Ideally, the toilet's rough-in should match the rough-in for your bathroom plumbing. However, even if a toilet is designed for a 10-inch rough-in, you can still install it into a space that has a 12-inch rough-in. Visually, it will appear differently than a toilet installed with the proper rough-in, but it should still function correctly.
How It Works
The rough-in distance is the distance from the wall behind the toilet, to the center of the drain opening that is underneath the toilet. This measurement is important because it tells you how much room the toilet needs to fit conveniently, allowing adequate space between the wall and the tank. A toilet with a 12-inch rough-in will definitely not fit into a space with a 10-inch rough-in, but a toilet with a 10-inch rough-in actually has extra space in a 12-inch rough-in space.
When a toilet with a 10-inch rough-in is placed in a 12-inch rough-in space there will be 2 inches of open space between the tank and wall. It won't affect how the toilet itself operates. However, the extra inches will cause the tank to be further away from the shutoff valve on the wall, so a longer connector may be needed to reach effectively. If the toilet paper holder is on the wall behind the toilet, users will need to reach a little further back to tear off the paper.
Different Rough-in Measurements
A 10-inch rough-in is the usual measurement installed in older homes. However, the 12-inch measurement is the standard for modern toilet plumbing. Most toilets that you encounter in retail outlets have 12-inch rough-in dimensions. The 10-inch rough-in measurements are still sometimes found in homes, and a lesser number of toilets are normally available for them. Another measurement used in the past, a 14-inch rough-in, is much more rare these days.
Determining the Rough-in
To determine the rough-in, the only item you really need is a measuring tool, such as a tape measure. Measure from the wall itself, not the baseboard, or the measurement will be shorter than the actual rough-in dimension. If the toilet is already in place, measure to the halfway point of the flange bolt, or the halfway point of the bolt cap, on the toilet bowl's base. If the floor is bare, with only the flange, which sits on top of the toilet drainpipe, measure from the wall to the midpoint of the flange opening.