Backsplash tile for a 12 ft run, 15″ high, 12×24″ in a diagonal layout

Tile to buy

18.1ft²

15 ft² of wall plus 21% — about 10 × 12×24″ tiles in diagonal

Tile to buy	18.1ft²
Tiles12×24″	10tiles
Wall area12 ft × 15″ high	15ft²
Waste allowance15% for diagonal + 6% for 3 outlets	21%
Outlets to cut aroundNEC wants one within 24″ of any point on the counter	3outlets
Edge trimtwo ends plus the exposed top edge	14.5linear ft
Layoutevery tile at the perimeter is a cut	diagonal
Heighta short splash — the wall above stays painted	15″

Run length: 12 ft
Height: 15 in
Tile: 12×24″
Layout: a diagonal layout

Backsplash waste is not floor waste

12 ft × 15″ is 15 ft² of wall. Buy 18.1 ft² — 21% 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 3 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 10 tiles in a diagonal layout.

Edge trim: the giveaway

14.5 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 15″ 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. 21% here — 15% for the diagonal layout and 6% for 3 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 15″ the top edge is exposed and needs finishing: 14.5 linear ft of metal trim or bullnose. A raw cut edge along the top is the single most obvious sign of a DIY backsplash.
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.