Do cases made for iPhone 6 fit the iPhone 6s?

I currently have the iPhone 6 and I'm getting the 6s soon. Can I still use the cases I have for my 6 on the 6s? I don't want to have to get all new ones!

Not necessarily. The 6s is a slightly different size so whether your fits depends upon the manufacturing tolerances of your existing cases and whether they have sufficient flexibility to stretch a little.

You'll only find out by trying them.

Most likely not. The iPhone 6 has a 4.7" screen versus the 5.5" for the 6 Plus. The 6 Plus is significantly larger.

Depends on the case. Some may, some may not since the 6s is different dimensionally than the 6.

Some cases are made with those dimensions in mind.