How to calculate lot sizes
Assumption is you have a $10 000 dollar account and you are risking 2% of your account per trade. In this case 2% of $10000 is $200.
we want to determine pip risk (difference between entry point and stop loss). For purposes of this demonstration we will use 50 pips.
Now determining your ideal position size
$ at Risk / (pip risk X pip value)
$200 / (50x$1)
4 Mini lots
One last example
Lets say you have a $100 account and risking 2% per trade. 2% of $100 is $2.
The pip risk is 20 pips. So determining the ideal position size.
$ at risk / (pip risk X pip value)
$2/ (20 x $0.10)