Finding a +EV bet is only half the problem. The other half is deciding how much to bet. Bet too little and you're leaving growth on the table. Bet too much and a normal losing streak can destroy your bankroll before your edge has time to materialize.
The Kelly criterion solves this problem with math. It tells you exactly what fraction of your bankroll to wager to maximize the geometric growth rate of your money over time.
Why Flat Betting Falls Short
Most recreational bettors use flat betting — the same dollar amount on every game. It's simple and prevents catastrophic losses, but it ignores the size of your edge. A bet with a 1% edge and a bet with a 6% edge deserve very different stakes. Flat betting treats them identically.
The Kelly criterion scales your bet size to your edge. When edge is high, you bet more. When edge is thin or zero, you bet less or nothing. This is how professional bettors and gamblers have operated for decades.
The Kelly Formula
Where:
f = fraction of bankroll to bet
b = net odds (decimal odds − 1, or payout per $1 wagered)
p = your estimated probability of winning
q = probability of losing (1 − p)
Every variable here is critical. b comes from the odds you're being offered. p comes from your model or probability estimate — this is the number you have to get right. If your probability estimate is wrong, Kelly will size your bets wrong too.
A Worked Example: +150 Underdog
Say you're looking at a game where the underdog is priced at +150. After devigging Pinnacle's line and running your own model, you estimate the underdog's true win probability is 42%.
p = 0.42 (your estimated win probability)
q = 0.58 (1 − 0.42)
f = (1.50 × 0.42 − 0.58) / 1.50
f = (0.63 − 0.58) / 1.50
f = 0.05 / 1.50
f = 0.0333 = 3.33% of bankroll
On a $10,000 bankroll, that's a $333 bet. If the underdog wins — which you estimate happens 42% of the time — you collect $333 × 1.5 = $499.50. If they lose, you lose $333. Over many bets at this edge, Kelly sizing maximizes your long-run growth.
Why Full Kelly Is Too Aggressive
Kelly assumes your probability estimates are perfectly accurate. They never are. Even sharp bettors with sophisticated models have estimation error. When you overestimate your edge and bet full Kelly, the consequences are brutal — your bankroll swings are enormous and drawdowns can hit 50% or more even while running +EV.
The standard professional approach is half-Kelly: multiply the Kelly fraction by 0.5.
From the example above: 3.33% × 0.5 = 1.67% of bankroll
On a $10,000 bankroll: $167 per bet
Half-Kelly achieves about 75% of the growth rate of full Kelly while cutting variance roughly in half. For most bettors, that tradeoff is very much worth it. You're still growing your bankroll at a rate proportional to your edge — you're just accepting slightly less maximum growth to protect yourself against model error.
What Kelly Is Not
Kelly is not a magic formula that makes you money. It only maximizes growth given a positive expected value. If you're betting on games with no real edge — if your probability estimates are just guesses — Kelly will tell you to bet a certain fraction, but that bet will still be -EV and you'll still lose money over time, just in a mathematically optimized way.
The formula is a bet sizing tool, not an edge finder. Get the edge right first.
Fractional Kelly and Risk Tolerance
You're not limited to half-Kelly. Some professionals use quarter-Kelly on markets where they have less confidence in their models. Others use full Kelly on high-conviction bets in markets they know deeply.
A reasonable framework:
- Quarter-Kelly (25%): New markets, uncertain models, or when you're testing a new strategy
- Half-Kelly (50%): Standard play across most markets — the professional default
- Full Kelly (100%): Only if your probability estimates have been validated across a large sample and you understand the variance implications
Kelly Across Multiple Simultaneous Bets
Kelly was designed for sequential bets, not parlays or simultaneous wagers. If you're betting on multiple games that kick off at the same time, the simple Kelly formula overestimates how much you should stake on each one — the bets compete for the same bankroll.
The safest approach: sum up all your planned Kelly stakes for a given slate, and if the total exceeds 20-25% of your bankroll, scale each bet down proportionally. This prevents over-leverage on busy game days.
Calculate Your Kelly Stake Automatically
Enter your odds and estimated win probability in JATSport's Calculator. It shows the full Kelly, half-Kelly, and quarter-Kelly stakes instantly — no math required.
Open Calculator