Two traders load the same system onto the same $50,000 account on the same morning. One runs it on one contract, the other on three. A year later the first is sitting in the range the backtest described, and the second has blown through two accounts. Same software, same signals, same market. The only thing they did differently was position sizing, and it decided everything.
Position sizing is the quietest decision in automated trading and the one that does the most damage when it is wrong. It has nothing to do with how good a trade looks or how confident you feel. It is arithmetic: how many contracts, tied to how much you can afford to lose, tied to the account you are trading.
Position sizing is a number, not a feeling
Sizing to confidence is discretionary trading's oldest mistake, and it survives into automated trading because it feels responsible. It is not. In a rule-based system there is no "this one looks good," because you do not see the trade before it happens and every trade is sized the same way in advance. The only lever you control is how many contracts ride on each signal, and you set it against your account size and the drawdown you can survive, never against a hunch about the next few trades.
Confidence is not a position size. Your account is.
The one thing to remember
Size to the drawdown you can sit through, not the profit you want. Contracts scale profit and loss at exactly the same rate, so the right number is the one that keeps the worst historical drawdown survivable on your account and inside your prop firm's rules.
What one contract actually risks
Rentabilio places its stop and its target in the market at the same instant as the entry, with the target set at twice the risk. On one micro contract that is roughly $250 of risk and $500 of target on a typical trade. The point of that fixed-risk design is that the loss on any single trade is defined before the trade is live, not discovered afterward.
The realized figures over the backtest came in a little tighter, because not every trade runs all the way to its stop or its target. The average winner was $354 and the average loser $193, a payoff ratio of 1.84. That combination, a system that wins less than half the time but wins bigger than it loses, is the whole engine, and how it holds together is covered in risk, reward and win rate.
Account size sets the ceiling on contracts
How many contracts you can trade is not a free choice. It is capped by the account. A $50,000 account runs one micro contract, which is what the backtest models. A $100,000 account allows up to two, a $250,000 up to five. Those are the sizes the prop firms permit, and the ceiling rises with the account because a larger account can absorb a larger drawdown.
| Account size | Contracts | Risk per trade | Target per trade | Scaled max drawdown |
|---|---|---|---|---|
| $50,000 | 1 micro | ≈ $250 | ≈ $500 | $4,379 (the tested figure) |
| $100,000 | up to 2 | ≈ $500 | ≈ $1,000 | ≈ $8,800 |
| $250,000 | up to 5 | ≈ $1,250 | ≈ $2,500 | ≈ $22,000 |
Read the last column, not the profit. The drawdown is what scales with you, trade for trade, and it is the number that ends accounts.
Double the contracts, double the drawdown
Here is the part the profit math hides. In the model, two contracts instead of one roughly doubles the profit line. It also roughly doubles the maximum drawdown, from the tested $4,379 to something closer to $8,800, and five contracts scale it toward $22,000 on paper. The pain grows exactly as fast as the gain, because index futures are linear: there is no contract count at which the reward outruns the risk.
So the question is never "how much could I make." It is "how deep a hole can I sit in without switching the system off or tripping the account's drawdown rule." A drawdown ends more accounts than a bad strategy does. Start with what drawdown is before you settle on a contract count.
Hypothetical performance. The $4,379 drawdown and the scaled figures above come from a backtest of Rentabilio over historical data, not from a live account. Simulated results carry no financial risk and cannot fully reflect real execution or slippage. Scaling drawdown by contract count is an approximation, and real drawdowns can be deeper. Past performance, real or simulated, does not guarantee future results.
Size it on your own account first
Put in your account balance and a contract count and see the backtest scaled to your numbers, the drawdown as well as the profit.
Why you start at the tested size
Every figure in the backtest was produced on one micro contract on a $50,000 account. The gross profit, the $4,379 drawdown, the 1.58 profit factor: all of it describes one contract. The moment you trade a different size, you are no longer running the tested system. You are running a leveraged version whose drawdown you have never seen.
Starting at the tested size is not timidity. It is the only way the numbers you looked at still describe what you are about to experience. Scale later, after the system has behaved as expected through at least one real drawdown, not before. How much you actually need to begin is worked out in how much money do you need, and the funded route in funded capital.
The mistakes that come from sizing to feeling
Almost every sizing failure is one of these three, and all three are the same error wearing different clothes.
- Sizing up to make it back. Three losses in a row, so you add contracts to recover faster. The system has no memory, and neither does the market. The next trade is not more likely to win, so all you have done is deepen the drawdown you were trying to escape.
- Sizing up because it feels safe. A hot stretch, so you leverage into it. Same mistake, reversed. A good run is not a mandate to add risk, it is the average showing up early.
- Overriding the size on a "good" trade. In an automated system you do not see the setup before it fires, and single-trade discretion is exactly what you paid the system to remove. Reinstalling it by hand defeats the point.
The system removes the decision to enter. Position sizing is the one real decision it leaves you, which is exactly why it should be the most boring part of your operation.
A worked example
Take a $100,000 account cleared for two contracts. Per the model, that is about $500 of risk and $1,000 of target per trade, and a worst backtested drawdown scaled to roughly $8,800. Now ask the only question that matters before you fund it: if the account fell $8,800 from a high over a few weeks, on your own money, would you leave it running?
If the answer is no, then two contracts is the wrong size for you, whatever the account permits. Drop to one. The permitted maximum is a ceiling, not a target, and the trader who treats it as a target is the one who does not survive the first ordinary bad stretch. The broader picture is in how much a trading bot actually makes, and the instrument itself in how to trade futures.
Frequently asked questions
How many contracts should I trade?
Start with the size your account permits and the backtest used: one micro contract on a $50,000 account, up to two on a $100,000 account, up to five on a $250,000 account. But the permitted maximum is a ceiling, not a recommendation. The right number is the largest one whose scaled drawdown you could sit through without switching the system off, which for most people is smaller than the ceiling.
Does trading more contracts increase my edge?
No. The per-trade edge stays exactly the same; more contracts simply multiply both the profit and the loss by the same factor. Doubling contracts roughly doubles the expected profit and roughly doubles the drawdown, from a tested $4,379 toward $8,800. You are not improving the system, you are amplifying it in both directions at once.
Why does the backtest use only one contract?
Because it models a $50,000 account, where one micro contract is the size the prop firms allow. Every published figure, from the gross profit to the drawdown to the 1.58 profit factor, describes that one contract. That is what makes the numbers reproducible: you can load the same strategy at the same size and print the same report, which you cannot do with a figure that has no contract count attached.
Can I increase size after a good month?
You can, but a good month is not a reason to. The system has no memory and the market does not owe you a repeat, so changing size based on recent results is discretionary sizing wearing an automated system's clothes. Scale on account size and tested behavior over time, not on a streak, or you will reliably be largest right before the average reasserts itself.
What is the risk per trade on one micro contract?
About $250, with a target near $500, because Rentabilio places the stop and the target in the market at entry and sets the target at twice the risk. The realized averages over the backtest ran a little tighter, an average loser of $193 and an average winner of $354, because not every trade reaches its full stop or its full target. The design risk is the number to size against.
In short: position sizing is how many contracts, decided by your account size and the drawdown you can survive, never by how a trade looks or how you feel. Rentabilio places a stop and target at entry with the target at twice the risk, roughly $250 at risk and $500 in target on one micro. A $50,000 account runs one contract, a $100,000 up to two, a $250,000 up to five, and every doubling of contracts doubles the drawdown too, from a tested $4,379 toward $8,800. Start at the size the backtest used, because it is the only size at which it still describes you.
Sources and further reading
- CME Group, "Micro E-mini Futures" contract specifications.
- CFTC, "Customer Advisory: Be Cautious of Trading Systems and Robots."
- NinjaTrader 8, "Strategy Analyzer" documentation.
