This calculator is valid for the 2014/15 tax year - if you want to calclulate for a different year, go to 2009/10 and 2010/11, 2011/12, 2012/13 or 2013/14.
To use the calculator enter the gross or net pay into one of the empty cells and click the calculator icon. Change anything you need to (check the 'other inputs' - you can change any value in a blue cell) and recalculate.