Kelly Criterion Calculator 2026 — Bet Sizing Made Simple
If there is one number that separates traders who compound their bankroll from traders who blow up, it is the size of their next bet. The Kelly Criterion is the math behind that number — a formula for sizing positions so you maximize long-run growth without taking ruinous risk. This guide explains what it is, where it comes from, the trap most people fall into, and how to actually use a Kelly Criterion calculator to size bets on Polymarket and other prediction markets in 2026.
What is the Kelly Criterion?
The Kelly Criterion is a position-sizing formula developed in 1956 by John L. Kelly Jr., a researcher at Bell Labs. It answers a deceptively simple question: given an edge, how much of my bankroll should I bet on each opportunity?
Bet too little and you waste your edge — your bankroll grows slower than it could. Bet too much and a single bad outcome can wipe you out, even when your edge is real. Kelly finds the sweet spot: the bet size that maximizes the expected logarithm of your wealth, which mathematically maximizes long-run growth rate.
The Kelly formula in plain English
For a binary bet (you win or you lose), the Kelly fraction is:
f* = (b·p − q) / b
Where:
- p = your estimated probability of winning
- q = 1 − p (probability of losing)
- b = the odds offered, expressed as net profit per $1 staked
- f* = the fraction of your bankroll to bet
For Polymarket, where you buy YES at price m and get back $1 if YES happens, the formula simplifies to:
f* = (p − m) / (1 − m)
The intuition is clean: the bigger your edge (p − m), the bigger the bet. If your edge is zero, Kelly tells you to bet zero. If your edge is negative, Kelly tells you to bet zero (the formula goes negative, meaning the bet is bad).
A worked example
Suppose Polymarket is offering YES at 50 cents and you genuinely believe the true probability is 60%. Your bankroll is $10,000.
- p = 0.60
- m = 0.50
- f* = (0.60 − 0.50) / (1 − 0.50) = 0.10 / 0.50 = 0.20
Full Kelly says bet 20% of your bankroll, or $2,000. That is mathematically optimal under the assumption that your probability estimate is exactly correct.
Why almost nobody bets full Kelly (and you shouldn't either)
The catch is in that assumption: full Kelly only works if your probability estimate is exactly right. In real life, your estimates are noisy. You think a market should be 60% but it might really be 55% or 65% — and you have no way to know. Full Kelly assumes a level of confidence you almost never have.
The volatility of full Kelly is brutal. Even with a real edge, expect drawdowns of 50%+ to be routine. One unlucky streak and a quarter of your bankroll is gone before lunch. Most professional bettors size between quarter Kelly and half Kelly, which means multiplying f* by 0.25 or 0.50.
Quarter Kelly trades roughly 75% of the long-run growth rate for a roughly 95% reduction in drawdown variance. That trade is worth it for almost everyone.
How a Kelly Criterion calculator helps
Computing Kelly by hand for a single bet is fast, but on Polymarket you face complications a textbook formula doesn't handle:
- The price moves as you size up. The 50¢ ask only fills the first slice. Stake more and you walk the order book — your effective entry climbs to 51¢, 52¢, eating into your edge. A real calculator should solve for the size that accounts for the average fill, not just the top-of-book price.
- Your estimate is uncertain. A serious tool treats your point estimate as the center of a distribution and sizes off a conservative bound, not the raw point. This is the Bayesian shrinkage layer.
- You want to see the simulation. Knowing the recommended bet is one thing. Seeing 10,000 simulated bankroll trajectories — including drawdowns and tail outcomes — gives you the gut conviction to actually pull the trigger.
Our free Kelly Criterion calculator handles all three. It implements walk-the-book sizing with a fixed-point solver, a Beta-posterior credible bound for robust shrinkage, and a 10,000-trial Monte Carlo of the resulting bankroll path. It auto-fills from live ProbBrain signals so you can stress-test real positions in seconds.
Pros and cons of the Kelly Criterion
Pros
- Maximizes geometric growth rate — provably optimal for long-run wealth
- Self-scaling: bigger edge means bigger bet, automatically
- Zero edge = zero bet; negative edge = zero bet (no rationalization)
- Mathematical foundation, not gut feel — defensible to yourself and partners
Cons
- Punishingly volatile at full Kelly — drawdowns of 50%+ are normal
- Sensitive to estimate error: garbage in, ruin out
- Assumes independent bets — correlated positions need adjustment
- Doesn't account for real-world frictions like fees and order book depth without extra machinery
Frequently asked questions
What is the Kelly Criterion in simple terms?
It is a formula for deciding what fraction of your bankroll to bet on each opportunity to maximize your long-term growth rate. The bigger your edge, the bigger the bet — but never so big that a single loss is catastrophic.
Why is full Kelly considered too aggressive?
Full Kelly assumes your probability estimate is exactly correct. In real markets your estimate is noisy, and the formula is extremely sensitive to overestimation — bet too much, lose too much. Quarter Kelly cuts variance dramatically while keeping most of the growth rate.
Can I use the Kelly Criterion on Polymarket?
Yes, and it's particularly well-suited because Polymarket's binary YES/NO structure maps directly to the textbook Kelly formula. The wrinkle is order-book depth: a real Polymarket Kelly calculator needs to walk the book and account for the fact that your effective entry price rises as you size up.
Bottom line
The Kelly Criterion is the cleanest answer math has to the question of how big to bet. It rewards real edge and punishes overconfidence. Use a fractional version (¼ Kelly is the sane default), size off a conservative belief rather than your raw guess, and account for the fact that markets get more expensive as you scale in. A proper Kelly Criterion calculator handles all of that for you in one screen.
Related reading
Size Your Polymarket Bets Like a Pro
Try our free Kelly Criterion calculator — Bayesian shrinkage, walk-the-book sizing, 10k-trial Monte Carlo. Built for serious Polymarket traders. Auto-fills from live ProbBrain signals.
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