Position sizing is the process of deciding how many units, shares, lots, or contracts to trade in a given position. It is one of the most important risk management decisions a trader makes, yet it is often either ignored or applied inconsistently. Getting it wrong is one of the most common causes of account blowups that were not caused by bad analysis.
What is the percentage risk model and how does it work?
The most widely used position sizing approach is the fixed percentage method: decide in advance the maximum percentage of your account you are willing to lose on a single trade if your stop-loss is triggered. Most experienced traders set this between 0.5% and 2% per trade.
At 1% risk per trade with a £10,000 account, you are risking £100 per trade. This means that even 20 consecutive losing trades — an extremely unlikely but possible run — would reduce your account by roughly 18% rather than wiping it out. Position sizing is what makes losing streaks survivable.
The rule is often called the “1–2% rule” in trading education, and the logic is simple: it is not whether you will have losing streaks (you will), but whether your account can survive them while you continue to execute your strategy. At 1% risk per trade, you need 100 consecutive losses to lose your entire account — a statistical near-impossibility. At 10% risk per trade, seven losses in a row removes more than half your capital.
How do you calculate position size? A worked example
Once you know your risk amount and your stop-loss distance, the position size follows from the maths:
Position size = Risk amount ÷ (Stop distance × Value per unit)
Here is a step-by-step worked example for a forex trade:
- Account size: £10,000
- Risk per trade: 1% = £100
- Currency pair: EUR/USD
- Stop-loss distance: 50 pips
- Pip value on a standard lot (100,000 units): approximately $10 per pip (roughly £8 at current rates)
- Position size: £100 ÷ (50 pips × £8) = £100 ÷ £400 = 0.25 standard lots
So with a £10,000 account risking 1%, a 50-pip stop on EUR/USD gives you a position of 0.25 standard lots (25,000 units). If the stop is hit, you lose approximately £100 — exactly 1% of your account. If you used a 25-pip stop instead, you could trade 0.5 lots for the same £100 risk. The size changes with the stop; the risk stays constant.
For a stock trade, the calculation is simpler:
- Account: £10,000. Risk: 1% = £100
- Share price: £50. Stop-loss: £47 (£3 stop)
- Position size: £100 ÷ £3 = 33 shares
Why must stop placement come before position sizing?
A common mistake is sizing a position and then placing the stop. The correct sequence is the reverse: first decide where the stop belongs based on market structure — a level where your trade thesis is clearly wrong — then calculate how many units you can buy given that stop and your risk limit. If the stop distance forces a position size so small it feels pointless, the trade should be reconsidered, not the stop moved closer to manufacture a bigger position.
Moving the stop in order to justify a larger position size is one of the most reliable ways to compound losses. The stop should be placed where the market will tell you that you were wrong; the position size should follow from that decision, not drive it.
What is the Kelly Criterion and should traders use it?
The Kelly Criterion is a formula that calculates the theoretically optimal fraction of capital to risk on each trade, based on your historical win rate and average win/loss ratio. The formula is:
Kelly% = W − (1 − W) / R
Where W is the win rate and R is the ratio of average win to average loss.
Example: a strategy with a 45% win rate and an average win 2.5 times the average loss.
Kelly% = 0.45 − (0.55 / 2.5) = 0.45 − 0.22 = 0.23, or 23% of capital per trade.
In practice, 23% per trade is far too aggressive for most traders. The common adjustment is fractional Kelly — using 25% to 50% of the Kelly recommendation. At half Kelly, the example above would suggest risking around 11–12% per trade, which is still aggressive by most standards and assumes your win rate and average win/loss figures are accurate over a large sample.
The Kelly formula is most useful as a theoretical check on whether a strategy has positive expected value and to understand the mathematical relationship between edge size and optimal bet size. Most active traders find the fixed-percentage method simpler to apply consistently, which matters more in practice than theoretical optimality. The Kelly formula also has no tolerance for estimation error — if your win rate or average win figures are slightly off, the suggested position size can be wildly wrong.
Fixed fractional vs fixed ratio: what is the difference?
The fixed percentage method described above is also called fixed fractional position sizing — each trade risks a fixed fraction of the current account balance. As the account grows, the absolute amount risked grows proportionally. As it shrinks, so does the amount risked. This creates a natural compounding effect on the upside and a cushioning effect on the downside.
Fixed ratio position sizing takes a different approach. Rather than risking a fixed percentage of equity, you increase your position size each time you accumulate a set profit “delta” — a fixed pound amount above your previous high. For example, you might start trading one contract and increase to two contracts only once you have made £2,000 in profits, then increase to three contracts once you have made another £2,000 from that plateau.
Fixed ratio tends to scale up more conservatively than fixed fractional during the early growth phase of a smaller account, which some traders prefer because it reduces the risk of giving back gains too quickly after a run of winning trades. The trade-off is that position growth is slower. For most retail traders, the fixed fractional (percentage risk) model is simpler and sufficiently robust.
How does volatility affect position size?
In highly volatile conditions, price swings are larger. A stop that would rarely be triggered in normal conditions may need to be wider during a volatile period. Some traders adjust position size dynamically using the Average True Range (ATR), which measures how much an asset typically moves over a given period.
An ATR-based approach works as follows: place your stop at 1.5× or 2× ATR below the entry price (for a long trade), then calculate how many units give you your target risk amount given that stop distance. During high-volatility periods, ATR expands, your stop widens, and your position size automatically decreases to keep the pound risk constant. During low-volatility periods, ATR contracts, your stop can be tighter, and your position size increases.
This means that during calm, trending conditions you will naturally trade larger positions. During volatile, choppy periods you will trade smaller ones. Both outcomes are sensible: volatility is the main driver of whether a stop will be triggered by noise rather than a genuine change in market direction, and ATR-based sizing accounts for that automatically.
How does position sizing work across different asset classes?
The calculation framework is the same across forex, stocks, and CFDs, but what changes is the value per unit of price movement. It is worth understanding the conventions for each.
Forex lots: A standard lot is 100,000 units of the base currency. A mini lot is 10,000 units. A micro lot is 1,000 units. For EUR/USD, one standard lot has a pip value of approximately $10. One mini lot has a pip value of $1. Most retail platforms allow fractional lot sizes, so you can trade 0.25 or 0.73 lots to achieve an exact risk amount.
CFD contract sizes: Contract sizes vary by instrument and broker. For a UK equity CFD, one contract is typically one share, with profit and loss calculated in pence or pounds per point of price movement. For a commodity CFD such as oil, one lot might represent 100 barrels. For index CFDs, one point of price movement might be worth £1 per contract on a standard size, or £10 on a larger contract. Always check the contract specification in your broker’s platform before running position size calculations.
Share quantities: For direct equity purchases, the maths is straightforward: risk amount ÷ (entry price − stop price) = number of shares. If you are buying shares at £5.00 with a stop at £4.70, your risk per share is £0.30. With a £100 risk budget, you buy 333 shares (£100 ÷ £0.30).
Leverage affects the margin required to hold these positions but does not change the underlying pound risk calculation — which is why position sizing and leverage awareness need to be considered together.
What are the most common position sizing mistakes?
Four mistakes appear repeatedly among traders who struggle with drawdowns:
Sizing based on gut feel rather than a calculation. The amount feels right in the moment but has no mathematical basis, and it tends to be too large when confidence is high — precisely when trades are most likely to be entered late in a move.
Using the same number of contracts or shares regardless of stop distance. A wider stop means more risk per unit. Trading the same fixed number of contracts with a 100-pip stop that you do with a 30-pip stop means you are taking more than three times the risk on the wider-stop trade, even though both feel like “one contract.”
Not accounting for spread and commission. If a broker charges a 2-pip spread and you are using a 10-pip stop, you are already 20% of the way to your stop before the trade begins. Including transaction costs in your risk calculation — adding the spread to your stop distance — gives a more accurate picture of actual risk.
Failing to reduce size during drawdown periods. If your account drops from £10,000 to £8,000, your 1% risk drops from £100 to £80 automatically under the fixed fractional method. Some traders resist this reduction and keep risking the original £100, which accelerates the drawdown rather than controlling it. The method only works if you apply it to your current equity, not your peak equity.
Trading psychology is frequently the reason traders abandon their sizing rules at the worst possible moments — when confidence is highest after a winning run, or when the temptation to “trade bigger to recover” is strongest after losses.
Related Reading
- Risk-reward ratios: what they mean and how to use them before entering a trade
- Stop-loss orders: where to place them and the trade-offs involved
- Leverage in trading: how it amplifies both gains and losses
- Trading psychology: how fear and greed affect decisions and what to do about it
- CFDs explained: how contracts for difference work and what makes them risky
Frequently asked questions
What percentage of my account should I risk per trade?
The standard range for active traders is 0.5% to 2% per trade. Below 0.5% and gains are too small to be meaningful relative to the time and effort involved. Above 2% and a normal losing streak of 8 to 10 trades causes disproportionate damage to the account. Many professional traders settle on 1% as a default and only deviate from it with a specific reason. Beginners in particular benefit from starting at 0.5% or even lower until they have a clear picture of how their strategy performs across a large sample of trades.
How do I calculate position size for a forex trade?
The formula is: Position size = Risk amount ÷ (Stop-loss distance in pips × Pip value per lot). The pip value depends on the currency pair and your account currency. For EUR/USD with an account in pounds, a standard lot pip value is approximately £8 at current exchange rates. A mini lot is approximately £0.80 per pip. Use your broker’s position size calculator or a dedicated forex lot size calculator to get an accurate figure for your specific pair, as pip values vary across currency pairs.
What is a lot in forex trading?
A lot is the standard unit of measurement for a forex position. A standard lot equals 100,000 units of the base currency. A mini lot equals 10,000 units. A micro lot equals 1,000 units. Most retail brokers allow fractional lot sizes, so you can trade 0.23 lots or 0.07 lots to match a specific risk amount. The lot size determines the pip value — and therefore how much money you make or lose per pip of price movement.
Can I use leverage and still apply position sizing properly?
Yes, and position sizing is arguably more important when trading with leverage. The process is the same: calculate the pound amount you are willing to lose (your stop distance times the value per unit), then determine how many contracts or units achieve that pound risk. Leverage determines how much margin is required to hold the position, but position sizing determines your actual risk. Treating leverage as a reason to skip position sizing is one of the most reliable ways to blow up a trading account.
