Hi, not a tile 'expert', but I have done much tiling in many homes. 10% extra is standard, which usually leaves you with a few pieces left over of a typical job in case you need to replace a piece or two.

In my house, for my back-splashes only, I didn't have but about 7 pieces left over in a typical subway pattern install. Some pieces do break during cutting, depending on which kind of tile you have. So, the 10% rule worked fine. The more complicated your design, then the more extra you need to figure for your installation. Example: If you install on the diagonal, then expect more wasted tiles; hence, maybe 15% extra would be needed to be ordered.

FLOOR TILING is a different story. FLOOR TILE, especially if is porcelain or ceramic, we always buy two or three extra full boxes of tile on top of the 10% overage for breakage/install because of experience.

Let me use my sister's house as an example.
She hired out her tile work. We counseled her several times on the proper method to install tile in her house with a slab foundation. We did not want her to have breakage. Even though she new 'how' to tile over concrete so she could properly supervise the installation, she did not supervise her tile guy. She did question his install technique when she interviewed tile guys, which he did 'say' the correct method, however he did not install the tile in the same such manner. She "trusted" him.

BIG MISTAKE. He did not prep the floor correctly. He also told her that "he would store her extra tile for her in his shop in case she ever had a need for repair". He gave her a verbal, one year warranty on install. She had paid for the two extra boxes as we suggested and her floors were beautiful.

Three months after moving in her house "it" happened. It was winter and we had some freezing weather for about a week. She heard a pop in her kitchen one night, but after investigation, couldn't find anything wrong.

The next morning she saw "it". She had a crack, now a 3/8" wide gap, running in the porcelain tile from where the tile starts running the entire length of the kitchen, under her island, sink, and through the breakfast room. I would say to fix this, she would need at least 22, 16" tile replaced to fix this mess.

She called the guy that installed her tile and he's out of business and her extra boxes of tile are gone. I think she had 10 tile to a box, so the two boxes would not have been enough anyway. She was/is still sick to her stomach over this mess. (Five years now and counting.)

What is she going to do now? She's decided to not worry about it until she sells the house. We have tried to find matching tile all over the place, but we can't find it. We come close, but not good enough for an acceptable fix.

So, I would suggest that you consider the worse case scenario for waste and if you had to replace some tile in each specific room, then buy that much plus 10% extra tile if it's on a floor.

One more scenario to think about. In our shower, we used travertine. We had a funky tall shower gizmo installed with 8 shower heads. JUST in case we ever had to replace the gizmo with another model and have new plumbling work done, we bought enough matching tile to fix this plumbing area TWICE. We decided that the cost of buying and storing a few boxes of tile is nothing compared to the major bucks redoing the entire shower! $200 worth of prevention is worth thousands of dollars of potential replacement and repair in the future should a leak or replacement become necessary.

I know you asked about a back-splash, but could not help sharing these other thoughts. Hopefully, this will help someone.

10% should be fine.

Thank you so much bill and sherrilyn for the advice. I appreciate it. Jaymie