Position sizing against a drawdown

The mistake that ends most evaluations

Almost every trader arrives with a sizing rule that sounds disciplined: "I risk 1% per trade." On a personal account that is sound. On a funded account it quietly stops meaning what you think it means, and the gap between the two is where most evaluations die.

Here is why. On a $100,000 account, 1% is $1,000. If the maximum drawdown is 10%, you have $10,000 of room — ten losing trades before the account closes. That sounds like plenty.

Now run it forward. You are $6,000 into the drawdown after a rough fortnight. Your remaining room is $4,000. But you are still risking $1,000, because 1% of the balance has barely moved. You are now risking a quarter of everything you have left on each trade. Four more losers and the account is gone.

The percentage never changed. What changed is what it was a percentage of. Your risk rule was anchored to a number that does not matter, while the number that does matter shrank underneath it.

The buffer is the only number that matters

Your buffer is the distance between your current equity and the level at which the account closes. Not the balance. Not the account size. The distance to the wall.

Everything in this guide follows from one rule: risk a fixed percentage of the buffer, recalculated before every session.

Do that and the arithmetic becomes self-correcting. As the buffer shrinks, your position size shrinks with it, which means you cannot reach the wall through ordinary losses — each loss is smaller than the last. As the buffer grows, size grows, so profit compounds. You have converted a rule that fails under pressure into one that adapts automatically.

Finding your buffer

You have at least two limits, and the buffer is the distance to whichever is nearer.

1. Distance to the daily loss limit. Today's starting balance (or equity, depending on the firm) minus the daily limit level.
2. Distance to the maximum drawdown. Current equity minus the total breach level.

Take the smaller. That is today's buffer.

Which one binds changes over the life of an account, and this catches people out. Early in a fresh evaluation, the daily limit usually binds: you have the whole drawdown available but only 4% or 5% of it for today. Deep into a drawdown, the total limit binds instead. On a trailing account after a strong run, the total limit can bind from the very first trade of the day.

A concrete case. A $100,000 account, 5% daily limit, 3% trailing total limit:

• Day one. Daily room $5,000. Total room $3,000. The total limit binds. Buffer = $3,000.
• After a good week, equity $104,000, high-water $104,000. Breach level $100,880. Total room $3,120. Daily room $5,200. Still the total limit. Buffer = $3,120.
• Mid-session after losing $2,400 today. Daily room $2,800. Total room $720. Total limit binds hard. Buffer = $720.

Note what happened in that third line. Your buffer fell by 77% during a single session. If your position size did not fall with it, you are now risking several times your intended percentage on the next trade.

The core calculation

Three steps.

Step 1 — risk amount. Buffer × your risk percentage. At 1% of a $3,000 buffer, that is $30.

Step 2 — risk per unit. Stop distance × value per unit. For forex: pips × value per pip per lot. For futures: ticks × tick value per contract.

Step 3 — size. Risk amount ÷ risk per unit.

Forex example. Buffer $10,000, risk 1% = $100. Stop 25 pips on EURUSD at $10 per pip per standard lot, so $250 per lot. Size = 100 ÷ 250 = 0.40 lots.

Futures example. Buffer $3,000, risk 1% = $30. Stop 12 ticks on MES at $1.25 per tick, so $15 per contract. Size = 30 ÷ 15 = 2 contracts.

The position size calculator does this in both directions, including the rounding discussed below.

What risk percentage to use

This is the question the calculation cannot answer for you, so work it out from your own losing runs rather than adopting a number from a forum.

At a 45% win rate — respectable for most strategies — the probability of an eight-loss run somewhere in a hundred trades is high. Not unlucky; expected. Twelve-loss runs happen over a career. Your sizing has to survive the runs your strategy actually produces, not the runs you hope for.

What each risk level survives, expressed as consecutive losses before the buffer is exhausted:

• 0.5% of buffer: roughly 40 losses. Extremely conservative; progress toward a target is slow but survival is close to assured.
• 1% of buffer: roughly 20 losses. The standard choice for funded accounts, and where most consistently funded traders sit.
• 2% of buffer: roughly 10 losses. Viable for a high win-rate strategy with a well-documented record. Uncomfortable otherwise.
• 5% of buffer: roughly 4 losses. Not a risk strategy; a bet on sequence.

Those counts are slightly pessimistic, because sizing off a shrinking buffer means each successive loss is smaller and the true count is longer. That is the mechanism working as intended — the rule stretches your survival precisely when you need it.

If you have never measured your worst losing run, do that before choosing a percentage. It is a five-minute exercise on any trade history and it is the single most useful number you will produce.

Static versus trailing changes the whole shape

Under a static limit the buffer grows with profit, so at a fixed percentage your position size grows too. Start with a $10,000 buffer risking $100. Make $5,000 and the buffer is $15,000, so you risk $150. The account compounds.

Under a trailing limit the buffer stays roughly constant, because the breach level rises with your equity high. Start with a $3,000 buffer risking $30. Make $6,000 and the buffer is still about $3,000, so you still risk $30. Six thousand dollars of profit bought you no additional capacity to be wrong.

This has a direct behavioural implication that is worth stating plainly: on a trailing account, do not increase size after a winning period. The instinct to press when things are going well is exactly wrong here, because your allowance did not grow. Traders who scale up after a good run on a trailing account are increasing risk against a fixed buffer, which is the fastest route to a breach.

The full mechanics are in static vs trailing drawdown.

Rounding, granularity and small accounts

The formula returns a fractional size. What you can actually trade is discrete, and the rounding is not neutral.

Always round down. A calculated 0.47 lots becomes 0.40, not 0.50. Rounding up on every trade quietly raises your risk above the level you chose, and it does so most on the trades where the stop is widest — the ones where you least want extra size.

Watch the minimum. On futures the minimum is one contract. If the calculation says 0.6 contracts, you cannot trade it. Taking one contract means risking 67% more than intended. On a small futures account this constraint, not your strategy, decides your effective risk.

Work it out before buying an account. Take the smallest tradeable size, multiply by your typical stop distance, and check the result against 1% of your expected starting buffer. If one contract risks more than 1% of buffer, the account is too small for that instrument — use micros if available, or size up the account. See passing on a small account.

Forex traders have it easier: micro lots give fine granularity, so rounding costs little.

Working backwards from the profit target

Sizing tells you what is survivable. It does not tell you whether the target is reachable at that size, and those are different questions that must both be answered before you buy.

Expectancy per trade, as a multiple of risk:

E = (win rate × reward-to-risk) − (1 − win rate)

A strategy winning 45% at 1.8R has E = (0.45 × 1.8) − 0.55 = 0.26R. To make 10% of the account at 1% risk per trade you need 10 ÷ 0.26 ≈ 38 winning-equivalents of net progress, which in practice means somewhere around a hundred trades taken.

Then check the other side. At 1% risk, an eight-loss run costs 8% of buffer — survivable. At 3% risk it costs 24% — survivable but painful. At 5% it costs 40%, and a second run in the same evaluation ends it.

Now the important part: sometimes there is no percentage that satisfies both. A 10% target against a 4% intraday trailing drawdown is close to impossible for most strategies, because the size needed to reach the target in reasonable time is the size that breaches on a normal losing run. When the maths says that, the answer is a different firm, not more discipline. See risk per trade for a 10% target.

The session routine

Five steps, ninety seconds, before your first trade of the day:

1. Note current equity.
2. Calculate the daily limit level and the total breach level. On a trailing account, use the current high-water mark, not the starting balance.
3. Buffer = the smaller distance.
4. Risk amount = buffer × your percentage.
5. Write both numbers down where you can see them while trading.

That last step matters more than it looks. The failure mode is not miscalculating; it is calculating correctly and then, three losses into a bad session, sizing off yesterday's number because you did not have today's in front of you.

Recalculate mid-session if you take a significant loss. A buffer that halves during the day means your size should halve too, immediately, not tomorrow.

When the number gets uncomfortably small

At some point the calculation returns a size that feels pointless — 0.02 lots, or a single micro contract. The instinct is to override it, because the trade "isn't worth taking" at that size.

That instinct is the mechanism failing at the exact moment it is most needed. A small calculated size is the arithmetic telling you something true: the account is nearly gone, and the correct response is to protect what remains, not to take a larger position to make the effort feel worthwhile.

Two legitimate responses. Take the small size, because a small win rebuilds the buffer and larger sizes follow automatically. Or stop for the day and come back with a fresh daily allowance. Both are fine. Sizing up is not.

If you find yourself repeatedly at a small buffer, the problem is upstream — either the risk percentage is too high for your strategy's losing runs, or the firm's structure does not suit your equity path.

The losing-day protocol

Buffer-based sizing already reduces exposure automatically, but one explicit rule is worth adding on top: after two consecutive losing days, halve the risk percentage until a green day.

This is not a superstition about streaks. It is a hedge against the possibility that something has changed — market conditions, or your own execution — that your per-trade sizing cannot detect. Halving costs you very little if nothing is wrong and saves an account if something is. Write it into your trading plan before you need it, because deciding it during a drawdown is deciding it under exactly the conditions that produce bad decisions.

Two failure modes to name

Sizing up to recover. After a losing sequence, the buffer is small and the required size is small, which means recovery is slow. The temptation is to double up. This is the single most common cause of a closed funded account, and buffer-based sizing exists specifically to make it visible: if you are taking a position larger than the calculation, you know it.

Carrying yesterday's size forward. Less dramatic and almost as costly. Your buffer changed overnight — through a trailing level update, a swap charge, or the daily limit resetting from a new balance — and the size you used yesterday no longer corresponds to your intended risk. Recalculating takes ninety seconds.

Funded accounts deserve smaller size than evaluations

This is counterintuitive and most traders do the opposite.

A breach during an evaluation costs the fee. A breach on a funded account costs the account, all unwithdrawn profit, and any accumulated scaling progress — which for a trader six months in is by far the largest of the three.

The consequences are asymmetric, so the risk should be too. A sensible pattern is to drop the risk percentage by a third when moving from evaluation to funded, and to raise it only after the first payout has cleared and the account has survived a losing week intact. See losing a funded account for what is actually at stake.

Worked example: three days on a trailing account

A $50,000 futures account, $2,000 trailing drawdown from the equity high, target $3,000. The trader risks a fixed 10% of the current buffer per trade — the point of the example is what that number does on its own.

Look at day two. The trader is up $200 on the day and their risk budget has fallen by a third, because the drawdown followed an intraday peak that existed for ten minutes and then locked in. Nothing went wrong; a normal give-back on an ordinary winning day cost 35% of the account's capacity.

This is why the buffer, not the balance, is the number to trade from, and why it must be recomputed against the peak rather than yesterday's close. A trader sizing off the $50,200 balance would still be risking $200 a trade on an account that can now only absorb nine of them.

Turning risk into units: pips, ticks and contracts

The buffer arithmetic gives you a dollar figure. Converting it into a position takes one more step, and the step differs by market.

Forex. Position size in lots equals risk in dollars divided by (stop in pips × pip value per lot). At $130 of risk with a 20-pip stop on EURUSD, where a standard lot is $10 a pip: 130 ÷ (20 × 10) = 0.65 lots.

Futures. Contracts equal risk in dollars divided by (stop in ticks × tick value). At $130 of risk with a 12-tick stop on MES, where a tick is $1.25: 130 ÷ (12 × 1.25) = 8.6, so eight contracts. On ES, where a tick is $12.50, the same calculation gives 0.86 — under one contract, which means the trade cannot be taken at that risk level.

Indices and CFDs. Contracts equal risk divided by (stop in points × value per point), with the added step of checking whether the quoted value per point is in your account currency.

Two implications worth internalising. Instrument granularity sets a floor on the risk you can take, so on small accounts the choice of instrument is a risk decision, not a preference. And a stop that is too tight to survive normal noise does not reduce risk — it converts risk into frequency, which the drawdown counts just the same.

Correlation: the position you doubled without noticing

Sizing each trade correctly and then taking three of them at once is the most common way a disciplined trader breaches a daily limit.

EURUSD and GBPUSD long are substantially the same trade. Long NQ and long ES is one position with two tickets. Short USDJPY and long gold have behaved as a single risk factor for long stretches. In each case the account has three positions and one exposure, and a single adverse move takes three stops at once.

The workable rule is to size by risk unit rather than by trade: decide what one unit is worth, and split it across correlated positions rather than repeating it. Two correlated trades at half size each is one unit; two correlated trades at full size is two units wearing one plan. If you want a hard number, treat anything above roughly 0.7 correlation as the same instrument for sizing purposes and check it occasionally rather than assuming — correlations move, and they move most in exactly the conditions that produce large days.

This matters more under a prop drawdown than on a personal account, because the daily loss limit measures the aggregate, and the aggregate is what correlation inflates.

The risk your stop does not cover

Position sizing assumes the stop fills where you placed it. Three situations where it does not, and all three are drawdown events rather than trading events.

Slippage on news. A stop in a fast market fills where liquidity exists, not where you asked. On a $130 risk with a 12-tick stop, a five-tick slip is a 40% overrun of your planned loss.

Weekend and session gaps. A position held over a gap has no stop at all in the interval; the fill is the open. This is the entire argument for the overnight and weekend rules that traders resent — the firm is managing the same exposure you are.

Equity-measured limits with open positions. Where the daily limit or the drawdown is measured on equity, an unrealised excursion breaches you before the stop is ever touched. Your effective stop is the smaller of your stop and your remaining buffer, always.

The adjustment is not complicated: size so that a stop overshooting by 50% still leaves the account trading, and treat any position carried through a scheduled event or a session close as costing double its nominal risk. That is a small tax on a minority of trades, paid to remove the scenario where one fill ends an account.

The one-page sizing sheet

Everything above, in the order you would actually run it, before the first trade of a session:

1. Peak equity. Read it, do not remember it.
2. Fail level. Peak minus the trailing amount, or the fixed static level.
3. Buffer. Current equity minus fail level. This is your account for today.
4. Daily ceiling. The smaller of the firm's daily limit and your own — and use your own.
5. Risk per trade. Buffer divided by the losing streak you must survive, floored at eight.
6. Units. Convert to lots or contracts with the stop you will actually use, then round down.
7. Correlation check. If the new trade duplicates an open one, split the unit rather than adding one.
8. Stop the day when the daily ceiling is hit, in full, without a recovery trade.

It takes ninety seconds and it is the difference between a trading plan and a hope. The evaluation is not testing whether you can find good trades; it is testing whether you can keep taking them for long enough for the good ones to arrive, and step three is what buys that time.

What to do next

Take your last two hundred trades. Find your worst losing run and your average adverse excursion. Choose a risk percentage that survives that run with room to spare. Then check, using the expectancy formula above, whether the target is reachable at that percentage — and if it is not, choose a different firm rather than a different percentage.

Then read daily loss limits and reset times, because the daily limit is what will bind first, and journaling during an evaluation, because logging your buffer at entry is what turns a failed evaluation into a diagnosis rather than a mystery.