The Kelly Criterion for Prop Firm Traders: How Much to Risk Per Trade

You have almost certainly been told to risk no more than 1% of your account on a single trade. It is the most repeated rule in trading, and also the least explained. Where does that number actually come from, and is it the right amount for a prop firm challenge?

The answer runs through a piece of maths called the Kelly criterion. Position sizing is where most challenges are won or lost: risk too much and a normal losing streak breaches the drawdown before your edge shows up; risk too little and the account crawls until you run out of time. Kelly finds the amount in between, and by the end of this article you will see exactly why that old 1% rule has stuck around.

An example about flipping a coin

Start with a game that sounds like free money. You are handed a coin that lands heads 60% of the time, and you can bet on heads as many times as you like at even odds: stake any amount, win the same amount on heads, lose your stake on tails. You have a clear edge. How much of your money should you put on each flip?

In a well-known experiment, researchers gave players exactly this game with real money and half an hour to play. The results were grim: about a third went broke, and only a fifth reached the maximum prize. People with a genuine 60% edge still lost, because they sized their bets on emotion, going all-in after a good run, or even betting on tails out of boredom.

The lesson is the one every prop trader eventually learns the hard way. Having an edge is not enough. How much you stake each time matters as much as whether you are right. Bet everything and a single tails wipes you out; bet a tiny amount and you barely grow. The Kelly criterion answers the question those players got wrong: what is the exact stake that grows your money the fastest without risking ruin?

What the Kelly criterion says

Kelly, published by a Bell Labs researcher in 1956, gives you the single fraction of your bankroll to risk that maximises long-run growth. In its classic form it looks like this:

Read in plain English: multiply your reward-to-risk by your win rate, subtract your loss rate, and divide by your reward-to-risk. If the top of that fraction is zero, you have no edge and Kelly tells you to bet nothing. If it is negative, the strategy loses money and you should not be trading it at all.

Working the coin example

Back to the 60% coin. The odds are even money, so b = 1. You win 60% of the time, so p = 0.6, and you lose 40%, so q = 0.4. Drop those into the formula:

Kelly says stake 20% of your bankroll on every flip. Not 100%, which risks ruin, and not 2%, which wastes the edge. Twenty percent is the fraction that compounds your money fastest over a long run of flips. That is the whole idea: turn your edge into a precise stake instead of a guess.

From coins to trades

Trades are just coin flips with your own odds. Suppose you win 55% of your trades and your winners are twice the size of your losers, a 2:1 reward-to-risk. Now b = 2, p = 0.55, and q = 0.45:

Kelly's answer is to risk 32.5% of your bankroll per trade. Hold onto that number, because in a moment it becomes the key to sizing a prop firm account correctly. But first, why you should never actually risk that much.

Why nobody trades full Kelly

Full Kelly maximises growth, but it pays for that with savage swings: drawdowns of 50% or more are routine. Worse, the formula assumes you know your win rate and reward-to-risk exactly, and you never do. Overestimate your edge even slightly and full Kelly tips from optimal into reckless.

So professionals scale it down. Because the growth curve is almost flat near its peak, half Kelly keeps around three quarters of the growth while roughly halving the volatility. Half Kelly, and often quarter Kelly, is the accepted standard. For the rest of this article, whenever we land on a Kelly number, the amount you would actually trade is half of it.

The prop firm twist: your bankroll is the drawdown

Here is the mistake almost everyone makes. Kelly sizes risk against your bankroll, the money you can truly afford to lose. On a personal account that is close to the whole balance. On a prop firm challenge it is not.

The instant you lose the maximum drawdown, the account is gone, even though most of the balance was never yours to risk in the first place. So your real bankroll is the drawdown buffer, and that is what Kelly must be applied to. The translation is simple:

Take a 50,000 dollar account with a 10% maximum drawdown. Your true bankroll is the 5,000 dollar buffer, not the 50,000. Apply the 32.5% Kelly fraction to the buffer, not the balance: 32.5% of 5,000 is 1,625 dollars per trade, which is 3.25% of the account. Then halve it for half Kelly, and you land near 1.6% per trade.

A worked example

Same 50,000 dollar account, 5,000 dollar buffer, 55% win rate and 2:1 reward-to-risk. Here is what each Kelly fraction means in real money and in account risk:

Look where the sensible answers land: half to quarter Kelly puts you at roughly 1 to 2% of the account per trade, exactly the old rule of thumb you have heard a hundred times. That is not a coincidence. The 1% rule endures because it is close to what Kelly recommends once you size against the drawdown and take a safe fraction.

Why to lean conservative on a challenge

Two things about a funded account push you toward the smaller end. A challenge is short and one-shot, so there is no long run for Kelly's edge to average out; a bad streak in the first weeks simply ends you. And on a trailing drawdown the losses are partly permanent, because the limit ratchets up behind your peak and never gives that ground back. On a tight trailing account, quarter Kelly, or a flat 1%, is the prudent choice.

Summary

1. Having an edge is not enough. How much you risk per trade matters just as much, and Kelly finds the optimal amount.
2. The formula: f* = (bp − q) ÷ b, using your reward-to-risk and win rate.
3. Never trade full Kelly. Half Kelly is the standard, quarter Kelly is safer still.
4. On a prop firm, apply Kelly to the drawdown buffer, not the balance: risk = f* × max drawdown %.
5. The answer usually lands near the 1 to 2% rule. Let the calculator do it, then check your odds in the simulator.

This post is for educational purposes only and does not constitute financial, investment, or trading advice. The Kelly criterion assumes you know your win rate and payoff exactly, which is never truly the case, so treat every figure as an estimate and size conservatively. Trading carries a significant risk of loss.