Percentage Betting & the Kelly Criterion: Staking a Share of Your Bankroll
Percentage betting stakes a fixed fraction of your current bankroll on each bet, so the dollar stake rises when you are winning and shrinks when you are losing. Unlike flat staking, the bankroll compounds — and unlike a progression, it never chases losses with a bet bigger than the last.
Tuned correctly, percentage staking becomes the Kelly criterion: the fraction that grows a bankroll as fast as mathematically possible given a real edge. This page covers how percentage staking compounds, why it almost never reaches zero, and how Kelly connects the idea to an actual edge. Use the Kelly calculator alongside it to size a specific bet.
How percentage betting works
Choose a percentage — say 2% — and stake that share of your current bankroll on each bet, recalculating every time. At $1,000 your bet is $20; win it at even money and the bankroll becomes $1,020, so the next 2% bet is $20.40. Lose it and the bankroll is $980, so the next bet is $19.60.
Because the stake is always a fraction of what remains, the bankroll shrinks geometrically rather than linearly during a losing run and can never quite hit zero from betting alone. That self-scaling is the core difference from flat staking.
- Pick a fixed fraction of bankroll per bet.
- Recalculate the dollar stake before every bet.
- Stakes rise after wins and fall after losses automatically.
- The bankroll compounds — growth and drawdowns are multiplicative.
How it compounds
Compounding cuts both ways. A winning run accelerates because each bet is a slice of a growing bankroll; a losing run decelerates the bleeding because each bet is a slice of a shrinking one. The result is a smoother ride through drawdowns than flat staking, at the cost of more complex bookkeeping.
The table shows how a $1,000 bankroll moves through alternating results at a 2% fraction. Note that even after several losses the bankroll is wounded, not dead — the next stake simply gets smaller.
| Bet # | Bankroll | Stake (2%) | Result | New bankroll |
|---|---|---|---|---|
| 1 | $1,000.00 | $20.00 | Loss | $980.00 |
| 2 | $980.00 | $19.60 | Loss | $960.40 |
| 3 | $960.40 | $19.21 | Win | $979.61 |
| 4 | $979.61 | $19.59 | Win | $999.20 |
| 5 | $999.20 | $19.98 | Win | $1,019.18 |
The Kelly criterion connection
Percentage betting becomes optimal when the fraction is set to your edge. The Kelly criterion gives that fraction: f* = (decimal odds × win probability − 1) ÷ (decimal odds − 1). Bet that share and your bankroll grows at the fastest possible long-run rate without risking ruin; bet more and growth slows and drawdowns deepen.
A fixed, edge-agnostic percentage (like a flat 2%) is really a blunt version of Kelly — fine when you cannot estimate your edge precisely, but it over-bets thin edges and under-bets fat ones. Because real probability estimates are never exact, most bettors use a fraction of Kelly — a half or a quarter — to keep most of the growth while sharply cutting volatility. The Kelly calculator works out the full and fractional stakes for any specific price and probability.
Strengths and the over-betting trap
Percentage staking’s strength is that it sizes risk to your bankroll automatically and resists ruin: you would have to lose almost every bet to approach zero. Its danger is over-betting. Set the fraction above your true Kelly edge — easy to do, since people overrate their edge — and you convert a positive-EV bet into a negative-growth one through sheer variance.
The honest rule: percentage staking only grows a bankroll if the bets are genuinely positive-EV, and only safely if the fraction is at or below Kelly. It is the best plan for a bettor with a measurable edge and a discipline problem waiting to happen for one without.
Pros
- Stakes scale automatically with the bankroll — no manual re-basing.
- Compounds gains during winning runs.
- Very low risk of literal ruin; stakes shrink as the bankroll does.
- Becomes mathematically optimal when tuned to your Kelly edge.
Cons
- Requires recalculating the stake before every bet.
- Over-betting above your true edge causes deep, volatile drawdowns.
- Needs an honest probability estimate to set the fraction well.
- Still negative-EV if the underlying bets have no edge.
Frequently asked questions
- What is the difference between percentage betting and flat betting?
- Flat betting stakes the same dollar amount every time; percentage betting stakes a fixed share of your current bankroll, so the dollar stake rises after wins and falls after losses. Percentage staking compounds and resists ruin, but needs a recalculation before each bet.
- How does percentage betting relate to the Kelly criterion?
- The Kelly criterion is percentage betting with the fraction set to your edge: f* = (decimal odds × win probability − 1) ÷ (decimal odds − 1). It grows the bankroll as fast as possible without risking ruin. A fixed percentage is a simpler, edge-agnostic approximation; most bettors use a fraction of Kelly to cut volatility.
- Can you go broke with percentage betting?
- Almost never to literal zero, because each stake is only a fraction of what remains — the bankroll shrinks geometrically instead of hitting zero. But over-betting above your true edge can still cause severe drawdowns that, on negative-EV bets, trend toward your bankroll evaporating.
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Educational content for informational purposes only. Not betting or financial advice. Please gamble responsibly. 21+ where applicable.