The first thing I had to do was figure out how high up the wall I wanted the tile to go. 48" is standard so I marked the wall and stood back to see how that looked. I want to finish the top with a bullnose. So the field tile would be ending at 45". I checked the room to see where this would land around windows, outlets, light fixtures, and the medicine cabinet - and it was all good. We would be well below them all, with no tricky tile workarounds.
The trickiest place was around the door of the freestanding shower. And I had determined I wanted to frame the doorway with white subway bullnose until it met the inside shower tile, which would be charcoal gray subway. I counted up my bullnoses by hand, making note of how many would curve on the 3" side and how many would curve on the 6" side. I needed 74 of the former and 43 of the latter.
Then I pointed my browser to squarefootage.org to calculate the to-be-tiled areas of each wall. This site's calculator is not fancy looking, but it's one of the few I've found that let's you input feet and inches.
My bathroom is 4' 10" x 8' 9". So I calculated that I would need 79.2 sq ft of beveled field tile, plus another 5.5 sq ft for overage. The rule of thumb is to add 10% extra to account for breakage during cutting as well as to keep a few in storage in case of future damage, when the tile you used may have long ago been discontinued. But my contractor advises that 7% should be sufficient.
The freestanding shower room needed 68.8 sq ft of gray field tile (we're tiling the ceiling too).
And the floors of both, which we're tiling in a handcrafted half hex in shades of gray, was 38.8 sq ft including overage.
The half hex floor is a special order that will take 8 weeks to get here. I double checked that with my contractor who said no worries. The floor is one of the last things to go in.