A backsplash is a thin wall: length times height, minus the window if the tile stops at the casing. Use the Square Foot Calculator, then the Tile Calculator with a 10 percent waste factor for stacked joints.
Outlets, corners, and a range hood eat extra cuts. Buy one extra box if the tile is a special order.
Dry-lay a row on the counter before you mix thinset so the pattern lands on a full tile at the focal point.