tiling - how to work out amount needed <<thick>>

[TheWheelsOnTheBusHaveFallenOff]

Tiles are 100mmx100mm.

Builder said we need about 8m sq of tiles but I think this is too much. Tiles have to be ordered so can't return excess.

Would like to have an idea of what I need before going back to the builder - how do I work out how many tiles I need? I assume that 100 tiles covers 1m sq?! I know I sound very thick!!

Yes, 100 tiles covers 1m sq, but you have to add an allowance for the shape you're tiling, eg if around windowsills, sockets etc in the kitchen there could be a fair bit of wastage. As a general rule you should add about 10% for wastage, but as I say if there are lots of fiddly bits, increase to maybe 15 or even 20% wastage.