What is the Kelly Criterion?

The Kelly Criterion is a mathematical formula for calculating the optimal fraction of a bankroll to bet on a favorable gamble. The formula is: f = (bp - q) / b, where b is the odds, p is the probability of winning, and q is the probability of losing.

Why do professionals use fractional Kelly?

Full Kelly betting can lead to large drawdowns due to variance. Most professionals use half-Kelly or quarter-Kelly to reduce volatility while still capturing much of the long-term growth advantage.

What if the Kelly Criterion gives a negative result?

A negative Kelly result means the bet has no mathematical edge — you have a negative expected value. You should not place this bet, as it will lead to loss over the long run.

Kelly Criterion Calculator

Calculate the optimal bet size using the Kelly Criterion formula. Enter your bankroll, odds, and estimated win probability to find the optimal wager.

Bet details

Decimal odds: 2,10(impl. prob. 47,6 %)

1.055.0010.0020.00

Estimated win probability: 52 %Edge: +4,4 %

1 %25 %50 %75 %99 %

Bankroll: 1 000 €

Kelly fraction

41,82 €

Recommended stake

(4,2 % of bankroll)

+46,00 €

Profit if win

−41,82 €

Loss if lose

+9,20 €

EV / €100 stake

Kelly fractions comparison

Full Kelly

83,64 €

8,4 %

½ Kelly

41,82 €

4,2 %

¼ Kelly

20,91 €

2,1 %

⅛ Kelly

10,45 €

1,0 %

✅ Value bet — positive edge

Edge: +4,38 % — every €100 stake returns an expected value of +9,20 € long-term. Full Kelly stake would be 83,64 € (8,4 %).

📐 Kelly formula explained

f* = (b × p − q) / b

b = 1,10

Net odds
(odds − 1)

p = 0,520

Win probability
(52 %)

f* = 8,4 %

Optimal fraction
of bankroll

Kelly The Kelly formula maximizes the logarithmic growth of the bankroll over time. Full Kelly is mathematically optimal but too risky in practice — most professionals use ½ or ¼ Kelly.

⚠️ Betting is gambling. Gamble responsibly. Help: peluuri.fi or tel. 0800 100 101 (free).

The Kelly Criterion Calculator works out the optimal bet size for you. Enter your estimated win probability and the odds, and it tells you the mathematically ideal fraction of your bankroll to stake.

How the calculator works and what it’s for▾

What the Kelly Criterion is based on

The Kelly Criterion is a formula designed to grow a bankroll as fast as possible over the long run without taking on an unreasonable risk of ruin. It weighs both the probability of winning and the odds on offer.

The bigger your edge relative to the odds, the larger the fraction the criterion recommends. When there is no edge, it suggests not betting at all.

What you enter and what you get

You provide your own estimate of the event's probability along with the odds. The calculator turns these into a recommended stake as a percentage of your bankroll.

The result shows how large a single bet should be relative to the funds you have available.

Who it is for

The calculator serves bettors and investors who want to manage risk and grow a bankroll steadily rather than staking random amounts.

It is especially suited to those who can assess probabilities realistically.

Common mistakes and tips

The biggest pitfall is an overly optimistic probability estimate, which leads to oversized bets. Many people therefore stake only a fraction of what Kelly recommends to reduce swings.

Remember the criterion assumes your estimate is accurate. A skewed estimate produces a skewed stake recommendation.

🔄 Reviewed June 2026