Hi Catherine,

$\frac{11}{16}$ is $0.6875$ so each tile has an area of $3.6875 \times 3.6875 = 13.598$ square inches. One square foot is $12 \times 12 = 144$ square inches and hence $28$ square feet is $28 \times 144 = 4032$ square inches. Also $\frac{4032}{13.598} = 296.5$ and hence you will need at least $297$ tiles. The actual number of tiles you will need depends on the shape of the back splash, the number of tiles you will need to cut and the amount of wastage there is.

Penny