Can Floor Tile be used on walls? Can Wall Tile be used on floors?

Generally speaking, any tile designated by the manufacturer as floor tile can be installed on vertical surfaces and countertops.  The durability required for floors is necessarily greater than for walls (which receive no foot traffic), so floor tile with Abrasion Resistance values from I – V is sufficiently durable for a typical wall installation.  Exterior wall applications must use tile that is Frost Resistant.  (See What Tile Should I Use Outdoors? FAQ) 

The reverse is not true, however.  Wall tile (identified by Abrasion Resistance 0) is not suitable for use on floors.  It is usually non-vitreous, i.e. not manufactured to withstand excessive impact, abrasion or freeze/thaw cycling. 

One caveat when considering floor tile for wall or countertop installation is the abrasiveness of the tile.  A highly abrasive product would be more difficult to clean on a wall or countertop since commercial floor scrubbers could not realistically be used for maintenance. Otherwise, continuing the chosen floor tile on walls adds great cohesion to an installation and continuity of design flow.