Position Sizing: The Very Basics of Money Management

Success in FX trading takes more than choosing strategic entry points. Effective money management is essential. This time I’ll explain “Position Sizing: The Very Basics of Money Management” in detail. Good money management lets you control risk and keep trading FX over the long term.

Index

Money Management and Why It Matters

Money management is about how you allocate your trading capital and how you manage risk. Many beginners focus on entry points and technical analysis skills, but among professional traders it is often said that “80% of success comes down to money management.”

Why is money management so important? The reason is simple. No matter how good your trading method is, poor money management means a single large loss could wipe out your entire account. On the other hand, even an ordinary method with a win rate of around 50% can grow your account over the long term if you manage money properly. It also helps you avoid a situation where you blow your capital all at once and end up blowing up the account right away. What matters is to prioritize “surviving” over “winning”.

The Basics of Position Sizing

Position sizing is the process of deciding how much capital to put at risk on each trade. The right position size is determined based on the following factors.

1. Setting Your Risk per Trade

The most basic rule is “what percentage of your total capital you are willing to risk on a single trade.” Many professional traders stick to a rule of never risking more than 1–3% of their total capital on one trade.

For example, with total capital of ¥1 million:

Risk per trade 1%: risk up to ¥10,000 on a single trade
Risk per trade 2%: risk up to ¥20,000 on a single trade

Beginners in particular should not force themselves to take on risk. I recommend starting at a safe level of around 1%. As your trading skills and experience improve, it’s important to find the risk percentage that suits your own strategy.

2. Working Backward from the Stop-Loss Level

In trading based on technical analysis, the stop-loss point is decided at the same time as the entry point. There is an appropriate stop-loss level derived from Dow Theory, horizontal levels and so on. From this stop-loss level, you calculate the position size you should take.

Position size = Risk amount ÷ (Stop distance × Value per pip)

For example, consider the following case.

Total capital: ¥1 million
Risk per trade: 1% (maximum risk of ¥10,000)
Stop distance: 50pips
For USD/JPY (1pip = approx. ¥0.01 per unit)

The calculation is:

Position size = ¥10,000 ÷ (50pips × ¥0.01) = 20,000 units (0.2 lot)

With this calculation, when your stop-loss level is farther away you reduce your position size, which lets you keep the risk you take on each trade constant. This is the essence of position sizing.

How to Think About the Risk-Reward Ratio

Closely related to position sizing is the “risk-reward ratio.” It expresses the ratio of reward (profit target) to risk (stop distance).

In technical trading, it is recommended to set a risk-reward ratio of at least 1:1.5 (ideally 1:2 or better). In other words, if you risk 20pips, you aim for a profit of at least 30pips (ideally 40pips) or more.

This allows you to make a profit over the long term even with a win rate of around 50%. For example:

10 trades at a risk-reward ratio of 1:2
5 wins: 5 × 40pips = 200pips
5 losses: 5 × 20pips = 100pips
Net: +100pips

Even with a 50% win rate, a good risk-reward ratio leaves you with a profit.

The Dangers of the Martingale Strategy

Position Sizing: The Very Basics of Money Management - illustration

When it comes to money management, I need to mention a method called the “Martingale strategy.” This is a method where you double your stake every time you lose, so that a single win eventually recovers all your losses.

For example:

1st trade: lose with ¥10,000
2nd trade: lose with ¥20,000
3rd trade: lose with ¥40,000
4th trade: win with ¥80,000 → profit ¥80,000 – ¥70,000 (cumulative loss) = ¥10,000

This method may look attractive at first glance, but in reality it is extremely dangerous. When losses keep coming in a row, the capital required grows exponentially, and eventually you run out of money. The Martingale strategy is like “Russian roulette” and is a system that is bound to fail in the long run.

A Practical Approach to Money Management

1. Trade Records and Regular Reviews

It is very important to record the risk amount, actual P&L, account balance and so on for each trade, and to review them regularly. Keeping a trading journal gives you the following benefits.

You can objectively check your own trading patterns and performance
You can derive the optimal risk per trade from statistics such as win rate and average profit/loss
You can spot problems early and fix them

Analyze your trading results regularly, for example once a month, and adjust your risk per trade and position sizing as needed.

2. Dealing with Losing Streaks

When you analyze your trade records, you may notice patterns of consecutive losses. In such cases, consider the following responses.

Temporarily lower your risk per trade (e.g. trade at half your usual size)
Recheck whether the market context suits your strategy
Take a break from trading for a while and spend time observing the market

These steps help prevent emotional “revenge” trades and help you regain calm judgment.

Calculating Risk per Trade Mathematically

Once you start winning trades, you’ll want to increase your position size to improve capital efficiency. In that case, a formula based on Balsara’s risk of ruin is useful. This formula derives the optimal risk per trade based on a given risk of ruin.

The formula for finding the risk per trade is as follows.

f = (1 / (1 + ln(α) / (2×ln(D)))) × ((p(b+1) - 1) / b)

For example, with

f: risk per trade (the loss allowed on one position, as a % of total capital)
p: win rate (e.g. 0.55 = 55% win rate)
b: risk-reward ratio (e.g. 1.5)
D: definition of ruin (e.g. 0.2 = ruin if ¥1 million falls to ¥800,000)
α: acceptable risk of ruin (e.g. 0.01% = 0.0001 = probability of ruin)

the risk per trade at which the probability of total capital falling to 80% is 0.01% or less is:

f = (1 / (1 + (-9.21034) / (2×(-1.60944)))) x ((0.55(1.5+1) - 1) / 1.5)
f = 0.259 × 0.25
f = 0.0647

This calculation means that a trader with an average risk-reward of 1:5 and a 55% win rate can safely risk up to 6.47% per position.

However, with the result above, four losing trades in a row would reduce your capital to 75%, and with a 55% win rate the probability of that happening is around 4%. In other words, it contradicts the acceptable risk of ruin of 0.01%.

This kind of contradiction can occur because of how the calculation logic works. So I think we should also consider a second risk of ruin: the probability of ruin from a continuous losing streak.

n = ln(α) / ln(1-p)
r = 1 - (1-D)^(1/n)

r: risk per trade (the loss allowed on one position, as a % of total capital)
p: win rate (e.g. 0.55 = 55% win rate)
D: definition of ruin (e.g. 0.2 = ruin if ¥1 million falls to ¥800,000)
α: acceptable risk of ruin (e.g. 0.01% = 0.0001 = probability of ruin)
n: number of consecutive losses corresponding to the risk of ruin α

Plugging the earlier example into this formula, the risk per trade drops from 6.74% to 1.91%.

I recommend adopting whichever of these two calculations gives the safer risk per trade.

Summary: Money Management Is the Key to Long-Term Success

In day trading based on technical analysis, money management is a valuable weapon for surviving in the market. Always keep the following points in mind.

Stick to an appropriate risk per trade: keep risk to around 1–2% of total capital per trade
Calculate position size from the stop-loss level: base your decisions on rules, not gut feeling
Secure a sound risk-reward ratio: aim for at least 1:1.5
Avoid dangerous methods like the Martingale strategy: even if they seem to work for a while, they fail in the long run
Keep trade records and analyze them regularly: optimize your money management based on data

Strictly following money management rules may feel unsatisfying in the short term, but in the long run it is the most important factor for “surviving to the end”. As your trading skills improve, your capital will grow naturally. Don’t rush; build a solid foundation of money management step by step.

Next time, I’ll explain “Is FX Trading Bound to Lose? The Reasoning and Logic.” Stay tuned!