ARMFForex lot size calculator
The forex lot size calculator converts a risk percentage into a trade size. Every value that affects the result is exposed as a field: the pip value, the leverage and the contract size differ between instruments and brokers, so they cannot be hard-coded as constants.
Lot calculator from the deposit
The base currency rate is needed only for margin: the collateral is counted from the notional in the base currency and converted into the account currency. For EUR/USD with an account in dollars that is the rate of the pair, for USD/JPY with a dollar account it is one, for gold it is the price of an ounce. The field does not affect the size or the loss at the stop. The pip value is exposed as a field because it differs: the familiar ten dollars per full lot hold only where the second currency of the pair is the dollar. The fractional part of the size is dropped to hundredths: an extra hundredth of a lot raises the loss above the percentage you set yourself.
How the size for a given risk is calculated
Any online lot calculator works by one and the same formula — the difference is only in which values it substitutes by default and how it rounds the result. Both values are taken apart below.
The formula is one, and the whole point is that the size comes out of a division rather than being chosen in advance.
| Value | Formula | In the example |
|---|---|---|
| Risk amount | equity × risk share | $50 |
| Cost of the stop on a full lot | stop length × pip value | $400 |
| Size | risk amount ÷ stop cost, down to hundredths | 0.12 lot |
| Pip value for the size | size × pip value | $1.20 |
| Notional | size × contract size | 12,000 EUR |
| Margin | notional ÷ leverage × rate to the account currency | $130 |
In the example the account is $5,000 with a limit of 1 %, a stop of 40 pips, a pip value of $10, leverage 1:100, the pair EUR/USD at a rate of 1.0850. The actual risk after rounding the size is $48 instead of the calculated $50: rounding down always gives a risk slightly below the planned one, and that is the correct direction of the error.
The main source of errors. The pip value is taken from memory. Ten dollars per full lot are right only when the dollar stands second in the pair — for example on the Aussie or the pound against the dollar. As soon as the dollar comes first, the value starts depending on the rate, and on pairs without the dollar it is calculated through a third currency altogether. The discrepancy in size reaches tens of percent.
Why a tight stop does not make a trade safer
Move the stop slider with the risk unchanged — the size changes inversely. The loss at the stop stays the same, but the notional of the position grows, and with it the required margin and the sensitivity to execution.
What changes when the stop is halved
- Loss at the stop
- It does not change — that is the whole point of calculating from risk.
- Size and notional
- They double. The position becomes twice as large.
- Required margin
- Doubles together with the notional.
- The cost of slippage
- Doubles: every pip costs twice as much.
- The share of the spread in the risk
- Doubles: the spread is the same while the distance is half.
- The probability of the stop triggering
- Grows: a tight stop falls inside ordinary price swings more often, and the win rate of the system drops.
Typical inputs and what they give
Four sets of parameters that cover most situations. All of them are calculated at 1 % risk and a pip value of $10 per standard lot. The margin is given for EUR/USD at a rate of 1.0850: the collateral is counted from the notional in the base currency and converted into the account currency.
| Deposit | Stop | Money risk | Size | Margin at 1:100 |
|---|---|---|---|---|
| $500 | 30 pips | $5 | 0.01 lot | $10.85 |
| $2,000 | 40 pips | $20 | 0.05 lot | $54.25 |
| $5,000 | 40 pips | $50 | 0.12 lot | $130 |
| $20,000 | 60 pips | $200 | 0.33 lot | $358 |
The first row is the boundary of applicability: with a deposit of $500 and a stop of 30 pips the calculated size equals the minimum, and any lengthening of the stop makes the trade impossible without breaking the risk rule. That is exactly the case where the calculator answers «no» — and the answer has to be accepted rather than bypassed by raising the percentage.
What to calculate next. After the size it makes sense to check the trade itself: the R/R calculator will show whether the target justifies this risk, and the losing streak calculator whether the account will survive a typical streak at the chosen percentage.
Frequently asked questions
How do you calculate the lot from the deposit?
Multiply the deposit by the risk percentage, then divide by the product of the stop in pips and the pip value. With a deposit of $5,000, risk of 1 %, a stop of 40 pips and a pip value of $10 you get 0.125 lot, which is rounded down to 0.12.
Where do you look up the pip value for my instrument?
In the instrument specification in the terminal: it states the contract size and the value of the minimum price step. Where the dollar stands second the value is constant, for the rest it is recalculated at the current rate.
Why is the size rounded down?
Rounding up raises the actual risk above the declared one. The difference on a single trade is small, but systematic rounding up means your real risk percentage is always slightly above the rule.
What to do if the margin is larger than the deposit?
It means the size is incompatible with your leverage: a position of that size will not open. The options are a longer stop (the size will shrink), a smaller risk percentage or an instrument with a smaller contract size.
How do you calculate the lot for USD/JPY in this calculator?
Enter the pip value from the specification — for a pair with the dollar as base it equals 10 units of the yen divided by the rate, about $6.7 at a rate of 150. The other fields stay the same.
Why does the size show a dash?
It means the calculated size is below the minimum step of 0.01 lot: the trade does not fit into your risk rule. The verdict under the calculator shows at which stop it would become possible.
What is the position notional and why is it in the calculation?
It is the size in units of the base currency: 0.12 lot is 12,000 units. The required margin is counted from the notional, so it is the notional rather than the risk amount that decides whether the free funds are enough.
How do you account for the spread in the size calculation?
Add it to the stop length: with a stop of 20 pips and a spread of 1.5 count it as 21.5. On long stops the correction fits inside the rounding, on tight ones it changes the size noticeably.
Why is there no entry price in the calculation?
It is not needed: the size depends on the distance to the stop, not on the absolute price level. The entry price will be needed in the risk-to-reward calculation, where three levels are compared.
Can the size be calculated for several positions at once?
The calculator handles one trade. For several positions divide the total risk limit between them and calculate each one separately — especially if the pairs share a common currency.
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