""My lounge is 10' x 15', how many floor tiles will I need?""
Fired tiles are nominal*** sizes anyway. The size is variable due to the shrinkage in the kiln. I am not sure if the nominal size also includes an allowance for a reasonable grouting gap - which is itself usually variable.
Obviously that matters more for small mosaic tiles than for 18 inch ones. Then again the big ones will probably be cut at the room edges - so the off-cut may save a tile elsewhere. Always best to include a decent number for wastage and as spares for repairs.
***"nominal" means the stated size - not the actual measured size of something. Like a resistor would be a nominal marked value like 4k7 - with an indicated percentage tolerance either way.