The shower measurement method
A shower has four tiled surfaces at most: three walls, the floor, and sometimes a curb or niche. Measure each one in inches, because walls are usually sized in inches, and convert at the end by dividing by 144. The formula for one wall is length times height divided by 144. The floor is length times width divided by 144. Add the walls and floor, then subtract the door or curtain opening, which is door width times tile height divided by 144. That opening is real area you do not tile, and on a narrow shower it is a big number.
Do not subtract the mixing valve, showerhead, or body sprays. They are small, the cuts around them waste tile anyway, and pretending they save area is how people come up short. Count niches and benches separately and add them, since their inside faces are tiled surface that does not show up in the flat wall math.
Worked example: a 60 by 32 inch alcove stall
Take the most common shower in American homes: a 60 inch wide, 32 inch deep alcove with tile running 72 inches high and a 24 inch door opening on the front.
Back wall: 60 times 72 is 4,320 square inches, divided by 144 gives 30.0 square feet. Two side walls: each is 32 times 72, which is 16.0 square feet, for 32.0 square feet total. Wall subtotal: 62.0 square feet.
Door opening: 24 times 72 is 1,728 square inches, divided by 144 is 12.0 square feet. Subtract it: 62.0 minus 12.0 is 50.0 square feet of wall tile.
Floor: 60 times 32 is 1,920 square inches, divided by 144 is 13.3 square feet. Add it to the walls: 50.0 plus 13.3 is 63.3 square feet of tile before waste. With the standard 10 percent allowance: 63.3 times 1.10 is 69.6, so buy about 70 square feet.
Converting square feet into tiles and boxes
Walls and floors often use different tile, so do the box math twice. Say the walls use 12 by 24 inch tile, which covers 2 square feet each: 50.0 square feet times 1.10 is 55.0 square feet, divided by 2 is 27.5, so buy 28 wall tiles. If the box holds 5, that is 28 divided by 5, which is 5.6, so 6 boxes.
Say the floor uses 2 by 2 inch mosaic sheets at 1 square foot per sheet: 13.3 times 1.10 is 14.6, so buy 15 sheets. If sheets come 5 to a box, that is exactly 3 boxes. Run the same two-step math for your sizes in the tile calculator by entering the tiled area as if it were a room: it will apply the waste factor and box count for you.
Where shower jobs usually go wrong
The classic error is forgetting the walls are the job. People measure the 60 by 32 footprint, get 13 square feet, buy two boxes, and discover the walls need five times that. The second error is measuring the door opening wrong: a door that swings or a fixed glass panel still has a frame zone you do not tile, and skipping the subtraction inflates the order. The third is using one tile size for both: shower floors need small mosaic that follows the drain slope, and one large format everywhere means the floor tiles rock on the slope.
Finally, buy one extra box of the wall tile beyond the math. Shower walls take more cuts than any floor, around the valve, the niche, the corners, and the door frame. That spare box is also your repair stock, because a shower is the last place you want a dye lot mismatch showing years later.