Definition
Fractional Kelly means betting only a set portion of the full Kelly stake to reduce bankroll swings. Full Kelly stake is:
f = (bp - q) / b
where f is the bankroll fraction to bet, b is decimal odds minus 1, p is your estimated win probability, and q is 1 - p.
Half Kelly uses 50% of the full Kelly stake. Quarter Kelly uses 25%.
Worked Example
A bettor has a $2,000 bankroll and likes a team at −110. At −110, the decimal odds are 1.9091, so b = 0.9091.
The bettor projects the team to win 55% of the time:
f = (0.9091 × 0.55 - 0.45) / 0.9091
f = (0.5000 - 0.45) / 0.9091
f = 0.055
Full Kelly says to risk 5.5% of bankroll, or $110.
Using Half Kelly, the stake is 2.75% of bankroll:
$2,000 × 0.0275 = $55
At −110, risking $55 wins $50 in profit if the bet wins.
Why It Matters
Fractional Kelly helps a bettor size wagers when they have a measured edge but want smaller drawdowns than full Kelly can create. It is most useful when projections are strong enough to estimate win probability, but still imperfect.