How to Calculate Profit or Loss?

When a Forex trader buy a currency, he/she will buy at the ask (offer) price.
When a Forex trader sell a currency, he/she will sell at the bid price.

Whenever a currency is purchased, there is a spread that a Forex trader requires to pay (enter but not exit).
Whenever a currency is sold by the Forex trader, there is no need for payment on the spread (not enter, but only during exit).

Example:
The rate being quoted is 1.4525 / 1.4530 (The Spread is 5). When USD is bought, 1.4530 will be the value that will work on; this value is the amount that the traders prepare to sell.

1 lot of $100,000 is bought 1.4530.

30 mins later, the price moves to 1.4550, the Forex trader sells the lot.

The new quote for USD/CHF is 1.4550 / 14555. To close the trade, the Forex trader must sell and take the 1.4550 as the closing price. This is the price that the Forex traders are prepared to buy at.

The difference between 1.4530 and 1.4550 is .0020 or 20 pips.

(.0001/1.4550) x $100,000 = $6.87 per pip
$6.87 x 20 pips = $137.40 of profit

Prerequisite
What is a lot?
What is a pip (point)?
Forex Quote: Bid & Ask Spread