Trading plan · Advanced

Position Sizing Models

Sizing decides what a single trade can cost, and it moves long-run results more than entry timing does.

Last reviewed 15 September 2026 by the MarketPro research desk · Editorial policy

In short

The three common sizing models are fixed percentage, volatility-adjusted and fixed unit. Fixed percentage risks the same share of the current balance on every trade and lets lot size absorb the stop distance, so risk stays constant as the account changes. Fixed unit sizing keeps the lots constant instead, which means a wide-stop trade can silently carry several times the intended risk.

What to remember

Under fixed lot sizing, a 120 pip stop costs nearly five times a 25 pip stop on the same account, with no decision made.
Percentage sizing recalculates from the current balance, so the dollar bleed decelerates through a drawdown.
An eight-loss run arrives roughly once every 263 trades at a 45% win rate, which is about once a year at 20 trades a month.
At 10% per trade, eight losses need a 132% gain to recover. At 1% they need 8.4%.
ATR is backward-looking, so volatility-adjusted sizing has you sized for yesterday on the day volatility triples.

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Three models on the same $5,000 account

Position sizing decides how much of the account a single trade can cost, and it influences long-run outcomes more than entry timing does. A genuine edge sized too aggressively still ends in a drawdown deep enough to stop you trading it. Here are the three common models applied to identical setups so the difference is visible rather than theoretical.

Account $5,000. Risk budget 1%, so $50 per trade. Three setups: EUR/USD with a 25 pip stop, EUR/USD with a 50 pip stop, and GBP/USD with a 120 pip stop.

Setup	Fixed percentage (1%)	Fixed unit (0.10 lots)	Risk under fixed unit
EUR/USD, 25 pip stop	0.20 lots, $50 risk	0.10 lots	$25, or 0.5%
EUR/USD, 50 pip stop	0.10 lots, $50 risk	0.10 lots	$50, or 1.0%
GBP/USD, 120 pip stop	0.042 lots, $50 risk	0.10 lots	$120, or 2.4%

Under fixed unit sizing the same trader takes five times as much risk on the third trade as on the first, without ever deciding to. The percentage model holds the risk constant and lets the lot size absorb the difference, which is what the position size calculator does in one step.

Why a percentage recovers and a fixed lot compounds the problem

The fixed percentage model recalculates from the current balance, so it scales in both directions. After a 20% drawdown the account is $4,000, 1% is $40, and each subsequent loss is smaller in dollars than the ones before it. The bleed decelerates on its own.

Fixed lot sizing does the opposite. The dollar risk stays at $50 while the balance falls, so the same trade that cost 1% at the start costs 1.25% at $4,000 and 2% at $2,500. Risk rises exactly as the account becomes less able to carry it. That is the arithmetic reason percentage sizing is the default in most written plans, and it has nothing to do with caution.

Volatility-adjusted sizing standardises the risk, not the result

The third model sets the stop from a volatility measure and the size from the stop. A typical rule is a stop at two times the 14-period ATR on the decision timeframe, then size so that distance costs 1%.

The effect is that a quiet EUR/USD session and a violent GBP/JPY one both produce a $50 trade, with the lot size doing the adapting. It removes a hidden variable: without it, the same lot size means different money risk on different days, and your equity curve mixes strategy performance with volatility drift. What it does not do is equalise outcomes. Wider stops mean fewer pips per lot and a different relationship to the spread, and an instrument whose ATR has tripled may simply be a market your setup was never tested in.

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Stress-test against eight losses, not against a good month

The useful test of any sizing model is what it does to the account during an ordinary bad run. At a 45% win rate, the expected number of trades before an eight-loss streak appears is about 263. Taking 20 trades a month, that is roughly once a year.

Risk per trade	After 8 straight losses	Gain needed to recover
0.5%	-3.9%	+4.1%
1%	-7.7%	+8.4%
2%	-14.9%	+17.5%
5%	-33.7%	+50.7%
10%	-57.0%	+132.3%

Read the last column carefully. The 1% row describes an annoying month. The 10% row describes an account that has to more than double to get back to where it started, on a losing run that arrives roughly once a year. Run your own win rate and reward ratio through the risk of ruin calculator and change only the risk figure to see how sharply the result turns.

Where each model quietly fails

Fixed percentage: recalculating after every trade means sizes drift downward through a drawdown, which is the point, but it also means the recovery is slower than the fall. Some plans recalculate weekly instead to avoid daily churn.
Volatility-adjusted: ATR is backward-looking. It expands after the move, not before it, so on the session where volatility triples you were sized for yesterday.
Fixed unit: simple and auditable, and acceptable only if your stop distances are genuinely similar across every setup you take. Most people's are not.
All three: they size the risk you intend, not the risk you get. A gap through your stop pays the gap, and leverage magnifies both directions, so the model is a plan for normal conditions rather than a cap on the worst one.

Two numbers to fix before choosing a model

First, the per-trade percentage. Between 0.5% and 2% is where most written plans sit, at the lower end while the strategy is unproven. Second, the account-level ceiling, meaning the drawdown at which you cut size or stop. Without the second number the first one is unbounded, because nothing in the model tells you when to stop applying it.

Write both into the plan with the calculation you will use, run it before every entry rather than estimating, and keep the model fixed for a full sample of trades. Sizing decisions made trade by trade on how confident you feel add a variable that is larger than most edges, and the drawdown calculator shows what that variable does over a bad run.

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Written and reviewed by the MarketPro research desk

MarketPro is a trading-signal and trading-education app operated by Harajuku Holdings LTD (Cyprus). Our signal desk publishes and tracks every trade idea in-app, and every page here is checked against the app's live behaviour before publishing. Last reviewed 15 September 2026. How we produce signals and content · About MarketPro

FAQ

Position Sizing Models: questions

What percentage of my account should I risk per trade?

Most written plans land between 0.5% and 2% of the current balance, with the lower end while a strategy is still unproven. The exact figure matters less than fixing it in advance and deriving the position size from it and the stop distance, so the number is not chosen while looking at a chart. Pair it with an account-level drawdown ceiling, otherwise nothing in the model tells you when to stop applying it.

Is fixed lot size ever acceptable?

Only if your stop distances are genuinely similar across every setup you take, which is rare once you trade more than one instrument. With a fixed 0.10 lots, a 25 pip stop risks 0.5% of a $5,000 account and a 120 pip stop risks 2.4%, so the risk taken varies by a factor of five without any decision being made. Its one advantage is that it is simple to audit.

How do I stress-test a position sizing model?

Apply it to a losing streak of realistic length rather than to an average month. At a 45% win rate, eight consecutive losses turn up about every 263 trades, so that is a reasonable test. Compute the resulting drawdown and the gain required to recover it. If the model produces a figure you would not keep trading through, the risk percentage is too high regardless of how the strategy looks in a good stretch.

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