Casters, or swivel casters, make heavy furniture and occupied chairs move easily in any direction. The caster wheels are commonly made of plastic that doesn't last as long as the furniture, so replacing casters is inevitable. The only measurement considered by many caster companies is the caster stem. If you are only replacing one out of four or five casters, the caster wheel height must also be considered. Find the right replacement caster size by measuring one of the casters on your furniture.
Find the height of caster wheels from the floor to the top of the nub on the caster stem. Place a plastic card parallel to the floor from the top of the stem nub to the edge of the caster wheel. Put a ruler between the floor and the card to measure wheel height.
Determine the stem length the furniture can receive, not the length of the current caster stems. Insert a pencil into the hole for a caster, eraser end first. Push the pencil in until it stops against the bottom of the hole. Mark the pencil neck at the edge of the hole with a pen. Remove the pencil and measure from the tip of the eraser to the ink mark on the pencil to determine how deep the hole is for receiving the stem of a caster.
Measure non-threaded caster stem diameters with wrenches. Three types of stems on casters are grip-neck stems, grip-ring stems and threaded stems. Place an open-end, or box, wrench around the middle of a grip-neck or grip-ring stem to measure its diameter. The two common measurements are 3/8 inch and 7/16 inch, so one of these two wrench sizes should fit the stem precisely.
Measure the non-threaded stem holes in your furniture with drill bits to find the stem diameters. Insert a 3/8- or 7/16-inch drill bit into the holes that receive caster stems. The bit size that fits the hole with a minimum amount of "play" -- wiggle room -- is the stem diameter size.
Take casters with threaded stems to a hardware store or home supply centre to find the stem measurement. Twist nuts over the stem until one fits. Use the labelled measurement of the fitting nut as the measurement of your threaded-stem caster.