heres a nice easy calculation for you. In your head convert cms to meters.
So 30 x 30 = 0.3 x 0.3
Which equals 0.09
1/0.09 = 11.11
So there are 11.11 tiles to a meter. Multiply this by the sq meterage of the room and you have the number of tiles you need.
Same with any other
tile...
30 x 60 = 0.3 x 0.6
== 0.18
1/0.18 = 5.56
So 5.56 Tiles per meter.
Also, if your room isn't square, then draw it out neatly on paper, and try and square off anything you can and work it out in sections. If its an angled wall, then i draw a rectangle so the wall would run through both corners of the rectangle, then when you work out the area of the rectangle, the area of the bit you want is half of that.
Its really hard to explain but really simple to show someone with a piece of paper and a pen. Hopefully ive made some sense though.
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oh, and another thing to add... when im working out tiles, if the tiles are less than33 x 33 then i allow 5% extra for wastage, and if theyre more than 33 x 33 i allow 10%. This normally works out pretty much spot on. If its mosaics, then its much less than 5 %.