Most online trading platforms automatically calculate the P&L of traders' open positions. However, it is useful to fully understand how this calculation is derived. 

Example : 

The current Bid/Ask price for USD/CHF is 1.6322/1.6327, meaning you can buy $1 US for 1.6327 Swiss Francs or sell $1 US for 1.6322. 

Suppose you decide that the US Dollar (USD) is undervalued against the Swiss Franc (CHF). To execute this strategy, you would buy Dollars (simultaneously selling Francs), and then wait for the exchange rate to rise. 

So you make the trade: purchasing US $100,000 and selling 163,270 Francs. (Remember, at 1% margin, your initial margin deposit would be $1,000.) 

As you expected, USD/CHF rises to 1.6435/40. You can now sell $1 US for 1.6435 Francs or buy $1 US for 1.6440 Francs. Since you're long dollars (and are short francs), you must now sell dollars and buy back the francs to realize any profit. 

You sell US $100,000 at the current USD/CHF rate of 1.6435, and receive 164,350 CHF. Since you originally sold (paid) 163,270 CHF, your profit is 1080 CHF. 
To calculate your P&L in terms of US dollars, simply divide 1080 by the current USD/CHF rate of 1.6435. 

Total profit: US $657.13