Measuring the diameter of your caster stem can be a bit tricky, especially if the end of the stem is rounded. (The end of your stem may be flat or rounded. This is not important. The two types of stems are interchangeable.)
It is important that the stems on the casters you will be replacing measure 7/16-inch in diameter.
One easy way to measure your caster stem is to use a 7/16-inch wrench. The edges of your caster stem should closely fit inside the wrench. (You can use either an open-ended wrench or a closed-ended wrench.) An adjustable wrench could also be used, but you will need to measure the wrench opening with a ruler and set the opening to 7/16-inch.
Another easy way to confirm that your caster stem is 7/16 of an inch in diameter is to cut a piece of thread or small string to a length of 1 3/8 inch (35 mm). You could even cut a small strip of paper to this length. The cut piece of string or paper should wrap around your caster stem, fitting perfectly if your stem has a diameter of 7/16 inch.
Or, if you don't like playing with little strings or pieces of paper, you can use a ruler. If your ruler does not show 16ths of an inch, remember, 7/16 of an inch is midway between 3/8ths and 1/2 of an inch.
Another item that you can use to measure the size of the stem holes is a drill bit. A 7/16-inch bit should fit into your stem hole, to a depth of at least a 1/4 inch. In some styles of stem holes, your drill bit will easily slide all the way into the holes, and may even feel a little loose in the hole (but not too loose!)
If you have a measuring square and a ruler, you can measure your stem directly using the following method: Place your stem on a cabinet or other flat surface that has a right angle. Then place an "L" shaped measuring tool (square) on the stem, such that one leg of the square is flush with the vertical edge of the cabinet, and the top of the square rests on the surface of the stem. Now use a ruler to measure the distance between the horizontal surface of the cabinet and the edge of your square. Take care that the edge of your square does not slip into the groove in your caster stem.
Please measure your caster stem before ordering!