Kelly Criterion Explained for Bettors

The Kelly criterion answers one question: given your estimated edge, what fraction of bankroll should you stake to maximise long-term growth — without risking ruin from one bad run?

The formula (decimal odds)

For decimal odds O and win probability p:

Kelly fraction = (O × p − 1) / (O − 1)

Stake = Kelly fraction × bankroll (often multiplied by a safety factor).

Numeric example

You believe a team has a 55% chance to win at decimal odds 2.00 (even money).

• p = 0.55, O = 2.0
• Kelly = (2.0 × 0.55 − 1) / (2.0 − 1) = 0.10 / 1.0 = 10% of bankroll

On a £1,000 bankroll, full Kelly suggests £100. Most bettors use half Kelly (5%, £50) or quarter Kelly to reduce variance.

Edge from implied odds

If your “fair” price is decimal 1.91 (implied 52.4%) and you get 2.00 (implied 50%), you have positive edge. Kelly converts that edge plus offered odds into a stake size.

Why not always full Kelly?

Full Kelly maximises geometric growth in theory. In practice:

• Your p estimate is wrong more often than you think
• Simultaneous bets correlate (same league, same day)
• Psychological drawdowns cause abandoning the system

Fractional Kelly (0.25–0.5×) is standard among professional bettors.

Common mistakes

• Using Kelly when you have no genuine edge estimate
• Applying Kelly to correlated parlays as independent events
• Ignoring bankroll that is not truly “risk capital”

Track Kelly in mybetrecord

Set bankroll and Kelly multiplier in settings. Enter personal implied odds on each bet to see a recommended stake alongside your actual stake — a sanity check, not a command.