Backsplash tile for a 9 ft run, 4″ high, 2×2″ mosaic in a diagonal layout

Tile to buy

3.6ft²

3 ft² of wall plus 19% — about 129 × 2×2″ mosaic tiles in diagonal

Tile to buy	3.6ft²
Tiles2×2″ mosaic	129tiles
Wall area9 ft × 4″ high	3ft²
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 ends plus the exposed top edge	9.7linear ft
Layoutevery tile at the perimeter is a cut	diagonal
Heighta short splash — the wall above stays painted	4″

Run length: 9 ft
Height: 4 in
Tile: 2×2″ mosaic
Layout: a diagonal layout

Backsplash waste is not floor waste

9 ft × 4″ is 3 ft² of wall. Buy 3.6 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 2×2″ mosaic that comes to roughly 129 tiles in a diagonal layout.

Edge trim: the giveaway

9.7 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 4″ 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 4″ the top edge is exposed and needs finishing: 9.7 linear ft of metal trim or bullnose. A raw cut edge along the top is the single most obvious sign of a DIY backsplash.
A 4″ splash is a strip, not a backsplash, and it draws a hard line across the wall at counter height. It is cheap and it protects the wall, but it dates a kitchen faster than almost anything else in it.
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.