Calculating Max Position Size

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Frederick Langemark
Frederick Langemark  


I have the following code...

maxLots = AccountFreeMargin() / MarketInfo(Symbol(),MODE_MARGINREQUIRED);
lotStep = MarketInfo(Symbol(), MODE_LOTSTEP);
maxLots = floor(((maxLots / lotStep) * lotStep) * 100) / 100;
maxLots = NormalizeDouble(maxLots, 2);

With $1,000 in Free Margin on an account with 50:1 leverage, it says I can open 0.36 lots of GBPUSD. However, when I try to open a position of that size, it fails due to insufficient margin.

Can anyone tell me what I am doing wrong?

All the best,

William Roeder
William Roeder  
Frederick Langemark: it says I can open 0.36 lots ... it fails due to insufficient margin.

Margin has nothing to do with lotstep. Risk depends on your initial stop loss, lot size, and the value of the pair. In code (MT4):

  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 You place the stop where it needs to be - where the reason for the trade is no longer valid.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. Account Balance * 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.
  4. You must normalize lots properly and check against min and max.
  5. You must also check FreeMargin to avoid stop out
Most pairs are worth about $10 per PIP. A $5 risk with a (very small) 5 PIP SL is $5/$10/5=0.1 Lots maximum.
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