Belts are NOT measured by diameter nor folding the belt in half. The most accurate way to measure the circumference of a v belt is with a v belt measuring tool or using a cloth tape measure (Not a steel tape measure or ruler). An additional option is to wrap a small diameter string/rope around the belt, then measure the length of the string.

Just use a small diameter string or rope, wrap it around the belt, and then measure its length. Make sure to add some tension to the belt and press the tape measure firmly against its back while measuring. Remember, belts stretch, but there's no universal value for this stretchiness.

V-belts are typically measured using three parameters: the width of the belt’s top, the depth of the belt, and the length of the belt. The length of a V-belt can be measured either by the inside diameter or the outside diameter, depending on the standard used.

You will notice that all V belts also sometimes referred to as wedge shaped belts are measured across the top think section and the depth of the belt. This means you would measure the widest part of the belt (thick section) and then from the widest part to the narrowest part of the belt.

If a v-belt is connected to a belt drive and the following is known: the diameter of each pulley and distance between pulleys, a v-belt is measured using an equation. If a v-belt is not on a pulley system, it can be measured using a measuring tape.