Example of chair leg that is of a solid material, i.e. wood or steel.

The bottom of the chair leg

MEASURE THE DIAMETER OF THE LEG CLARIFICATION!
Take a measurement from the outer sides of the bottom, make sure the ruler is paced in center of the bottom circle. The furniture leg's diameter will be having the exact size as the outside pad's diameter.

Example of an outside furniture pad.

Example of chair leg that is like a tube, primarily steel legs of steel of iron.

The bottom of the chair leg

MEASURE THE DIAMETER INSIDE THE LEG CLARIFICATION!
The furniture leg's inside diameter will be having the exact size as the pad's diameter.

Example of an inside furniture pad.

Furniture leg

The bottom of the chair leg

MEASURE THE DIAMETER ON THE BOTTOM CLARIFICATION!
The pad to be fitted on-top doesn't neccessarily have to match the exact diameter, it's ok if the pad is matching or having a bit smaller size.

Example of an on-top furniture pad.

Ruler.