One of the most honest things a trader ever said to me was five words long: “I made a lot of money and lost all of it.”. The problem was he didn’t correctly understand the risk of ruin.
He was not unlucky. He was not trading a bad strategy, either. He was trading a decent strategy at a size that guaranteed the account would eventually die, and he had no idea that was what he was doing. The market did not take his money. His position size did.
Risk of ruin is the number that would have told him. It is also the number almost nobody calculates before they start, because it lives in the gap between “how much can I make” and “what could go wrong”, and most traders never look in that gap at all.
This article shows you what risk of ruin actually is, the formula behind it, and what the real numbers look like across the position sizes traders actually use. Every figure below comes from a 100,000-run Monte Carlo simulation, so you get evidence rather than a rule of thumb.
What Is Risk Of Ruin?
Risk of ruin is the probability that your account falls to a level you cannot recover from, before your edge has enough time to pay you.
The concept comes from gambling and insurance mathematics, where it describes the chance of a bankroll hitting zero. In trading it is more useful with a practical threshold instead of zero, because if you size positions as a percentage of your account, you never quite reach zero. You just get small enough that it no longer matters.
So the working definition for a trader is this: the probability that your equity falls to some fraction of your starting capital, given three things you control or can measure.
- Your edge – the expectancy of your system, measured in R
- Your risk per trade – the percentage of capital you put at risk on each position
- Your ruin threshold – the loss level at which you are finished, whether financially or psychologically
Change any one of those three and the answer moves. Change your edge and it moves violently.
What Is The Risk Of Ruin Formula?
The classic closed-form risk of ruin formula, for a system with a fixed win rate and a 1:1 payoff, is:
Risk of ruin = ((1 – E) / (1 + E)) ^ U
Where E is your edge as a decimal (win rate minus loss rate) and U is the number of units of capital you are prepared to lose before you stop.
It is a clean formula and it is genuinely useful for building intuition. It is also too simple for real trading, for three reasons:
- It assumes every win and every loss is the same size. Real systems have a distribution of outcomes, not two fixed ones.
- It assumes a fixed bet size in dollars. Most systematic traders size as a percentage of current equity, which changes the maths entirely.
- It assumes your trades are independent. In a real portfolio you hold several correlated positions at the same time, so a bad day hits many of them together.
That is why the tables below come from simulation rather than the formula. Simulation lets us model fixed fractional position sizing, which is what a systematic trader actually does, and it lets us ask questions the formula cannot answer.
How The Numbers Below Were Produced
Every table in this article comes from the same model, so you can compare them directly.
- 100,000 simulated traders per cell, each trading 1,000 trades
- Fixed fractional sizing: each trade risks a set percentage of current equity, so wins compound and losses shrink your next position
- A win multiplies equity by (1 + risk x payoff); a loss multiplies it by (1 – risk)
- Trades are independent, and the win rate and payoff are held constant
That last point matters, and I will come back to it. Independent trades make these numbers optimistic. Treat them as a floor.
This is probability mathematics, not a backtest of any particular strategy. It does not claim any system produced these returns. It answers a narrower and more useful question: if a system has these characteristics, what happens to the account?
How Much Does Risk Per Trade Change Your Risk Of Ruin?
Here is the probability of your account falling to half its starting value within 1,000 trades, across five system profiles and six position sizes.
| System profile | Win rate | Payoff | Expectancy | 0.5% | 1% | 2% | 3% | 5% | 10% |
|---|---|---|---|---|---|---|---|---|---|
| Trend following | 40% | 2.5:1 | +0.40R | <0.1% | <0.1% | <0.1% | 0.1% | 2.5% | 20.5% |
| Mean reversion | 65% | 0.8:1 | +0.17R | <0.1% | <0.1% | <0.1% | <0.1% | 0.4% | 8.7% |
| Breakout | 35% | 3.0:1 | +0.40R | <0.1% | <0.1% | <0.1% | 0.5% | 5.2% | 29.3% |
| Coin flip, no edge | 50% | 1.0:1 | 0.00R | <0.1% | 3.9% | 36.9% | 61.8% | 85.2% | 98.5% |
| Slight loser | 45% | 1.0:1 | -0.10R | 14.6% | 90.5% | 99.7% | 100% | 100% | 100% |
Read the top three rows first. A system with a real edge, traded at 1% or 2% risk per trade, has a risk of ruin so low it does not register. The same system at 10% risk has a one-in-five chance of halving the account. The strategy did not change. Only the size did.
Now read the bottom two rows, because they are the more important ones.
Can Position Sizing Save A System With No Edge?
No. Position sizing changes how long a losing system takes to kill your account. It does not change the destination.
Look at the coin flip row. That is a system with exactly zero expectancy, which is roughly what you have after costs if you are trading without a tested edge. At 1% risk it has a 3.9% chance of halving the account. At 2% it is 36.9%. At 5% it is 85.2%.
Then look at the slight loser: a 45% win rate at 1:1, which is a -0.10R expectancy. That is a system that is only slightly bad. At 1% risk per trade it destroys half the account 90.5% of the time.
There is a trap in that row worth naming. At 0.5% risk the slight loser shows only 14.6%, which looks like small position sizes fixed the problem. They did not. Trading smaller made the bleed slower, so 1,000 trades were not enough to finish the job. Extend the horizon and that number climbs toward certainty. A negative expectancy system traded small is still a negative expectancy system. You are choosing the pace of the loss, not avoiding it.
This is the single most useful thing risk of ruin teaches, and it is why the order of operations matters so much. Find a real edge first. Size it second. Traders almost always do this backwards, tuning their position size to make a backtest look survivable when the actual problem is that the edge was never there. If you are not sure whether yours is real, start with what a trading edge actually is and then measure it with the trading expectancy calculator.
Is The 2% Rule Good Enough?
No. “Never risk more than 2% per trade” is the most repeated number in trading education and it is weak guidance. It was never a considered answer to the question. It is a guard rail designed to stop people doing something obviously insane, like putting 10% on a single position.
Here is the ladder I actually use, with the simulated numbers from this article beside it so you can see the arithmetic behind each judgement.
| Risk per trade | My assessment | Median max drawdown | Risk of ruin at -50% |
|---|---|---|---|
| More than 10% | Guaranteed suicide | 85.6% | 20.5% |
| 5% to 10% | Wild and reckless | 57.9% | 2.5% |
| 1% to 5% | Still too much for most traders | 14.9% to 57.9% | under 2.5% |
| Below 1% | Getting into safe territory | 14.9% | under 0.1% |
| 0.5% or less | Where I trade, often much less | 7.6% | under 0.1% |
Those drawdown figures are for a system with a genuinely good edge. That is the point. Even when everything goes right, 5% risk per trade means living through a 57.9% typical drawdown, and no one keeps following a system through that.
There is a second problem with the 2% rule that is more dangerous than the number itself. It quietly assumes your stop loss always gets filled where you put it.
It does not always get filled. A stock can gap down 47% overnight on an earnings miss, which has happened to me. It can be suspended and halted. It can be delisted with no warning. In each of those cases your “2% risk” becomes whatever the market decides, and here is the cruel part: the tighter your stop, the bigger the position you took for that same 2%. Tight stops are where catastrophic risk is manufactured.
That is why risk per trade is only one of two limits. Cap the total exposure to any single stock separately, as a percentage of the portfolio, and you are protected against the gap your stop cannot catch.
The compounding version of the same idea is simpler still. Ten losses in a row at 0.5% risk leaves you down about 5%, which is an ordinary week. Ten losses in a row at 10% risk leaves you with less than 35% of your capital.
What Counts As “Ruin” For A Real Trader?
Ruin is not a margin call. For nearly every trader I have worked with, ruin is the point where they stop following the system, and that arrives long before the money runs out.
Here is the same trend following system at different ruin thresholds.
| Risk per trade | Fall to -25% | Fall to -50% | Fall to -75% |
|---|---|---|---|
| 0.5% | <0.1% | <0.1% | <0.1% |
| 1% | <0.1% | <0.1% | <0.1% |
| 2% | 1.5% | <0.1% | <0.1% |
| 3% | 6.2% | 0.1% | <0.1% |
| 5% | 20.2% | 2.5% | 0.1% |
| 10% | 50.2% | 20.5% | 4.6% |
At 10% risk per trade, a system with a genuinely good edge has a coin-flip chance of dropping 25%. Most traders abandon a strategy well before that. One prospect told me plainly, “I can tolerate between 25 to 30 percent drawdown, I don’t want to go more than that.” That is a completely reasonable limit, and it means his personal ruin threshold is -25%, not -100%.
Set your threshold where you will actually quit, not where the broker closes you out. Then size so the probability of reaching it is small enough that you can keep trading through the bad stretch.
Risk Of Ruin vs Maximum Drawdown: What Is The Difference?
Risk of ruin measures a fall from your starting capital. Maximum drawdown measures the worst fall from any peak along the way. The second number is always worse, and it is the one you have to live through.
This distinction catches people out. A system can have a near-zero risk of ruin and still hand you a drawdown that feels like ruin at the time, because after a long winning run the fall is measured from a much higher peak.
| Risk per trade | Median max drawdown | Worst 5% of runs |
|---|---|---|
| 0.5% | 7.6% | 11.5% |
| 1% | 14.9% | 21.8% |
| 2% | 27.9% | 39.6% |
| 3% | 39.3% | 53.8% |
| 5% | 57.9% | 74.1% |
| 10% | 85.6% | 95.5% |
Same system, same edge, every row. At 2% risk the typical trader in this simulation lives through a 27.9% drawdown, and the unlucky one in twenty sees 39.6%. At 5% risk the median is 57.9%.
Notice how far apart the two measures sit. At 5% risk the chance of falling to half your starting capital is only 2.5%, but the median peak-to-trough drawdown is 57.9%. Nearly everyone experiences a fall that would end most trading careers, while almost nobody is “ruined” by the strict definition. If you only ever calculate risk of ruin, you will badly underestimate how uncomfortable your own system is going to be. Pair it with a proper understanding of drawdown, and with the risk-adjusted performance metrics that tell you whether the return was worth the ride.
What Happens If Your Backtest Overstated Your Edge?
This is where most real ruin comes from, and it is the part the calculators never show you.
Every number in the tables above assumes you know your win rate and payoff exactly. You do not. You have an *estimate*, produced by a backtest, and if that backtest was over-optimised then your live edge is smaller than the number you sized your positions against.
Here is what happens when the payoff stays at 2.5:1 and only the win rate degrades from the tested 40%, with risk held constant at 2% per trade.
| Actual win rate | Expectancy | Risk of ruin (-50%) | Median max drawdown | Worst 5% |
|---|---|---|---|---|
| 40% (as tested) | +0.40R | <0.1% | 27.9% | 39.6% |
| 36% | +0.26R | 0.2% | 35.0% | 50.5% |
| 33% | +0.16R | 2.7% | 44.2% | 64.2% |
| 30% | +0.05R | 31.9% | 61.1% | 84.3% |
| 28% | -0.02R | 76.6% | 79.1% | 94.3% |
| 26% | -0.09R | 98.0% | 92.7% | 98.3% |
Ten percentage points of win rate is the difference between a risk of ruin under 0.1% and one of 31.9%. You did not change your position size. You did not change your rules. The only thing that changed is that the edge you measured was not the edge you had.
This is why over-optimisation is a risk management failure and not just a modelling error. Every parameter you tune to make the equity curve prettier moves you down this table without telling you. The trader still believes they are in the top row, sizing accordingly, while actually trading the fourth.
Two habits protect you here. First, build systems with few parameters and test them properly, which is the whole subject of trading system optimization and robust backtesting. Second, size your positions using a deliberately degraded version of your backtested numbers. If your test says 40%, size as though it is 33%. If the system still looks acceptable at that win rate, you have a genuine margin of safety.
Why Is Your Real Risk Of Ruin Worse Than Any Calculator Says?
Every risk of ruin calculator on the internet, including the maths in this article, assumes your trades are independent. Real portfolios do not work that way, and the gap between the two is where accounts actually die.
Four things make live risk worse than the simulated number:
Positions are correlated. If you hold 10 stock positions at 2% risk each, you do not have ten independent 2% bets. In a market-wide selloff they fall together, and you are much closer to a single 20% bet than the maths assumes.
Gaps jump your stops. Your risk per trade is defined by where your stop sits. A stock that gaps down 30% overnight on an earnings miss does not care where you put it. Capping your maximum position size for any one stock is the control for this, and it is separate from your stop loss.
Leverage multiplies everything. Every figure above assumes no leverage. At 2x, read the row for double your risk per trade. Every major fund blow-up I have studied was running somewhere between 4 and 30 times leverage, and those were professionals with risk teams. Above about 1.5x starts getting aggressive for a private trader. Beginners should use none.
Systems degrade. Edges decay as markets change. The win rate that held for the last decade is not guaranteed for the next one, which puts you back on the degradation table above.
And your own risk per trade drifts upward. This one is behavioural rather than mathematical, and it is the most common. Traders increase exposure when they are up, feeling confident after a good run, which means the inevitable drawdown arrives when position sizes are at their largest. The dollar loss is what ends them. The moment you feel certain enough about one trade to go bigger than your rules allow, you have stopped being impartial about it.
The honest way to use these numbers is as a best case. If a position size only looks survivable in the idealised model, it is not survivable in the market.
How Do You Reduce Your Risk Of Ruin?
In order of how much difference each one makes:
- Get a real edge before you size anything. Everything else is arithmetic on top of expectancy. A negative expectancy system cannot be sized into a positive one.
- Assume your backtest is optimistic. Size against a degraded win rate, not the tested one.
- Keep risk per trade at 1% or below. The tables show why. I run systems at 0.5% and lower, and the right number for you comes out of backtesting against your own objectives, not from a rule of thumb. The mechanics are covered in position sizing, and if you want the theoretical ceiling, the Kelly Criterion explains why trading anywhere near “optimal” size is far more punishing than it sounds.
- Cap total portfolio heat. Risk per trade is meaningless if you hold 20 positions at once. Limit the total percentage of capital at risk across all open trades.
- Cap maximum position size per stock. Protects you from the overnight gap that your stop cannot.
- Diversify across genuinely uncorrelated systems. Several systems with different profit drivers, markets and directions will not all draw down at the same time. This is the most reliable way to cut drawdown without giving up return, and it is the core argument for building a portfolio of trading systems rather than one.
- Set your ruin threshold honestly, then respect it. The wider risk management framework puts these controls together.
If you want proof that skipping these controls is not a beginner problem, the hedge fund graveyard documents 62 professional blow-ups by people with risk committees and compliance departments. They should have known better. If they got this wrong with all that infrastructure, the case for you doing the arithmetic before your next trade is fairly strong.
Frequently Asked Questions
What is risk of ruin in trading?
Risk of ruin is the probability that your trading account falls to a level you cannot recover from before your edge has time to pay you. It is driven by your system’s expectancy, your risk per trade, and the loss level you define as ruin.
What is the risk of ruin formula?
The classic formula is ((1 – E) / (1 + E)) ^ U, where E is your edge as a decimal and U is the number of capital units you can lose before stopping. It assumes fixed bet sizes and equal win and loss sizes, so it is useful for intuition but understates real risk for a portfolio of correlated positions.
What is a good risk of ruin percentage?
For a system with a genuine tested edge, keeping risk per trade at 1% or below drives risk of ruin below 0.1% in simulation. The more useful target is your maximum drawdown, because you will stop trading from drawdown pain long before mathematical ruin.
Is risking 2% per trade too much?
For most traders, yes. The 2% rule exists to stop people risking 10%, not because 2% is a considered answer. In simulation, a system with a good edge traded at 2% risk still produces a typical peak-to-trough drawdown of around 28%. Below 1% is safer territory, and 0.5% or less is where I trade.
Can position sizing fix a losing trading system?
No. Smaller position sizes slow the rate of loss but do not change the outcome. A system with negative expectancy loses money at any size, so the edge has to come first.
Is risk of ruin the same as maximum drawdown?
No. Risk of ruin measures a fall from your starting capital, while maximum drawdown measures the worst fall from any equity peak. Maximum drawdown is always the larger number and is what you actually experience.
How many trades does it take to know if my edge is real?
More than most traders think, and the degradation table above shows why it matters. A modest overestimate of your win rate moves your risk of ruin by orders of magnitude, so size conservatively until you have a large live sample.
Where To Go From Here
Risk of ruin is arithmetic. You can run the numbers for your own system this week, and if the answer is uncomfortable, the fix is usually smaller size, better diversification, and a harder look at whether the edge survived testing.
Doing that properly across a portfolio of systems is what the Trader Success System is built for. It walks you through finding an edge you can verify, sizing it so the drawdowns stay inside what you can tolerate, and combining uncorrelated systems so no single stretch of bad luck takes you out.
Remember – You are only one trading system away!
