Grout line spacing is really personal preference. On floors and for that size tile, I would probably choose 1/16" or 1/32". I would also try to either match the color of the tile with the grout color or go as dark as possible. I have travertine in my own kitchen and I did this exact combination I just described, I used 1/16". The grout color is a tobacco tan color and has held up quite well. Have your installer mock up a square of your material, with both spacers and see which one you like better. Hope that helps.
Charmean Neithart

Thank you! That helps a lot :)

Thank you, good points- we are laying it in a diamond pattern and I don't know if it is rectified or not. We bought it at Home Depot. All boxes are from the same dye lot.

Now this is just my opinion but some tiles look better with a larger grout line, spanish tile, mexican tiles, random slate or limestone tiles. If this in not the look you are trying to achieve, go with the small joint possible .
If you look at the picture I attached you can see that the joints are tight and not noticable these are 16 x 16 tiles,
I have supplied many porcelain look alikes in 20 x 20 and butt jointed the tiles so the joint isn't noticable. I would suggest you go with the smallest joint possible, nothing worse than having a great floor only to notice the grout lines. Your installer should be able to select random tiles to make sure they are square, if there not then go a little bigger as Deborah has suggested but try and stay as tight as possible.

Sorry I posted the wrong picture, this floor isn't grouted yet. I am sure you can find many nice looking floors.

I doubt seriously she has rectified tile. Poorgirl, will you be laying the tile yourself or are you having an installer do it. It sounds as though you are. Laying a straight ahead tile is difficult enough for many DIY, but laying on the diagonal, measuring and snapping a plumb line can be a difficult process. It has to be correct. If it's off from the onset, it will only grow worse as you progress with your job. Also, if you are doing it yourself, I think a larger grout line will give you a little more leeway. I know you didn't ask for advice except for what size grout is best. You have good answers. I just like to know if someone is fully prepared to lay a floor on their own. Good luck!

Hi Karen, I sell i don't install tile, it's mknop1 that was inquiring on the joint size. My husband can do many things including building but when it comes to stone,he leaves it for the professionals, to many things can go wrong if you don't know what your doing.

Thanks for telling me I got the wrong "poor girl". :) So, you understand when not to DIY. I just wanted to do the caveat thing because there's nothing worse than tackling a job that should be left to the professionals. Even they screw up from lack of knowledge or will sometimes do what's easy. Drives me nutz!

Karen, to funny, I'm sure there is more than 1 "poor girl" . There are lots of things that go wrong with tile DIY, I have seen jobs that the tiles were not butter backed and were lifting off the floor, what a mess, direction of veining running different directions. installers are worth every cent they make I think they can make or break your project just as any other professional.

I do apologize, my wording concerning installation was misleading. A hired professional is installing the tile. I would not attempt to do it myself and am more than happy to leave it to a pro!
I really appreciate the replies from each one of you. Thanks for your advice and suggestions!

I laid a lot of tile and hired a lot out. I would go with Deborah Butlers advice

We are going with 3/16ths. So far so good. The tiles are not rectified.

3/16" is a goodchoice for non-rectified tile.
It should give you the look you are going for.

Torry Manzo
Renaissance Kitchen and Home

Large format tiles are a bit tricky. These, more than either of the other two, are more dependent upon the tile itself. While most people will purchase larger format tiles specifically because they do not want a lot of grout lines, sometimes the tile will not allow it. Although this is rarely a problem, you need to be aware of it and make sure you check the tiles before you try to go with a very small grout line.

The easiest way to check larger format tiles is simply to measure corner to corner in each direction to ensure squareness. As long as they are the same in both directions, they’re square. Then measure several different tiles from different boxes. With a very good tile you should get exactly the same measurement every time. If that’s the case, you can use a 1/16″ grout line and not have any problems.

Large format tiles with 1/16″ or 1/32″ grout lines and a grout that matches the tile color closely looks great! If done correctly it will almost look like a single large slab of tile.

what is a "narrow" 8mm grout line? Is it close to 1/32"? Is there a ".8mm" grout line?

tile companies recommend a grout width for their tile patterns. Rectified tile can use very narrow grout lines. The grout, however, has to be correctly mixed and applied, to get down into the more narrow lines.

In addition the color of the grout is very important; you should look for a grout color as close as possible to the base color of the tile.

We are building a new house and having trouble trying to decide between a 18 X 18 square
tavertine tile or a pattern in travertine for the bathroom floors. Any suggestions?

I have 21 X 21 rectified tile I plan on using them on the wall and floor. I am not sure what of what size grout lines to use. My contractor suggest 1/8" grout lines. Any suggestions?

Michael. read Deborah Butlers answer, basically if the tile is not perfect then go with wider joints.