Riding mowers have a variety of deck sizes and shapes. Some are square, while others are round, and still others are oblong. The deck is typically the widest part of the riding mower, and indicates how wide a cut the machine can make. One, two or three blades may be beneath the deck. It is important to understand the exact deck width to know if the mower can fit through gates in your yard, or into storage areas, or between permanent obstacles. Always measure the deck of any riding mower you're considering buying.
Examine the placement of the pulleys on the topside of your mower's deck to determine if any will obstruct a straight path from one side of the deck to the other. If there is no obstruction, feed your tape measure through the pulleys, across the center of the deck, and hook it on the opposite side. For a round deck, locate the widest area of the circle, or the diameter, and extend your tape measure across that area. Note the distance on your tape measure. This is your deck size.
Chock the rear wheels of the mower, and raise the front of the mower about 6 inches off the ground with a floor jack if your mower's deck is not accessible from the topside, or if the pulleys prevent a straight path for your tape measure.
Lie down on the ground next to the elevated mower, and pass the tape measure beneath the blades, hooking it on the opposite side of the deck. Note the distance on the tape measure. This is your deck size.