The Kelly criterion in gambling and crash games
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The Kelly criterion is a formula for sizing bets as a share of your bankroll based on your edge, and for crash games it gives a blunt answer: at a 97% RTP there’s no player edge, so pure Kelly says don’t bet. What gamblers actually borrow from Kelly is the useful part: betting a fixed percentage of your current balance. That proportional style shrinks your stake after losses and grows it after wins, and in our 10,000-session test it was one of the calmest systems of all. Here’s the formula, the honest result and the version worth using. Part of our crash game strategy guide.
The Kelly formula
f = (bp − q) ÷ b
- b = net odds (a 2x cash out pays 1 to 1, so b = 1)
- p = win chance (0.97 ÷ 2 = 0.485)
- q = loss chance (0.515)
For Aviator at 2x: f = (1 × 0.485 − 0.515) ÷ 1 = −0.03. A negative fraction means the bet has a negative expected value, so Kelly sizes it at zero. The same goes for every target, because every target returns about 97%. The cash out calculator shows the expected return per bet.
What players borrow: proportional betting
Instead of a fixed £5, bet 2% of whatever your balance is right now. From £250 that’s £5; drop to £200 and it’s £4; climb to £300 and it’s £6. It’s the core idea of Kelly, sizing to your bankroll, without pretending you have an edge.
What 10,000 sessions show
2% of current balance, 2x cash out, £250 start, 100 rounds: median £226.55, best 10% at £299.25, worst 10% at £177.90, and zero sessions ran out of money. The average session wagered £484.50. Flat £5 bets were almost identical (median £230, best 10% £300). The difference shows up in long sessions: proportional stakes can never quite reach zero.
Kelly vs the progressions
| System (2x, 100 rounds) | Median | Ran dry |
|---|---|---|
| Proportional 2% | £226.50 | 0% |
| Flat betting | £230 | 0% |
| Martingale | £0 | 50.2% |
| Anti-Martingale | £220 | 1.0% |
All systems compared in the best betting system for crash games, and the sizing rules are in our bankroll management guide.
Try proportional betting
Test it in the simulator below (choose the proportional system), then use it in Aviator or JetX. Practise free in the free-play crash game.
Play smart: round your stake to the nearest easy number each time. Proportional betting only works if you actually recalculate.
Strategy simulator
- Median final bankroll
- Unlucky (10th percentile)
- Lucky (90th percentile)
- Chance of going bust
- Median max drawdown
- Average rounds played
Example: 1000 sessions of 100 rounds, bankroll £250, base bet £5, target 2x, 3% edge
| Strategy | Median final bankroll | Unlucky (10th percentile) | Lucky (90th percentile) | Chance of going bust |
|---|---|---|---|---|
| Proportional (Kelly-style) | £235 | £185 | £299 | 0.0% |
| Fixed bet | £240 | £180 | £300 | 0.0% |
Run it a few times with different settings to find the approach that fits your bankroll and playing style.
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Frequently asked questions
What is the Kelly criterion?
A formula for sizing bets as a fraction of your bankroll: f = (bp − q) ÷ b, where b is the net odds, p the win chance and q the loss chance.
Does the Kelly criterion work for crash games?
Strictly, Kelly says to bet only when you have an edge. Crash games pay back about 97%, so the house has the edge and pure Kelly says zero. The useful part is the idea of betting a fraction of your current balance.
What does Kelly say about Aviator at 2x?
b = 1, p = 0.485, q = 0.515, so f = 0.485 − 0.515 = −0.03. A negative result means no bet.
What is proportional betting?
Staking a fixed percentage of your current balance each round, for example 2%. Bets shrink after losses and grow after wins.
Is proportional betting safe?
It’s one of the calmest systems we tested: no simulated 100-round session ran dry because the stake shrinks as the balance falls.
What percentage should I bet?
1–2% of your current balance keeps swings small. Higher percentages make sessions far swingier.
By Daniel Foster · Crash Game Maths & Mechanics Analyst
Daniel focuses on the mathematics and mechanics behind crash games. His reviews examine RTP disclosures, volatility, multipliers, game mechanics and the mathematical principles behind different crash titles.
Fact-checked by Alex Morgan
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