Kelly Criterion Calculator for Sports Betting — How to Size Bets with Your Real Bankroll

Most bettors size their bets the same way every time: flat stakes, round numbers, gut feel. It feels safe and consistent. In reality, it’s leaving significant expected value on the table — and creating unnecessary ruin risk on bad streaks.

The Kelly Criterion calculates a bankroll fraction from offered odds and an estimated win probability. Its growth result depends on those inputs being accurate; a calculator cannot verify the estimate for you.

Before sizing a position, use the fair-odds calculator to examine the market probability you are comparing with your own estimate.

What is the Kelly Criterion?

John L. Kelly Jr.'s 1956 paper established the long-run growth framework behind the criterion.^[1] Applying it to a bet requires an estimated probability, and estimation error can make the resulting stake too large.

The core formula for sports betting:

The Kelly % is the fraction of your bankroll to stake on this bet. If Kelly outputs 5%, you stake 5% of your current bankroll — not 5% of your starting bankroll.

A Worked Example

You want to bet on a team at +150 American odds (decimal: 2.5). You estimate the true win probability is 48%.

In this illustrative calculation, the full-Kelly fraction gives about $133 on a $1,000 bankroll or $200 on a $1,500 bankroll. Those amounts assume the 48% probability estimate is accurate and omit any extra cost or risk limit.

Full Kelly vs. Half Kelly vs. Quarter Kelly

Under its stated assumptions, full Kelly maximizes expected logarithmic growth, but it can produce large drawdowns and is sensitive to probability error. Fractional approaches reduce the calculated stake:

• Full Kelly (1×): Theoretically optimal, practically brutal. For professional traders with very high confidence in their probability estimates.
• Half Kelly (0.5×): Uses half the full-Kelly stake; the resulting risk and growth depend on the actual probability and payoff distribution.
• Quarter Kelly (0.25×): Conservative. Good starting point for new bettors or markets with higher model uncertainty (e.g., player props, prediction markets).

The right fraction depends on how confident you are in your probability estimates. If your model has high calibration error, using Full Kelly amplifies the error — fractional Kelly is the appropriate hedge.

Why Most Kelly Calculators Are Broken

Search “Kelly criterion calculator” and you’ll find dozens of standalone tools that let you enter odds and a probability and output a Kelly percentage. They’re mathematically correct but practically useless for two reasons:

• They’re not connected to your real bankroll. Kelly % means nothing without a current bankroll figure. A 5% Kelly stake is $50 on a $1,000 bankroll and $250 on a $5,000 bankroll. A standalone calculator that uses a manually entered bankroll is almost always stale.
• They don’t track your probability calibration. Your Kelly input is only as good as your estimated win probability. Without a record of your past probability estimates vs. actual outcomes, you have no way to know if your model is calibrated — or whether you’re feeding garbage into the formula.

A Kelly calculator that’s disconnected from your performance history is a formula with no feedback loop. The formula is correct. The inputs are the problem.

How evrai’s Kelly Calculator Works

evrai integrates the Kelly Calculator directly with your live bankroll and bet history. When you open the calculator:

• The bankroll field pre-fills with your current balance from Settings — or your live Kalshi balance if you’ve connected via API (Trader plan)
• You enter the odds and your estimated win probability
• The calculator outputs Full Kelly, Half Kelly, and Quarter Kelly stake amounts in dollar terms — not just percentages
• Historical calibration data from your logged bets surfaces alongside the calculator, so you can check whether your model has been accurate in similar situations

The Kalshi integration is particularly powerful: your Kelly calculator always reflects your real prediction market capital, not a figure you set months ago and forgot to update.

Kelly Sizing Across Multiple Simultaneous Bets

A common question: if Kelly says 8% on Bet A and 6% on Bet B, and you’re placing them simultaneously, do you stake 14% of your bankroll total?

No. The classical Kelly formula assumes sequential bets. For simultaneous bets, the conservative approach is to treat each bet’s Kelly allocation as a fraction of the remaining bankroll after previous allocations, or to use fractional Kelly more aggressively (Quarter Kelly works well here).

For simultaneous positions, review total exposure and correlation before adding individual stake fractions. A fixed cap cannot guarantee protection from a correlated loss.

Kelly and Prediction Markets: The Binary Contract Adaptation

For Kalshi and Polymarket binary contracts, the Kelly formula adapts slightly:

evrai’s Kelly Calculator handles both formats natively — sportsbook odds mode and prediction market cents mode — so you don’t need to manually convert between formats.

The Bottom Line on Kelly Sizing

Kelly is not a guarantee. Its theoretical growth result assumes the probabilities and payoffs used in the formula are correct. Error in either input can change the appropriate stake substantially.

The discipline required is not in the formula — it’s in building the habit of estimating win probability before every bet, and maintaining an accurate bankroll figure at all times. evrai builds both of those habits into its interface.