Measure the height of the wall with the tape measure. A standard wall will typically measure 8 feet.
Divide 8 by 3 for a result of 2.67 feet. Convert the feet to inches by multiplying by 12 for a result of 32.04 inches. If the wall has a light switch you will need to use a different method.
Measure from the floor to the bottom of the light switch. Subtract at least one inch from the measurement if you are not using a top cap. Make a note of the measurement on a piece of paper. This is the height of the wainscoting.
Subtract the height of the cap, if used, plus 1/2 an inch from the measurement in step 3 to determine the height of the wainscoting panel.