hmm make that a centimetre then....oh and 5mm - 10mm sharp grit and 2mm screen the grit first if ya really wanna be cool...this stuff also works in breaking the lifecycle of those horrible tiny black flying insects...i forget the name...that colonise houseplants and apparently love the comp screen in office environments...the eggs are laid on moist compost...the grit interrupts the life-cycle...hey presto..!!...no more tiny annoying flies...shove the grit in the base of the pot to cover the crocks also...mixed units i was brought up on threepenny bits and only know hot in fahrenheit and cold in centigrade..!!