This is even easier though http://www.plantedtank.net/substratecalculator.html
to convert cm to inches, divide the cm by 2.5 To conver kg to lbs, multiply kg by 2.2.
Actually for your 100x50cm tank you would need 83lbs/38kg for a 3" deep bed. I would highly recommend you use 3" of substrate and you can have it deep in the back of the tank. Generally you should use 1.5-2" in the very front and slope it back to about 3"+ in the back, 4" is ok to.