how do i calculate spread commissions for forex broker.

The spread is not a comission, is the difference between the buying price and the selling price. If the price at a certain time is: 1.4120/1.4122 (the spread is 3 pips, or 0.0003 If you go short (sell), the price you will be getting, is 1.4120, if you go long (buy) you are getting the price of 1.4122. So if you instantly you want to close your trade, you will have to pay the other price, for buying or selling, depending if you were short or long. So you only have to "pay" once this comission, eventhough it is not a comission, it´s just the difference between the buy and sell price. For a better example, EUR/USD is 1.4120/1.4122 You think that the EUR will be stronger than the USD, so you buy EUR, you buy let´s say, 10000 euros, so you will have to pay 10000*1.4122, that´s 14120 dollars (if you have a 100:1 leverage you will only need 141 dollars to do this trade). Ok, as you thought, the euro is going better, now the quotes say: 1.4200/1.4202, so you want to close the trade, you are going to sell the 10000 euros you bought, you are going to sell them at the other price, so 1.4202 You sell them for 14202 dollars, the profit is 14202-14120, you are winning 82 dollars, the price moved 82 pips