Backsplash tile for a 9 ft run, 36″ high, 12×24″ in a diagonal layout

Tile to buy

32.1ft²

27 ft² of wall plus 19% — about 17 × 12×24″ tiles in diagonal

Tile to buy	32.1ft²
Tiles12×24″	17tiles
Wall area9 ft × 36″ high	27ft²
Waste allowance15% for diagonal + 4% for 2 outlets	19%
Outlets to cut aroundNEC wants one within 24″ of any point on the counter	2outlets
Edge trimtwo vertical ends — the top runs under the cabinets	6linear ft
Layoutevery tile at the perimeter is a cut	diagonal
Heightfull height from counter to cabinets	36″

Run length: 9 ft
Height: 36 in
Tile: 12×24″
Layout: a diagonal layout

Backsplash waste is not floor waste

9 ft × 36″ is 27 ft² of wall. Buy 32.1 ft² — 19% over.

That allowance is higher than a floor's for a specific reason. A narrow strip has far more edge per square foot than the same area spread across a floor, and every outlet is four cuts and most of a wasted tile. There are about 2 outlets in a run this long, because the NEC wants one within 24 inches of any point on the counter.

At 12×24″ that comes to roughly 17 tiles in a diagonal layout.

Edge trim: the giveaway

6 linear ft. A raw cut edge along the top or at the ends is the single most obvious sign of a DIY backsplash, and it is the one thing you cannot fix later without pulling tile off.

Buy it all at once

Tile is made in colour lots. A second box bought three weeks later can be visibly different against the first under kitchen lighting, and kitchen lighting is unforgiving — it comes from directly above and rakes across the wall.

Start from the middle of what people look at

Usually the cooktop or the sink. Work outward from there so the cut tiles land at the ends of the run where nobody looks, rather than dead centre behind the range.

Notes

Backsplash waste is not floor waste. A 36″ strip has far more edge per square foot than the same area on a floor, and every outlet is four cuts and most of a wasted tile. 19% here — 15% for the diagonal layout and 4% for 2 outlets.
On a diagonal, every tile around the perimeter is a cut, and the offcuts rarely fit anywhere else. This is why the same tile costs more to lay at 45°.
At 36″ this runs from the counter to the underside of the wall cabinets, so the top edge is hidden and only the two vertical ends need trim — 6 linear ft.
Start the layout from the most visible point — usually the centre of the cooktop or the sink — and work outward, so the cut tiles land at the ends where nobody looks.
Buy the whole job in one order. Tile is made in colour lots, and a second box bought three weeks later can be visibly different against the first under kitchen lighting.

Nearby sizes

What a 25 lb Bag of Grout Actually CoversA bag's coverage figure is real. It is also stated for one specific tile size, joint width and thickness — change any of the three and the number moves by a lot.
Grout for Subway Tile: Three Times the 12-Inch FigureSubway tile is the highest grout consumption per square foot of any common format. If you sized the order from a floor tile figure, you are about a third of the way there.
Grout for 12 × 24 Tile: Why the Big Format Uses LessLarge format tile is the one place where the bigger product costs you less of something. Grout is that something, and the saving is exactly a quarter.
Sanded or Unsanded: The Joint Width That Decides ItThe dividing line is 1/8 inch and it is not a style guideline. Using the wrong one on either side of it produces a specific, predictable failure.