how to calculate position size based on money


Let's say I have 50$ to spend on single position. It is 1/100 of my account - this is my strategy (EA).

How can I calculate size of position in lots based on FIAT?

I'd like to use the same EA for different markets - I don't want to hardcode position size.

Well, the lot size depends on the distance between your opening price and your stop loss.

If you have a stop loss with a distance of 50 points, then every point may be worth of 1$.

So now you need to get the value of one point in your account currency, based on 1.0 standard lot. Then you can calculate the lot size you would use for your position.

So let's say eurusd and contract size 100.000

A contract size of 100.000 equals to 1.0 lot.

There you go.
  1. Risk depends on your initial stop loss, lot size, and the value of the symbol. It does not depend on margin and leverage. No SL means you have infinite risk. Never risk more than a small percentage of your trading funds, certainly less than 2% per trade, 6% total.

    1. You place the stop where it needs to be — where the reason for the trade is no longer valid. E.g. trading a support bounce, the stop goes below the support.

    2. AccountBalance * percent/100 = RISK = OrderLots * (|OrderOpenPrice - OrderStopLoss| * DeltaPerLot + CommissionPerLot) (Note OOP-OSL includes the spread, and DeltaPerLot is usually around $10/PIP but it takes account of the exchange rates of the pair vs. your account currency.)

    3. Do NOT use TickValue by itself - DeltaPerLot and verify that MODE_TICKVALUE is returning a value in your deposit currency, as promised by the documentation, or whether it is returning a value in the instrument's base currency.
                MODE_TICKVALUE is not reliable on non-fx instruments with many brokers - MQL4 programming forum (2017)
                Is there an universal solution for Tick value? - Currency Pairs - General - MQL5 programming forum (2018)
                Lot value calculation off by a factor of 100 - MQL5 programming forum (2019)

    4. You must normalize lots properly and check against min and max.

    5. You must also check FreeMargin to avoid stop out

    6. For MT5, see 'Money Fixed Risk' - MQL5 Code Base (2017)

    Most pairs are worth about $10 per PIP. A $5 risk with a (very small) 5 PIP SL is $5/$10/5 or 0.1 Lots maximum.

  2. Search for a GUI/Trade Assistant EA, like mine: Indicators: Money Manager Graphic Tool - Risk Management - Articles, Library comments - MQL5 programming forum - Page 8 #80 (2022.02.16).