Backsplash tile for a 21 ft run, 15″ high, 6×6″ in a stacked layout

Tile to buy

31.5ft²

26.3 ft² of wall plus 20% — about 126 × 6×6″ tiles in stacked

Tile to buy	31.5ft²
Tiles6×6″	126tiles
Wall area21 ft × 15″ high	26.3ft²
Waste allowance10% for stacked + 10% for 5 outlets	20%
Outlets to cut aroundNEC wants one within 24″ of any point on the counter	5outlets
Edge trimtwo ends plus the exposed top edge	23.5linear ft
Layoutgrid lines up in both directions — least cutting, least forgiving of a wall out of square	stacked
Heighta short splash — the wall above stays painted	15″

Run length: 21 ft
Height: 15 in
Tile: 6×6″
Layout: a stacked layout

Backsplash waste is not floor waste

21 ft × 15″ is 26.3 ft² of wall. Buy 31.5 ft² — 20% 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 6×6″ that comes to roughly 126 tiles in a stacked layout.

Edge trim: the giveaway

23.5 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. 20% here — 10% for the stacked layout and 10% for 5 outlets.
Grid lines up in both directions — least cutting, least forgiving of a wall out of square.
At 15″ the top edge is exposed and needs finishing: 23.5 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.