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Kelly Criterion Calculator

Work out the Kelly bet or position size from your win rate and payoffs, and a safer fractional Kelly size.

Updated November 4, 2026. Free, no sign-up; everything runs in your browser and nothing you type is stored.

Kelly criterion calculator

Win probability (%)Net odds b (win per $1 risked)Trade: loss at stop, % of positionTrade: gain at target, % of positionAccount size ($)Kelly fraction you'll use (%)

Kelly bet, binary odds

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–Kelly position, stop/target trade

–Position size to use

–Expected log growth

The Kelly criterion

The Kelly criterion (J. L. Kelly Jr., 1956) is the bet size that maximizes the long-run growth rate of capital when you have an edge. For a bet that pays b to 1 with win probability p (q = 1 − p):

f* = (b × p − q) ÷ b

For a trade that gains a fraction g of the position at its target and loses a fraction a at its stop, the Kelly fraction of capital is f* = p ÷ a − q ÷ g. If f* is zero or negative there's no edge and the right bet is nothing.

Worked examples

A coin that wins 55% at even odds: f* = (0.55 − 0.45) ÷ 1 = 10% of bankroll.
A trade with a 55% win rate, a 20% target and a 10% stop: f* = 0.55 ÷ 0.10 − 0.45 ÷ 0.20 = 325% of capital in the position, which risks 32.5% of the account at the stop.

Why most people use half Kelly

Full Kelly assumes you know p and the payoffs exactly; overestimate your edge and you bet too much, and drawdowns are deep even when you're right. Half Kelly keeps about three-quarters of the growth rate with far smaller swings, which is why fractional Kelly is common. Use it with the position size calculator and check strategy math with the options profit calculator.

Common questions

What is the Kelly criterion formula?

f* = (b × p − q) ÷ b, where b is the net odds, p the win probability and q = 1 − p.

Why use half Kelly?

It keeps most of the growth with much smaller drawdowns and is safer if your edge is overestimated.

What if the Kelly fraction is negative?

There's no edge, so the optimal bet is zero.

Sources

More investing and trading calculators: JEPI Dividend Calculator · JEPQ Dividend Calculator · MSTY Dividend Calculator · Options Profit Calculator

General information, not financial, tax or legal advice. Check important numbers with the official source before acting on them.