ARMFPosition size calculation on forex
Position size calculation on forex is the only place where a risk percentage turns into a specific number of lots. The formula is the same for every currency pair; only the pip value changes. We take the calculation apart step by step, three ways to get it wrong and the check of the result through margin.
How to calculate position size in trading: the formula
lot = (equity × risk share) ÷ (stop distance × pip value per full lot)
The numerator is the amount you are prepared to lose. The denominator is how much one lot loses when the stop is hit. The division gives the number of lots at which these two values coincide.
| Value | Where it comes from | Example |
|---|---|---|
| Equity | The current state of the account, not the starting deposit | $5,000 |
| The risk percentage | Your own rule, the same for every trade | 1 % |
| Money risk | equity × the percentage | $50 |
| The stop in pips | The distance from the entry to the idea invalidation level | 40 pips |
| Pip value on a full lot | From the instrument specification | $10 |
| Cost of the stop on a full lot | stop × pip value | $400 |
| Size | money risk ÷ the cost of the stop on a full lot | 0.12 lot |
The exact quotient here is 0.125 lot, and the rounding must be downwards: 0.12 gives an actual risk of $48, while 0.13 gives $52, that is 1.04 % instead of the declared percentage. Rounding up looks harmless right up until it becomes a habit.
How the stop distance changes the size
The most useful property of the formula: with the risk amount unchanged, the size is inversely proportional to the stop distance. A tight stop does not make a trade safer — it increases the size by exactly the same factor.
| Stop | Cost of the stop on a full lot | Size at a risk of $50 | Position notional |
|---|---|---|---|
| 10 pips | $100 | 0.50 lot | 50,000 |
| 20 pips | $200 | 0.25 lot | 25,000 |
| 40 pips | $400 | 0.12 lot | 12,000 |
| 80 pips | $800 | 0.06 lot | 6,000 |
| 160 pips | $1,600 | 0.03 lot | 3,000 |
The loss at the stop is almost identical in every row — about $50. But the notional of the position differs by a factor of seventeen, and with it the required margin, the cost of slippage in pips and the sensitivity to a widening spread. That is exactly why «a small stop means small risk» is a false statement: the risk is the same, but the vulnerability to execution is not.
The calculation step by step
Five actions that take about fifteen seconds and are performed before every trade. Skipping any of them means your actual risk is unknown to you.
Not the starting deposit and not the balance: equity includes the result of already open positions and therefore describes the account at the moment of the calculation.
from the terminalYou get the amount you are prepared to lose on this trade. It does not depend on how convincing the idea looks.
ruleIn pips; on stops shorter than 20-30 pips add the spread: the entry goes on one side of the quote and the exit on the other.
in pipsFrom the instrument card in the terminal. With the dollar in second position the number is constant, in other cases it is recalculated at the rate.
specificationThe risk amount is divided by the product of the distance and the pip value, and the result is rounded down to the size step — usually to 0.01 lot.
down to 0.01 lotThree mistakes in the calculation
The check through margin
After calculating the size it is useful to make a second calculation — how much of the funds that size will take up. The formula: margin = notional ÷ leverage, where the notional = size × contract size. For a pair whose base currency differs from the account currency the notional is converted at the current rate.
| Leverage | Margin for 0.12 lot | Margin for 0.50 lot | Share of a $5,000 deposit |
|---|---|---|---|
| 1:30 | $400 | $1,667 | 8 % / 33 % |
| 1:100 | $120 | $500 | 2.4 % / 10 % |
| 1:200 | $60 | $250 | 1.2 % / 5 % |
| 1:500 | $24 | $100 | 0.5 % / 2 % |
The notional: 0.12 lot is 12,000 units of the base currency, 0.50 lot is 50,000. The calculation is given for a pair whose base currency is the dollar; for the others multiply the notional by the rate of the base currency against the dollar.
The point of the check is not to choose high leverage but to see how much free funds are left. Used margin does not take part in absorbing a loss, and the more of it there is, the closer the account is to a forced close of positions when price moves against you.
Frequently asked questions
How does position sizing on forex differ from calculations on other markets?
In essence not at all: in both cases the size comes from dividing the risk amount by the stop distance. What differs is the unit of size — lots instead of shares or contracts — and the fact that the pip value has to be taken from the instrument specification.
How do you calculate position size if the stop is given in price rather than in pips?
Convert the price difference into pips: for a pair with four decimal places divide the difference by 0.0001, for yen pairs by 0.01. For example, an entry of 1.0850 and a stop of 1.0810 give 0.0040 ÷ 0.0001 = 40 pips.
Which amount should the risk percentage be taken from?
From equity. The balance does not see open trades, so with a hanging minus a calculation from it allows a larger lot exactly when the account is already loaded.
Does the spread have to be included in the stop distance?
On stops shorter than 20–30 pips, definitely: a spread of 1.5 pips with a 15-pip stop is 10 % of the risk. On long stops the correction fits inside the rounding error of the size.
How do you calculate the size when several positions are open?
The risk on each is calculated separately, but the total risk and the total margin are checked together. If the pairs are linked — for example EUR/USD and GBP/USD — they are best counted as one position: when the dollar moves, the stops will most likely trigger at the same time. More on this in the article on correlation and total risk.
What to do if the size comes out below 0.01 lot?
That is an answer, not an error: the trade does not fit into your risk rule. The options are a shorter stop from a different idea, an instrument where a pip costs less, an account with a smaller size step or a larger deposit. Raising the risk percentage «just this once» is not an option: that is exactly how rules stop working.
How do you calculate the size for USD/JPY?
The same formula, a different pip value. For a pair with the dollar as base it equals 10 units of the quote currency divided by the rate: with USD/JPY around 150 that is about $6.7 per standard lot. At a risk of $50 and a stop of 40 pips the size comes out at 0.18 lot against 0.12 on EUR/USD.
How do you calculate the size on gold or indices?
The formula does not change, but the contract size and the price step are different, so the pip value is calculated differently too. All three values are taken from the instrument specification: the contract size, the minimum price step and its value.
Should the size be rounded up or down?
Always down to the size step. With a calculated 0.125 lot you take 0.12: the actual risk becomes slightly smaller than planned. Rounding up means your real risk percentage is systematically above the rule.
Does the size have to be recalculated when the deposit changes?
Yes, if the risk is set as a share of equity — and that is the recommended option. After a 20 % drawdown the same 1 % gives a smaller amount and a smaller size: it is exactly this property that slows the fall of the account.
Does leverage change the position size?
The lot, no — it comes from the risk amount and the stop distance. Leverage decides whether the free funds cover that size: margin is counted as the notional divided by the leverage, and at 1:30 the same trade takes up more than three times as much of the funds as at 1:100.
Read next
