Backsplash tile for a 18 ft run, 15″ high, 3×6″ subway in herringbone

Tile to buy

29.3ft²

22.5 ft² of wall plus 30% — about 234 × 3×6″ subway tiles in herringbone

Tile to buy	29.3ft²
Tiles3×6″ subway	234tiles
Wall area18 ft × 15″ high	22.5ft²
Waste allowance20% for herringbone + 10% for 5 outlets	30%
Outlets to cut aroundNEC wants one within 24″ of any point on the counter	5outlets
Edge trimtwo ends plus the exposed top edge	20.5linear ft
Layouttwo cuts on most edge tiles; order generously	herringbone
Heighta short splash — the wall above stays painted	15″

Run length: 18 ft
Height: 15 in
Tile: 3×6″ subway
Layout: herringbone

Backsplash waste is not floor waste

18 ft × 15″ is 22.5 ft² of wall. Buy 29.3 ft² — 30% 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 5 outlets in a run this long, because the NEC wants one within 24 inches of any point on the counter.

At 3×6″ subway that comes to roughly 234 tiles in herringbone.

Edge trim: the giveaway

20.5 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. 30% here — 20% for the herringbone layout and 10% for 5 outlets.
Herringbone is the expensive layout. Almost every tile at the perimeter takes two cuts, and a mis-cut cannot be reused elsewhere the way a straight cut often can. Order the full 20%, and order it all at once.
At 15″ the top edge is exposed and needs finishing: 20.5 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.