Causes

• A toilet bowl can be loose because the flange underneath the bowl is cracked or broken. Or because the flange's closet bolts, which anchor the bowl to the floor, may be loose or damaged. Also, the bowl may not have been installed on the flange correctly.
  
  

Prevention & Solution

• Twist the nuts on the closet bolts by hand to see if they turn. Tighten loose nuts with an adjustable wrench, but be sure to not overtighten. If the bowl is still loose, remove the bowl and inspect the bolts and flange for any damage. Replace the bolts and/or flange, if necessary. Also, check the floor around the area of the flange for any damaged or loose tiles.
  
  

Other Considerations

• If the bowl is very loose or rocks, it's likely that you also need to replace the wax seal underneath the toilet, even if tightening the nuts on the closet bolts remedies the looseness. Whenever you remove or lift the bowl, even for just a quick inspection underneath, replace this wax ring.