You think a team has a 45% chance. The bookie has them at 6/4, which only says 40%. That’s a value bet. But how much should you put on it? The Kelly criterion calculator gives a straight answer: with a £1,000 bankroll, full Kelly is £83.33, and half Kelly, which most people use, is £41.67.
Drop your estimate to 35% and the stake goes to zero. No edge, no bet. That’s half the value of Kelly right there.
What the Kelly criterion calculator does
Kelly is a formula for bet size that grows your bankroll as fast as possible over the long run, if your probabilities are right. Bet too little and you leave money on the table. Bet too much and the losing runs knock you back further than the wins push you forward.
Kelly % = (b × p − (1 − p)) ÷ b b = decimal odds − 1 p = your chance it wins
At 2.50 and 45%: (1.5 × 0.45 − 0.55) ÷ 1.5 = 8.3% of your bankroll.
Why almost nobody bets full Kelly
That “if your probabilities are right” is carrying a lot of weight. Nobody’s estimates are perfect, and Kelly punishes overconfidence hard. If you think 45% and the truth is 41%, full Kelly is betting way too much for the real edge.
Full Kelly also swings a lot. Losing a third of your bankroll on the way to long-run growth is normal, and most people don’t enjoy that. Half Kelly gives you about three quarters of the growth with much smaller swings. Quarter Kelly is calmer still. If you’re new to value betting, start at quarter.
Where your probability comes from
The calculator can’t tell you if 45% is right. That’s the hard part, and it’s on you. A few honest ways to get a number:
- Your own model, tested over hundreds of bets, not ten.
- A sharp bookie’s price with the margin taken out, compared with a soft bookie’s price.
- The exchange price, which is usually close to fair.
“I’ve got a feeling” is not a probability. Kelly turns a feeling into an oversized bet faster than anything.
Kelly sits nicely next to the odds calculator, which shows the bookie’s implied chance for any price.
Kelly assumes you’ll be betting for a long time. Make sure it’s money you can leave in play. BeGambleAware.org is free and confidential if it stops being fun.