What is the Kelly criterion in betting?

The Kelly criterion is a formula that tells you what fraction of your bankroll to wager to maximize long-term growth, given your edge and the odds. The bigger your edge, the larger the bet; the smaller the edge, the smaller the bet. It mathematically balances growing your bankroll against the risk of ruin.

What is fractional or half Kelly?

Fractional Kelly means betting a fixed fraction of the full Kelly stake — commonly half (half Kelly). Because real-world probability estimates are imperfect and full Kelly is very volatile, most professional bettors use half or quarter Kelly to cut the swings dramatically while keeping most of the long-term growth.

Why not just bet the same amount every time?

Flat betting ignores the size of your edge — it risks the same amount on a razor-thin edge as on a huge one. Kelly sizing scales your stake to your advantage, which grows your bankroll faster over the long run while protecting it from large bets on small edges.

Bankroll

The Kelly Criterion: How Much to Bet

Finding a +EV bet is only half the job. The other half is sizing it — bet too big and one cold streak busts you, bet too small and you leave growth on the table. The Kelly criterion solves for the sweet spot.

Updated June 2026 · ~5 min read

The problem Kelly solves

Say you've found a genuine 5% edge. How much of your bankroll should ride on it? Bet your whole roll and a single loss wipes you out — even a sure-thing strategy goes to zero if you ever risk everything on a bet that can lose. Bet a single dollar and you'll never grow. Somewhere between "too reckless" and "too timid" is a mathematically optimal stake. That's what the Kelly criterion finds.

Developed by physicist John Kelly at Bell Labs in 1956, it's the bet size that maximizes the long-term growth rate of your bankroll. Bigger edge → bigger bet. Thinner edge → smaller bet. It scales your risk to your advantage, automatically.

The formula

For a single bet, Kelly says wager this fraction of your bankroll:

f = (bp − q) / b f = fraction of bankroll to bet b = decimal odds − 1 (your profit per $1 staked) p = your true probability of winning q = 1 − p (probability of losing)

Work an example. You bet an underdog at +150 (decimal 2.50, so b = 1.5). You believe its true probability is 45% (p = 0.45, q = 0.55):

f = (1.5 × 0.45 − 0.55) / 1.5 = (0.675 − 0.55) / 1.5 = 0.125 / 1.5 = 0.083 → 8.3% of bankroll

Full Kelly says bet 8.3% of your roll. On a $2,000 bankroll, that's a $166 bet. Notice the edge here is healthy, so Kelly sizes up. On a razor-thin 1% edge, the same formula would tell you to bet a tiny sliver — which is exactly the point.

Kelly is only as good as your probability estimate. Feed it an inflated edge and it will tell you to bet dangerously large. Garbage probability in, reckless stake out — which is why an accurate, devigged sharp line matters so much.

Why pros bet fractional (half) Kelly

Full Kelly is mathematically optimal only if your probabilities are exactly right. In the real world they never are — you're estimating, and estimates have error. Full Kelly is also savagely volatile: it's normal to see your bankroll cut in half on the way to growth. Most people can't stomach that, and overbetting a slightly-wrong edge can flip you from +EV to going broke.

The fix professionals use is fractional Kelly — betting a fixed fraction of the full Kelly number, most commonly half (or even quarter) Kelly:

Strategy	Stake on our example	Volatility
Full Kelly	8.3% ($166)	Very high
Half Kelly	4.2% ($83)	Far smoother
Quarter Kelly	2.1% ($42)	Conservative

The trade is remarkable: half Kelly keeps about three-quarters of the growth rate while cutting the volatility roughly in half. For imperfect human edges, that's almost always the better deal — and it builds in a safety margin against overestimating your edge.

Why not just flat-bet?

Flat betting — the same stake every time, say "one unit" — is simple and far better than betting randomly. But it ignores edge size: it risks the same amount on a 1% edge as on a 10% edge. Kelly fixes that by pouring more onto your strongest spots and less onto your weakest, which compounds meaningfully faster over thousands of bets. Flat betting is a fine floor; Kelly is the ceiling.

How it fits the +EV workflow

Stake sizing is the last step of a complete process. To bet a correct Kelly fraction you need a trustworthy probability and a real edge:

Remove the vig from a sharp line to get a true probability.
Find a book whose price beats that fair value — a +EV bet.
Plug the edge into Kelly, then bet a fraction of it.
Confirm you're beating the closing line over time.

Doing this by hand on every bet is a chore. Iron Marker computes the fractional-Kelly stake for every +EV bet it surfaces, using the same devigged probability it found the edge with — so the right bet size is already on the screen.

The right stake, already calculated.

Iron Marker finds +EV bets against Pinnacle's sharp line and shows you the fractional-Kelly stake for each one — no spreadsheet required. $39/mo, 7-day free trial.

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Iron Marker is an analytics tool, not a sportsbook, and this guide is educational — not betting or financial advice. The Kelly criterion does not guarantee profit and assumes an accurate edge estimate. Never bet more than you can afford to lose. Must be 21+. Problem gambling? Call 1-800-GAMBLER.