How EA position sizing works: fixed lot vs percent-risk
Resumen — The sizing rule decides how much a losing trade costs you — and it matters more than the entry logic. The exact formula every percent-risk EA runs, two worked examples, a fixed-lot vs percent-risk comparison table, and why relative drawdown is the only backtest number that transfers to your own account.
How EA position sizing works: fixed lot vs percent-risk
TL;DR — Position sizing decides how much you lose when an EA is wrong, and it matters more than entry logic.
- Fixed lot means you type the volume once and it never changes. Percent-risk means the EA computes volume from equity, stop distance and pip value on every trade.
- The formula every percent-risk EA uses is: lots = (equity × risk%) ÷ (stop distance × value per point per lot).
- Percent-risk automatically shrinks size in a drawdown; fixed lot does the opposite and turns a bad month into a margin call.
- Our published reports all use percent-risk on a 10,000 USD simulated account, which is why relative drawdown, not net profit, is the number that transfers to your account.
What is position sizing in an EA, exactly?
Position sizing is the step where an EA converts a trade idea into a lot size. It is the only part of the system that controls the size of a loss — the entry controls whether you lose, the sizing controls whether it hurts.
Most Expert Advisors expose it as one or two inputs, typically something like InpFixedLot and InpRiskPercent. On every EA we publish, InpFixedLot = 0 is the default, and a zero there is a switch, not a blank: it tells the EA to ignore fixed volume and size from account equity instead. There is no EA in our store that ships with a hard-coded lot size as the default.
How does an EA actually calculate the lot size?
With one formula, run before every order:
lots = (account equity × risk percent) ÷ (stop-loss distance in points × value per point for one lot)
Two worked examples on the same 10,000 USD account at 1 percent risk, so the risk budget is 100 USD each time:
- EUR/USD, 25-pip stop. One standard lot moves about 10 USD per pip, so a full lot would risk 25 × 10 = 250 USD. Lots = 100 ÷ 250 = 0.40 lots.
- XAUUSD (gold), 8.00 USD stop. One lot is 100 ounces, so a full lot risks 8.00 × 100 = 800 USD. Lots = 100 ÷ 800 = 0.125, rounded down to 0.12 lots by the broker's volume step.
Same account, same risk setting, same EA — and the volumes differ by more than three times. That is the whole point: with percent-risk sizing, lot size is an output, never a setting you tune for a bigger number.
Fixed lot or percent-risk: which should you use?
Percent-risk for anything live, fixed lot only for controlled testing. Here is the honest trade-off:
| Fixed lot | Percent-risk | |
|---|---|---|
| Where volume comes from | Typed once by you | Recomputed on every trade |
| When the account grows | Risk percent silently falls | Risk percent stays constant |
| When the account shrinks | Risk percent silently rises | Position size shrinks with equity |
| Behaviour inside a drawdown | Amplifies it | Damps it automatically |
| Moving to a different account size | Must be re-tuned by hand | Transfers directly |
| Prop-firm daily-loss control | Manual arithmetic every day | Rule-based and repeatable |
| Reasonable use case | One fixed account, or isolating a variable in a test | Live trading, prop challenges, any account that changes size |
The row that costs people money is the third one. Suppose you set 0.10 lots on a 10,000 USD account with a 25-pip stop: that is 25 USD, or 0.25 percent per trade. Lose 60 percent of the account and that same 0.10 lots is now 0.63 percent per trade. Fixed lot does not hold your risk steady — it quietly increases it exactly when you can least afford it.
Why do two traders get different results from the same EA?
Because they are not running the same risk. An EA sold as "profitable" is a sequence of trade signals; the equity curve that comes out of it is that sequence multiplied by a sizing rule. Change the risk percent from 1 to 3 and you have roughly tripled both the profit and the drawdown of the identical signal stream.
This is why a screenshot of someone's account tells you almost nothing without the risk setting attached, and why we publish full reports with the deposit and leverage stated on the proof page rather than curated profit numbers.
What does percent-risk sizing mean when reading our backtests?
It means the percentage columns are portable and the dollar columns are not. Every report below ran on the same harness: 10,000 USD deposit, 1:100 leverage, percent-risk sizing.
| EA | Symbol · TF | Trades | Net P&L (10k account) | Profit factor | Relative DD |
|---|---|---|---|---|---|
| Aurora Gold Breakout | XAUUSD · H1 | 1,446 | +14,443.56 | 1.27 | 15.56% |
| Gold Grid M1 | XAUUSD · M1 | 62,575 | +31,110.70 | 1.69 | 42.91% |
| Index ORB | USTEC · M15 | 237 | +2,723.30 | 1.38 | 6.42% |
| JPY Asia Ranger | USD/JPY · H1 | 70 | +1,013.02 | 1.37 | 3.92% |
Run these on a 100,000 USD account at the same risk percent and the Net P&L column would scale roughly ten times, subject to minimum lot steps and margin. The Relative DD column would barely move. So when you plan, plan against the drawdown column.
Read row two carefully. Gold Grid M1 has the highest profit factor and the highest net profit in this table — and a 42.9 percent relative drawdown that makes it unsuitable for any funded account with a 10 percent overall limit, at any risk setting that still trades meaningfully. Highest return and highest usable return are different questions.
How do prop-firm rules change the sizing math?
They replace your risk budget with theirs. A firm with a 5 percent daily loss limit on 100,000 USD gives you 5,000 USD of daily rope; at 1 percent risk per trade that is five consecutive losers before a breach, which is an ordinary week for most strategies. Sizing for a challenge usually means dropping to 0.25–0.5 percent per trade and capping concurrent positions, not finding a better entry. We cover the specific numbers in EA risk settings for a prop challenge, and the per-firm limits on our partners page.
If you trade manually and want the same discipline without an EA, Risk Sizer applies this identical formula to a drag-and-drop stop level.
Verdict: fixed lot sizing is a testing convenience, not a trading method — percent-risk is the only sizing mode that keeps risk constant as an account grows and shrinks, and drawdown percentage is the one backtest figure that survives the move to your own account size.
FAQ
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q: What is the position sizing formula used by an EA?
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a: lots = (account equity × risk percent) ÷ (stop-loss distance in points × value per point for one lot). On a 10,000 USD account at 1 percent risk with a 25-pip EUR/USD stop, that gives 100 ÷ 250 = 0.40 lots. The same settings on gold with an 8.00 USD stop give 0.12 lots, because the value per point is different.
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q: Is fixed lot or percent-risk better for an EA?
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a: Percent-risk for live trading, fixed lot only for isolating variables in a test. Fixed lot keeps volume constant while your equity moves, so your real risk per trade rises during a losing streak and falls during a winning one — the opposite of what you want. Percent-risk holds the percentage steady and de-risks automatically in a drawdown.
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q: Why does the same EA show different drawdown for different traders?
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a: Because drawdown scales with the risk percent, not with the strategy. The signal stream is identical; a trader at 3 percent risk sees roughly three times the drawdown of a trader at 1 percent on the same trades. Always compare EAs at the same risk setting, and treat relative drawdown as the portable number rather than the profit total.
Risk disclaimer: Forex and CFD trading with leverage carries a high risk of loss and can exceed your deposit. All figures above are historical backtest simulations on demo data and do not guarantee future results. Position sizing controls risk; it does not remove it. Nothing here is financial advice — see our risk disclosure and validate any configuration on a demo account before risking real funds.