What causes a prop firm drawdown breach?

The short answer

Evaluations are rarely lost to bad trading. They are lost to a rules breach — and the specific mechanism is almost always the same one: an evaluation carries several independent termination rules, each moving on its own clock, and at any given moment exactly one of them is nearest to ending the account. The trader is usually watching a different one.

I call the nearest one the binding barrier. It is not fixed. A daily-loss limit binds in the morning and stops binding at reset; a trailing floor binds after every new high and ratchets upward without coming back down — where “high” means an equity peak including unrealised profit under an intraday rule, and a daily balance under an end-of-day one; a static floor binds in a deep, slow drawdown. The rule quoted in the marketing is rarely the rule that kills the account.

So the diagnosis is not “I traded badly”. It is: which barrier was binding when I breached, and in what unit was I measuring my risk while it did?

Enumerate the barriers before you trade

The first step is not risk management. It is transcription. Open the rulebook and write down every rule that can end the account, and for each one record four things:

The anchor. What is the floor measured from? The starting balance, the day’s opening balance, or the high-water mark. This single choice determines everything that follows.

The clock. When does it reset, and when does it move? A daily limit resets at a stated time in a stated timezone — which is frequently not your timezone, and the boundary is where a surprising number of breaches sit. A trailing drawdown floor does not reset at all; it ratchets. Whether it ever stops is a property of your rule text: some stop once the floor reaches the starting balance, others state no ceiling and keep climbing for as long as the account does.

The valuation basis. Does the rule read closed balance or equity including open positions? Under an end-of-day trailing rule, an unrealised spike is irrelevant. Under an intraday equity rule, the same spike permanently raises your floor. The two variants share a name and behave nothing alike.

The measurement points. Is the rule evaluated at the close of each bar, on every tick, or on the platform’s own sampling? A rule evaluated on ticks can be crossed inside a candle that closes back above the floor.

That transcription is the artifact. It takes an hour and it is the only part of this article that is not optional.

Convert every barrier into one unit

Once transcribed, each barrier becomes a number in a single common unit: how much adverse movement, at your current sizing, ends the account through this rule. Not a percentage of balance. Not a lot count. The distance to death, in the currency the account is denominated in.

Restate all of them after every position change and after every new equity high. The binding barrier is simply the smallest of those numbers, and it is the only one that matters for the next decision. Traders who do this once at the start of an evaluation are computing a quantity that expires within hours.

Two properties make the restatement non-negotiable rather than fussy:

• Under a ratcheting floor, profit consumes room. Every new high on whichever basis the rule samples drags the floor up behind it, so the account’s own success reduces the excursion it can survive. Sizing that conditions on balance moves in exactly the wrong direction: the balance grew, so the size grows, while the room shrank.
• Under a daily rule, room is refunded on a schedule you did not choose. A position held across the reset is exposed to two consecutive daily allowances with no intervening flat period, which is a different risk profile from the one the day-trader intuition assumes.

Correlation: five tickets, one trade

The second mechanism is exposure that is counted wrongly. Five positions in correlated instruments are not five risks. They are one large risk wearing five tickets, and it will move against you as one.

Effective exposure is the quantity that matters, and it is not the sum of the ticket sizes. A book that feels diversified because it holds several symbols can carry a single dominant factor — one currency on both sides of several pairs, or several instruments that are all short volatility in different costumes. In an adverse move, the correlation the book displays on a quiet Tuesday is not the correlation the event produces; correlations across risk assets have a habit of converging exactly when it is most expensive.

The practical test is arithmetic rather than intuition: for each barrier, ask what a simultaneous adverse move across the whole book does to distance-to-floor. If the answer is “I have never computed that”, the book’s real size is unknown, and the risk of ruin against the nearest barrier is unknown with it.

The path, not the outcome

The third mechanism is that breaches are path-dependent and P&L is not.

Two sequences of trades with identical final balances can differ completely in whether the account survived, because a barrier only has to be touched once. This is why an end-of-day equity curve is a poor instrument for diagnosing a breach: it shows where the account finished each day, and the breach happened somewhere the curve does not plot.

Three path effects are worth naming because they are routinely absent from the trader’s reconstruction:

• Gaps and vacuum fills. A stop is an instruction, not a guarantee. Where the market gaps through the level, the fill happens where a print existed, which may be a long way below the floor you were defending. Any risk plan that assumes the stop fills at the stop has assumed the problem away.
• Slippage at the worst moment. Execution cost is not constant across the session, and the moments when a breach is being decided are not the calm ones.
• The intra-bar excursion. If the rule is evaluated on ticks, what matters is the worst price reached, not the price at any close you looked at.

The Binding Barrier

The single-sentence version of everything above: at every moment of an evaluation, one rule is closest to ending it — that is the binding barrier — and a breach is what happens when the trader’s attention and the binding barrier are on different rules.

It reframes the question productively. “Am I risking too much?” has no answer without a benchmark. “Which barrier is binding right now, how far away is it in money, and what would a correlated adverse move do to that distance?” has an arithmetic answer, and it is available before the trade rather than after the post-mortem.

The reframe also explains why generic risk advice underperforms here. A fixed percentage-per-trade rule is calibrated against balance, which is not the quantity being judged. Under a ratcheting floor the same percentage is conservative early and reckless at the peak, and the trader experiences that transition as bad luck.

Where the measured version publishes

The transcription and the arithmetic are both doable by hand. Doing them live, restating distance-to-floor after every fill and every new equity high while a position is open, is where hand-computation fails — which is precisely when the number matters.

That is the gap the Prop-Evaluee Risk Guardian is being built to close: halt distance against the evaluation’s actual rules rather than the trader’s recollection of them, correlation-adjusted exposure so the size you feel matches the size you hold, and risk-of-ruin surfaces published with their assumptions visible. The Stress Harness is the companion for the path problem — replaying recorded shocks against the book you hold now, filling where prints existed rather than where your stop sat. Both are pre-launch, and both have outputs marked NOT YET PUBLISHED. The single-point version of the Guardian’s risk-of-ruin surface — pass or breach odds from rules you type and an edge you declare, with the binding barrier counted on every breached path — is free and on this site: the prop evaluation survivor.