn you if your Kelly bet is too small to place.

Click Calculate to see your results. Click Reset to clear all fields and start over.

What Is the Kelly Criterion?

The Kelly Criterion is a math formula that tells you how much of your bankroll to bet. It was created by John L. Kelly Jr. in 1956. The goal is simple: grow your money as fast as possible without going broke.

The formula looks at two things: the odds the sportsbook gives you and your real chance of winning. If your chance of winning is higher than what the odds suggest, you have an edge. The Kelly Criterion uses that edge to find the exact bet size that grows your bankroll the most over time.

How the Formula Works

The basic Kelly formula is f* = (bp − q) / b, where b is the net odds you receive on the bet, p is your probability of winning, and q is your probability of losing (1 − p). The result, f*, is the fraction of your bankroll you should wager.

If the formula gives a number at or below zero, it means you have no edge. In that case, the correct bet is no bet at all.

Why Bettors Use Fractional Kelly

Full Kelly betting is aggressive. It assumes your win probability is exactly right, and it isn't afraid of big swings. Most sharp bettors use a fraction of the Kelly amount — often half Kelly — to reduce risk. You give up a small amount of growth in exchange for much smoother results and fewer losing streaks that drain your bankroll.

Key Things to Know

Your win probability must be honest. The Kelly Criterion only works if your estimate is accurate. If you overestimate how often you win, the formula will tell you to bet too much, and you will lose money over time.
Each bet is treated on its own. The formula assumes every wager is independent and does not overlap with your other active bets.
Never round up. If you need to round your bet to a clean dollar amount, always round down. Betting more than Kelly recommends increases your risk without increasing long-term growth.
A 2.5% bankroll cap is a common rule among professional bettors. Even when Kelly suggests a larger stake, many pros never risk more than 2.5% of their bankroll on a single bet.

The Kelly Criterion is used in sports betting, poker, investing, and any situation where you make repeated decisions with an edge. It is one of the most trusted bankroll management strategies because it balances growth and survival — you bet enough to make your edge count, but not so much that a bad run wipes you out.

Formulas used

Raw Kelly Fraction

f^{*} = \frac{b \cdot p - q}{b}

Fractional Kelly Adjustment

f_{adj} = f^{*} \times m

Kelly Dollar Bet

\text{Bet} = \text{Bankroll} \times f_{adj}

Expected Value of Wager

EV = \text{Bet} \times (p \cdot b - q)

Average Fortune Growth Rate

G = p \ln(1 + f^{*} \cdot b) + q \ln(1 - f^{*})

Frequently asked questions

What is the Kelly Criterion formula?

The Kelly Criterion formula is f* = (bp − q) / b. In this formula, b is the net odds you get from the sportsbook, p is your chance of winning, and q is your chance of losing (1 − p). The answer, f*, is the fraction of your bankroll you should bet. If the answer is zero or less, you should not bet at all.

How do I estimate my win probability?

Look at stats, trends, injuries, and matchups to form your own opinion on how likely a bet is to win. Compare your estimate to the implied probability from the sportsbook odds. If your number is higher, you may have an edge. Always be conservative. It is better to guess too low than too high, because overestimating your win probability makes the calculator tell you to bet too much.

What does the calculator mean when it says Do Not Bet?

It means the Kelly formula returned a number at or below zero. This happens when the sportsbook odds do not offer enough value at your estimated win probability. You have no mathematical edge, so the smartest move is to skip the bet entirely.

What is the difference between full Kelly and half Kelly?

Full Kelly (1.00) bets the exact amount the formula recommends. Half Kelly (0.50) bets half that amount. Half Kelly gives up a small amount of long-term growth but cuts your risk and bankroll swings by a lot. Most experienced bettors prefer half Kelly or less because it protects them during losing streaks.

What fractional Kelly multiplier should I use?

Most sharp bettors use a multiplier between 0.25 and 0.50. If you are very confident in your win probability estimates, you can go higher. If you are new to betting or unsure of your edge, stay closer to 0.25. A lower multiplier means smaller bets, less risk, and smoother results.

Can I use this calculator for parlays or multi-leg bets?

This calculator is built for single bets only. The Kelly formula assumes one independent wager at a time. For a parlay, you would need to calculate the combined odds and combined win probability first, then enter those values. However, most bettors apply Kelly to straight bets, not parlays.

Why does the calculator round my bet down instead of up?

Rounding up would make you bet more than the Kelly-optimal amount. Betting more than Kelly recommends increases your risk without adding long-term growth. Rounding down keeps you at or below the ideal stake, which is always the safer choice.

What is the 2.5% professional cap shown in the results?

Many professional bettors never risk more than 2.5% of their bankroll on a single bet, no matter what Kelly says. This cap adds an extra layer of protection. The calculator shows this benchmark so you can compare it to your Kelly bet and decide if you want to use the cap as a ceiling.

What is expected value and why does it matter?

Expected value (EV) is the average amount you would win or lose per bet if you placed the same wager many times. A positive EV means the bet is profitable in the long run. The calculator shows your EV in dollars and as a percentage of your stake so you can see exactly how much edge your bet has.

What is the average fortune growth rate?

The growth rate tells you how fast your bankroll is expected to grow per bet when you use the Kelly-optimal stake. A higher growth rate means your money compounds faster over many bets. This is the main thing the Kelly Criterion is designed to maximize.

How do I convert American odds to decimal odds?

For positive American odds like +150, divide by 100 and add 1. So +150 becomes 2.50. For negative American odds like -200, divide 100 by the number (without the minus sign) and add 1. So -200 becomes 1.50. You do not need to convert manually — this calculator handles all three formats for you.

Does the Kelly Criterion guarantee I will make money?

No. The Kelly Criterion finds the best bet size if your win probability is correct. It does not guarantee wins on any single bet or even over a short stretch. It is a long-term strategy. You can still have losing streaks. The formula only works over many bets, and only when your edge is real.

What happens if I overestimate my win probability?

The calculator will tell you to bet more than you should. Over time, this leads to losses instead of growth. Overestimating your edge is the most common and most costly mistake bettors make with the Kelly Criterion. Always use a conservative estimate.

Can I use this calculator for betting exchanges like Betfair?

Yes. Enter the odds from the exchange and your estimated win probability. Keep in mind that exchanges charge a commission on winnings, so you should adjust your net odds to account for that fee before entering them into the calculator.

What should I enter for the minimum bet field?

Enter the smallest bet your sportsbook allows. This is optional. If the Kelly bet comes out lower than your minimum, the calculator will warn you. In that case, it is usually better to skip the bet rather than bet more than Kelly recommends.

Can I use the Kelly Criterion if I have multiple bets at the same time?

This calculator assumes each bet is independent and placed one at a time. If you have several open bets, your true exposure is higher than what the formula shows. Some bettors lower their Kelly fraction even more when they have multiple active wagers to avoid overexposing their bankroll.

What bankroll amount should I enter?

Enter the total amount of money you have set aside only for betting. Do not include money you need for rent, bills, or savings. Your betting bankroll should be money you can afford to lose. The Kelly Criterion sizes your bets as a percentage of this amount.

Why is my Kelly bet so small?

A small Kelly bet means your edge is small. The odds are close to your true win probability, so there is not much value in the bet. This is normal. The Kelly Criterion is conservative by design — it only recommends large bets when your edge is large.