Kelly Criterion Calculator With Risk of Ruin
This Kelly criterion calculator gives the stake that grows a bankroll fastest for any win chance and decimal odds, and then shows what that stake risks. For quarter, half, full and double Kelly it works out the exact risk of ruin, the chance of finishing ahead and the typical bankroll after the number of bets you choose.
Kelly criterion and bankroll calculator
Edge
+10.00%
per unit staked
Kelly stake
10.00%
of bankroll per bet
Full Kelly bet
100.00
half Kelly: 50.00
Growth per bet
0.502%
at full Kelly, on average
200 flat bets from a bankroll of 1,000 (exact)
| Stake size | Bet | Risk of ruin | Finish ahead | Median bankroll | Expected result |
|---|---|---|---|---|---|
| Quarter Kelly | 25.00 | 0.0036% | 91.1% | 1,500 | +500 |
| Half Kelly | 50.00 | 1.14% | 91.0% | 2,000 | +1,000 |
| Full Kelly | 100.00 | 11.93% | 84.8% | 3,000 | +2,000 |
| Double Kelly | 200.00 | 35.22% | 64.0% | 4,200 | +4,000 |
Stakes are fixed at the size shown for every bet. Risk of ruin is the chance the bankroll drops below one stake at any point, including after the last bet. All figures are calculated exactly, not simulated.
The Kelly criterion formula
With a win chance p, a loss chance q = 1 − p, and decimal odds d, the net odds are b = d − 1 and the Kelly stake, as a share of your bankroll, is:
f* = (b × p − q) ÷ b
A bet that wins 55% of the time at even money, decimal odds 2.00, gives 10%. If the result is zero or negative, the bet has no edge and the Kelly answer is to stake nothing, however the bet is sized.
Why most people bet half Kelly
Full Kelly maximises long-run growth, but only if your win chance is exactly right, and the ride is rough. In the example, full Kelly grows the bankroll about 0.50% per bet on average, while half Kelly still grows it 0.38% with half the stake. Double Kelly grows it not at all: overbetting by a factor of two turns an edge into a slow loss.
Risk of ruin shows the same thing more bluntly. From a bankroll of 1,000 over 200 flat bets, the chance of dropping below one stake is:
- Quarter Kelly (25 a bet): 0.0036%.
- Half Kelly (50 a bet): 1.1%.
- Full Kelly (100 a bet): 11.9%.
- Double Kelly (200 a bet): 35.2%.
Because an estimated edge is usually too optimistic, a fraction of Kelly is the practical choice for anyone who is not certain of their numbers.
Using it as a risk of ruin calculator
When a bet has no edge, the table switches to stakes of 1%, 2%, 5% and 10% of your bankroll, which makes it a plain bankroll calculator. The risk of ruin column is exact: it counts every sequence of wins and losses rather than sampling some of them. On a 1.90× coin flip, which loses 5% of each stake on average, betting 50 at a time from 1,000 ends in ruin within 200 bets 36.5% of the time, and betting 100 does 72.4% of the time.
No stake size turns a negative edge positive. If you want to see what a doubling system does instead of flat stakes, try the martingale simulator.
Frequently asked questions
What does the Kelly criterion calculate?
The share of your bankroll to stake on a bet so that the bankroll grows as fast as possible over many bets, given your win chance and the odds.
What does a negative Kelly percentage mean?
The bet has no edge: at those odds and that win chance you lose money on average. The Kelly criterion then says to bet nothing.
Should I use full Kelly or half Kelly?
Half Kelly keeps most of the growth with much less risk of ruin, and it protects you if your win chance is overestimated. Most bettors who use Kelly use a fraction of it.
How is risk of ruin calculated here?
Exactly. For a fixed stake, the bankroll after any number of bets depends only on how many were won, so the calculator tracks the probability of every win count bet by bet and adds up the paths that fall below one stake.
Does the Kelly criterion work for casino games?
Only as a warning. Casino games have a house edge, so Kelly always recommends staking nothing; the table still shows how quickly different stake sizes use up a bankroll.
Play within your limits. Paid rounds cost Telegram Stars or Gram, and every game on Gift Club is built so the house keeps a margin — over enough rounds the maths runs against you, by design. Stars cannot be converted back into money. Treat paid play as entertainment you have budgeted for, set a limit before you start, and play Spark rounds whenever you just want to play. Gift Club is 18+.
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