The main reason you want to calculate your position size is to manage your risk, of course. There are two parts to doing this effectively: trade size and a stop loss.
Trade size is simply the number of micro lots, mini lots or standard lots you're trading at any given time. A stop loss is the number of pips you're willing to lose on a given trade.
These two ideas are directly connected when it comes to managing risk.
If you make one larger, then the other has to be smaller to maintain the same level of risk. For example, the larger your trade size, the smaller your stop loss must be (and vice versa).
Here’s a handy position size calculator you might find useful:
Just select the appropriate currency, account size, risk ratio (I suggest 1% for most trades), stop loss, contract size and currency pair. It’s really easy once you’ve tried it.
Please remember to remain within an appropriate risk level when trading. Otherwise you could wipe out your account after just a handful of losses.