Free calculator

Kelly Criterion Calculator for Sports Betting

Enter book odds, your win probability and bankroll to get full, half and quarter Kelly stakes.

If edge is zero or negative the stake is zero. Fractional Kelly is the usual practical choice when your probability is an estimate.

Calculator

Full Kellyn/a
Half Kellyn/a
Quarter Kellyn/a
Half-Kelly staken/a

How do you calculate Kelly stake for sports betting?

f* = p − (1−p)/(decimal−1). Half Kelly is f*/2. Stake = fraction × bankroll. Negative edge means stake nothing.

Why use half Kelly for betting?

Your win probability is an estimate. Half or quarter Kelly cuts variance when that estimate is wrong.

Can I use fractional odds with Kelly?

Convert to decimal first (decimal = fractional + 1), or use the odds converter on this site.

The formula

For decimal odds: b = decimal − 1, q = 1 − p
Full Kelly fraction f* = p − q/b = p − (1−p)/(decimal−1)
Half / quarter Kelly = f*/2 or f*/4. Stake = f × bankroll.
Negative f* means no bet.

Worked example

Win probability 55%, decimal odds 2.2 (so b = 1.2), bankroll £1,000.

f* = 0.55 − 0.45/1.2 = 0.175 = 17.5% of bankroll.

Half Kelly = 8.75%. Stake = £87.50.

Quarter Kelly = 4.375% = £43.75.

Methodology

How it calculates

  • Full Kelly fraction from win probability and decimal odds (or payoff ratio b), then half and quarter Kelly.
  • Stake = fraction × bankroll when a bankroll is entered.

Formula

For decimal odds: b = decimal − 1, q = 1 − p
Full Kelly fraction f* = p − q/b = p − (1−p)/(decimal−1)
Half / quarter Kelly = f*/2 or f*/4. Stake = f × bankroll.
Negative f* means no bet.

Assumptions

  • Assumes a known edge. Estimation error makes fractional Kelly the usual practical choice.

Edge cases

  • Negative or zero edge => Kelly fraction and stake are 0 (no bet).
  • Half / quarter Kelly are f*/2 and f*/4.
  • Same Kelly maths as the parent tool; aimed at sports-betting stake sizing language.

Sources

None. Pure maths; no external rate or spec feed.

FAQ

Why use half Kelly for betting?

Full Kelly maximises long-run growth only if your probability is exact. Estimation error and variance make half or quarter Kelly the usual sports-betting choice.

Can I use fractional odds?

Convert to decimal first (decimal = fractional + 1), or use the odds converter. This Kelly tool takes decimal odds or a win/loss payoff ratio b.

Is positive Kelly a guarantee you win?

No. It is a sizing rule given your edge assumption. Variance still hurts, and a wrong probability makes Kelly too aggressive.

What if my edge is negative?

Kelly says stake nothing. The calculator shows 0% and a warning.

Not financial advice. This calculator is for education only. Double-check results before you make money decisions. Past moves do not predict future ones.