This slate has a much rougher surface than the brazilian black slate giving a more charactered appearance. To calculate the number of tiles required per meter square e.g 60 x 30cm tile = .6 x .3=.18 then 1divided by .18=5.55 rounded off to 5.5 tiles per meter square(alowing an average 5mm gap between tiles) simple! if you want us to check you're calculation give us a call.