Risk management
Lot Sizing Methods for Expert Advisors: Fixed Lot, Risk Percentage and Volatility (ATR) Compared
By Benjamin Lee · Updated · 7 min read
The lot size input is the most important setting on any Expert Advisor, and the one with the most ways to get wrong. Some EAs use a fixed lot, some risk a percentage of the account, some scale with the balance, and some adjust to volatility. Each method gives a very different experience in a drawdown and in a winning run. This guide explains the common lot sizing methods, the formula behind each, how they behave in practice with euro examples, and how to choose the one that fits your account and your nerves.
Key takeaways
- Fixed lot is simple but ignores changes in the account and in volatility.
- Risk percentage sizes each trade from the stop distance, so the money at risk stays steady.
- Balance-scaled lots grow with the account but do not look at the stop.
- Volatility (ATR) sizing shrinks positions when markets are wild and grows them when calm.
- Whichever method you use, check the money at risk on one losing trade against your written limit.
Why lot size matters more than the entry
Two traders can run the same EA with the same entries and exits and have opposite results, because one sized well and the other did not. A lot that risks 1 percent per trade turns a ten-trade losing run into a 10 percent drawdown. A lot that risks 5 percent turns the same run into a 40 percent one, and recovering from it takes far more than a 40 percent gain. The drawdown guide shows the arithmetic. Sizing is how you keep the strategy alive through its worst stretch.
The four common methods
| Method | How the lot is chosen | Strength | Weakness |
|---|---|---|---|
| Fixed lot | You type a number, such as 0.05 | Simple and predictable | Risk changes with stop distance and as the account moves |
| Risk percentage | Lot is calculated so a stop-out loses a set share of the account | Money at risk stays steady | Needs a stop loss; tiny stops give huge lots |
| Balance-scaled (per amount of balance) | A set lot for each block of balance, such as 0.01 per 1,000 euros | Grows and shrinks with the account | Ignores stop distance and volatility |
| Volatility based (ATR) | Lot shrinks when the average range is large, grows when it is small | Adapts to market conditions | More complex; the ATR setting matters |
Fixed lot
A fixed lot means the EA always trades the same size. It is easy to understand and easy to test, which is why many vendors default to it. The weakness is that the money at risk is not fixed. If the EA uses a 5 dollar stop on gold on one trade and a 15 dollar stop on the next, a fixed 0.05 lot risks 25 dollars then 75 dollars. And when the account shrinks, the same lot is a bigger share of what is left.
Fixed lots suit small accounts where the minimum lot is already near your risk limit, and strategies with a constant stop. If you use one, set it from your risk limit and the typical stop, and review it as the balance changes.
Risk percentage
The EA calculates the lot so that a stop-out loses a chosen percentage of the account. The formula is the money at risk divided by the loss per lot at the stop. The money at risk is the account balance times the percentage. The loss per lot is the stop distance times the value of one unit of price per lot.
| Account | Risk | Instrument and stop | Loss per 1.00 lot at stop | Lot size |
|---|---|---|---|---|
| 2,000 euros | 1% (20 euros) | EURUSD, 20 pip stop | about 174 euros | about 0.11 |
| 2,000 euros | 1% (20 euros) | EURUSD, 50 pip stop | about 435 euros | about 0.04 |
| 2,000 euros | 1% (20 euros) | XAUUSD, 5 dollar stop | about 435 euros | about 0.04 |
| 2,000 euros | 1% (20 euros) | XAUUSD, 15 dollar stop | about 1,304 euros | about 0.01 |
Risk percentage is the method most professionals prefer, because it ties the position to the stop. Its pitfalls are a very small stop, which produces a very large lot, and a stop that the broker cannot honour in a gap, which makes the real loss bigger than planned. Many EAs add a cap on the maximum lot to prevent the first problem. Use the lot size calculator and the gold lot size calculator to check the result.
Balance-scaled lots
Some EAs offer a lot per amount of balance, for example 0.01 lots for each 1,000 euros. The lot rises as the account grows and falls as it shrinks, so exposure stays in rough proportion to the account. This is a compromise: it reacts to the balance but not to the stop. If the strategy's stop distance is steady, it works well. If it varies a lot, the risk per trade will vary too.
Volatility-based sizing with ATR
The Average True Range (ATR) measures how far price typically moves over a period. Volatility sizing sets the stop as a multiple of ATR and then sizes the lot so that stop risks a fixed percentage. When the market is calm, ATR is small, the stop is tight and the lot is larger. When the market is wild, ATR is large, the stop is wide and the lot is smaller. In effect the position risk stays steady while the market changes character.
This is more robust than a fixed stop, because a stop of 5 dollars on gold is wide on a quiet day and noise on a volatile one. The cost is complexity: the ATR period and the multiplier become settings that need testing. The risk is overfitting them. Our overfitting guide explains how to check.
Methods to treat with caution
- Martingale. Doubling after a loss. It can recover small losses and wipe the account on a long run. See grid and martingale risks.
- Anti-martingale without limits. Increasing size after wins can give back gains in one trade if there is no cap.
- Full Kelly or aggressive growth formulas. These assume you know the true edge, which you do not. They give large positions and large swings.
- Lot per equity, including floating profit. Sizing from equity lets open profits inflate the next position, which raises the risk when you are most exposed.
How to choose
Step 1: Write your risk per trade
A figure in percent and in euros, such as 1 percent or 20 euros.
Step 2: Find the EA's stop distance
Read the manual and look at the backtest's typical and largest stops.
Step 3: Compute the lot
Use the formula or a calculator. Check it against the broker's minimum lot and lot step.
Step 4: Pick the method that keeps the money at risk steady
Risk percentage or ATR sizing if available, a carefully chosen fixed lot otherwise.
Step 5: Add a cap
Set a maximum lot and a maximum number of open trades, so that an unusual input cannot produce an oversized position.
Step 6: Review monthly
Check the lot against the account balance. Adjust only according to the rules, not after a streak.
Examples by account size are in how much money you need to run an EA, and the full set of risk topics is in the MT5 Expert Advisors guide.
Sources and further reading
- MetaTrader 5 Help: Testing ReportWhat each figure in the tester's report means, including drawdown.
- MetaTrader 5 Help: Basic Principles of Trading OperationsOrders, deals and positions, and how netting and hedging accounts differ.
- Bailey, Borwein, López de Prado and Zhu: Pseudo-Mathematics and Financial Charlatanism (Notices of the AMS, May 2014)A plain-language account of how backtest overfitting produces strategies that fail out of sample.
Frequently asked questions
What is the best lot sizing method for an EA?
For most traders, risk percentage or volatility-based sizing keeps the money at risk steady. A fixed lot can work for small accounts and constant stops. The best choice is the one whose result on a losing trade fits your written limit.
How do I calculate the lot size from risk percent?
Multiply the balance by the risk percentage to get the money at risk, then divide by the loss per lot at the stop distance. Check the result against the broker's minimum lot and lot step.
Is a fixed lot safe?
It is predictable, but the money at risk changes with the stop distance and with the account size. Review the lot when the balance changes and make sure one stop-out fits your limit.
What does ATR lot sizing do?
It sets the stop as a multiple of the market's average range and sizes the lot so that stop risks a fixed share of the account. Positions get smaller in volatile markets and larger in calm ones.
Should I use a bigger lot after wins?
Only by rule and in small steps. Let the lot follow the balance through a risk percentage rather than raising it after a winning run, which is how gains are lost.
Expert Advisors in these styles
Apply the checks above to any of these before you buy: read the manual, run your own backtest, and start on demo.
Keep reading
- Risk and sizingHow Much Money Do You Need to Run an EA? Euro ExamplesThe smallest account that can run an Expert Advisor safely: the 1% risk method, minimum lots on EURUSD and gold, margin, costs and euro examples from 500 to 10,000.
- Gold EAsXAUUSD EA Settings: Lot Size, Stop Loss, Spread, HoursWhat each XAUUSD Expert Advisor input does and how to size it: lot size, stop loss, spread filter, slippage, trading hours and magic number, with euro examples.
- Forex EAsEURUSD EA for MT5: Settings, Sessions and SpreadHow to run an Expert Advisor on EURUSD in MT5: best sessions, spread and news filters, pip value in euros, swap, server time and a safe testing plan.
- Pillar guideMT5 Expert Advisors: The Complete Guide for BeginnersEverything about MetaTrader 5 Expert Advisors in one place: how they work, strategy types, risk, backtesting, buying safely, gold, EURUSD, prop firms and Europe.